Monitoring Fungicide Sensitivity of Cercospora beticola of Sugar Beet for Disease Management Decisions

Preventing multi-resistance: New insights for managing fungal adaptation

Sustainable crop protection is vital for food security, yet it is under threat due to the adaptation of a diverse and evolving pathogen population. Resistance can be managed by maximising the diversity of selection pressure through dose variation and the spatial and temporal combination of active ingredients. This study explores the interplay between operational drivers for maximising the sustainability of management strategies in relation to the resistance status of fungal populations. We applied an experimental evolution approach to three artificial populations of Zymoseptoria tritici, an economically significant wheat pathogen, each differing in initial resistance status. Our findings reveal that diversified selection pressure curtails the selection of resistance in naïve populations and those with low frequencies of single resistance. Increasing the number of modes of action most effectively delays resistance development, surpassing the increase in the number of fungicides, fungicide choice based on resistance risk, and temporal variation in fungicide exposure. However, this approach favours generalism in the evolved populations. The prior presence of multiple resistant isolates and their subsequent selection in populations override the effects of diversity in management strategies, thereby invalidating any universal ranking. Therefore, the initial resistance composition must be specifically considered in sustainable resistance management to address real-world field situations.

Evaluating Rhizobacterial Antagonists for Controlling Cercospora beticola and Promoting Growth in Beta vulgaris

Cercospora beticola Sacc. is an ascomycete pathogen that causes Cercospora leaf spot in sugar beets (Beta vulgaris L.) and other related crops. It can lead to significant yield losses if not effectively managed. This study aimed to assess rhizosphere bacteria from sugar beet soil as a biological control agent against C. beticola and evaluate their effect on B. vulgaris. Following a dual-culture screening, 18 bacteria exhibiting over 50% inhibition were selected, with 6 of them demonstrating more than 80% control. The bacteria were identified by sequencing the 16S rRNA gene, revealing 12 potential species belonging to 6 genera, including Bacillus, which was represented by 4 species. Additionally, the biochemical and molecular properties of the bacteria were characterized in depth, as well as plant growth promotion. PCR analysis of the genes responsible for producing antifungal metabolites revealed that 83%, 78%, 89%, and 56% of the selected bacteria possessed bacillomycin-, iturin-, fengycin-, and surfactin-encoding genes, respectively. Infrared spectroscopy analysis confirmed the presence of a lipopeptide structure in the bacterial supernatant filtrate. Subsequently, the bacteria were assessed for their effect on sugar beet plants in controlled conditions. The bacteria exhibited notable capabilities, promoting growth in both roots and shoots, resulting in significant increases in root length and weight and shoot length. A field experiment with four bacterial candidates demonstrated good performance against C. beticola compared to the difenoconazole fungicide. These bacteria played a significant role in disease control, achieving a maximum efficacy of 77.42%, slightly below the 88.51% efficacy attained with difenoconazole. Additional field trials are necessary to verify the protective and growth-promoting effects of these candidates, whether applied individually, combined in consortia, or integrated with chemical inputs in sugar beet crop production.

GWAS reveals a rapidly evolving candidate avirulence effector in the Cercospora leaf spot pathogen

The major resistance gene BvCR4 recently bred into sugar beet hybrids provides a high level of resistance to Cercospora leaf spot caused by the fungal pathogen Cercospora beticola. The occurrence of pathogen strains that overcome BvCR4 was studied using field trials in Switzerland conducted under natural disease pressure. Virulence of a subset of these strains was evaluated in a field trial conducted under elevated artificial disease pressure. We created a new C. beticola reference genome and mapped whole genome sequences of 256 isolates collected in Switzerland and Germany. These were combined with virulence phenotypes to conduct three separate genome-wide association studies (GWAS) to identify candidate avirulence genes. We identified a locus associated with avirulence containing a putative avirulence effector gene named AvrCR4. All virulent isolates either lacked AvrCR4 or had nonsynonymous mutations within the gene. AvrCR4 was present in all 74 isolates from non-BvCR4 hybrids, whereas 33 of 89 isolates from BvCR4 hybrids carried a deletion. We also mapped genomic data from 190 publicly available US isolates to our new reference genome. The AvrCR4 deletion was found in only one of 95 unique isolates from non-BvCR4 hybrids in the United States. AvrCR4 presents a unique example of an avirulence effector in which virulent alleles have only recently emerged. Most likely these were selected out of standing genetic variation after deployment of BvCR4. Identification of AvrCR4 will enable real-time screening of C. beticola populations for the emergence and spread of virulent isolates.

Efficacy and Profitability of Fungicides for Managing Frogeye Leaf Spot on Soybean in the United States: A 10-Year Quantitative Summary

Frogeye leaf spot (FLS), caused by Cercospora sojina, is an economically important disease of soybean in the United States. Data from 66 uniform fungicide trials (UFTs) conducted from 2012 to 2021 across eight states (Alabama, Arkansas, Illinois, Iowa, Kentucky, Louisiana, Mississippi, and Tennessee) were gathered and analyzed to determine the efficacy and profitability of the following fungicides applied at the beginning pod developmental stage (R3): azoxystrobin + difenoconazole (AZOX + DIFE), difenoconazole + pydiflumetofen (DIFE + PYDI), pyraclostrobin (PYRA), pyraclostrobin + fluxapyroxad + propiconazole (PYRA + FLUX + PROP), tetraconazole (TTRA), thiophanate-methyl (TMET), thiophanate-methyl + tebuconazole (TMET + TEBU), and trifloxystrobin + prothioconazole (TFLX + PROT). A network meta-analytic model was fitted to the log of the means of FLS severity data and to the nontransformed mean yield for each treatment, including the nontreated. The percent reduction in disease severity (%) and the yield response (kg/ha) relative to the nontreated was the lowest for PYRA (11%; 136 kg/ha) and the greatest for DIFE + PYDI (57%; 441 kg/ha). A significant decline in efficacy over time was detected for PYRA (18 percentage points [p.p.]), TTRA (27 p.p.), AZOX + DIFE (18 p.p.), and TMET + TEBU (19 p.p.) by using year as a continuous covariate in the model. Finally, probabilities of breaking even were the greatest (>65%) for the most effective fungicide DIFE + PYDI and the lowest (<55%) for PYRA. Results of this meta-analysis may be useful to support decisions when planning fungicide programs.

Resistance to QoI and DMI Fungicides Does Not Reduce Virulence of C. beticola Isolates in North Central United States

Cercospora leaf spot (CLS) is a destructive disease limiting sugar beet production and is managed using resistant cultivars, crop rotation, and timely applications of effective fungicides. Since 2016, its causal agent, Cercospora beticola, has been reported to be resistant to quinone outside inhibitors (QoIs) and to have reduced sensitive to demethylation inhibitors (DMIs) in sugar beet growing areas in North Dakota and Minnesota. Isolates of C. beticola resistant to QoIs, DMIs, and both QoIs and DMIs were collected from fields in Foxhome, Minnesota, in 2017. Fitness of these resistant isolates was compared with that of QoI- and DMI-sensitive isolates in laboratory and greenhouse studies. In the lab, mycelial growth, spore production, and spore germination were measured. The results showed that resistant isolates had significantly less mycelial growth and spore production than sensitive isolates, while no significant difference in spore germination was detected. In the greenhouse, six leaf-stage sugar beets were inoculated with a spore suspension made from each resistant group and incubated in separate humidity chambers. CLS disease severity was evaluated visually at 7, 14, and 21 days after inoculation (DAI), and the areas under disease progress curve (AUDPC) were calculated. Resistant isolates had significantly smaller AUDPC but still caused as high disease severity as the sensitive ones at 21 DAI. Although QoI- and/or DMI-resistant isolates had a relatively slower disease development, they still caused high disease severity and need to be factored in disease management practices.

An in-field heat treatment to reduce Cercospora beticola survival in plant residue and improve Cercospora leaf spot management in sugarbeet

Introduction

Sugarbeets account for 55 to 60% of U.S. sugar production. Cercospora leaf spot (CLS), primarily caused by the fungal pathogen Cercospora beticola, is a major foliar disease of sugarbeet. Since leaf tissue is a primary site of pathogen survival between growing seasons, this study evaluated management strategies to reduce this source of inoculum.

Methods

Fall- and spring-applied treatments were evaluated over three years at two study sites. Treatments included standard plowing or tilling immediately post-harvest, as well as the following alternatives to tillage: a propane-fueled heat treatment either in the fall immediately pre-harvest or in the spring prior to planting, and a desiccant (saflufenacil) application seven days pre-harvest. After fall treatments, leaf samples were evaluated to determine C. beticola viability. The following season, inoculum pressure was measured by monitoring CLS severity in a susceptible beet variety planted into the same plots and by counting lesions on highly susceptible sentinel beets placed into the field at weekly intervals (fall treatments only).

Results

No significant reductions in C. beticola survival or CLS were observed following fall-applied desiccant. The fall heat treatment, however, significantly reduced lesion sporulation (2019-20 and 2020-21, P < 0.0001; 2021-22, P < 0.05) and C. beticola isolation (2019-20, P < 0.05) in at-harvest samples. Fall heat treatments also significantly reduced detectable sporulation for up to 70- (2021-22, P < 0.01) or 90-days post-harvest (2020-21, P < 0.05). Reduced numbers of CLS lesions were observed on sentinel beets in heat-treated plots from May 26-June 2 (P < 0.05) and June 2-9 (P < 0.01) in 2019, as well as June 15-22 (P < 0.01) in 2020. Both fall- and spring-applied heat treatments also reduced the area under the disease progress curve for CLS assessed the season after treatments were applied (Michigan 2020 and 2021, P < 0.05; Minnesota 2019, P < 0.05; 2021, P < 0.0001).

Discussion

Overall, heat treatments resulted in CLS reductions at levels comparable to standard tillage, with more consistent reductions across year and location. Based on these results, heat treatment of fresh or overwintered leaf tissue could be used as an integrated tillage-alternative practice to aid in CLS management.

Seedborne Cercospora beticola Can Initiate Cercospora Leaf Spot from Sugar Beet (Beta vulgaris) Fruit Tissue

Cercospora leaf spot (CLS) is a globally important disease of sugar beet (Beta vulgaris) caused by the fungus Cercospora beticola. Long-distance movement of C. beticola has been indirectly evidenced in recent population genetic studies, suggesting potential dispersal via seed. Commercial sugar beet "seed" consists of the reproductive fruit (true seed surrounded by maternal pericarp tissue) coated in artificial pellet material. In this study, we confirmed the presence of viable C. beticola in sugar beet fruit for 10 of 37 tested seed lots. All isolates harbored the G143A mutation associated with quinone outside inhibitor resistance, and 32 of 38 isolates had reduced demethylation inhibitor sensitivity (EC₅₀ > 1 µg/ml). Planting of commercial sugar beet seed demonstrated the ability of seedborne inoculum to initiate CLS in sugar beet. C. beticola DNA was detected in DNA isolated from xylem sap, suggesting the vascular system is used to systemically colonize the host. We established nuclear ribosomal internal transcribed spacer region amplicon sequencing using the MinION platform to detect fungi in sugar beet fruit. Fungal sequences from 19 different genera were identified from 11 different sugar beet seed lots, but Fusarium, Alternaria, and Cercospora were consistently the three most dominant taxa, comprising an average of 93% relative read abundance over 11 seed lots. We also present evidence that C. beticola resides in the pericarp of sugar beet fruit rather than the true seed. The presence of seedborne inoculum should be considered when implementing integrated disease management strategies for CLS of sugar beet in the future.

The Influence of Wetting Agent and Type of Nozzle on Copper Hydroxide Deposit on Sugar Beet Leaves (Beta vulgaris L.)

Protective fungicides are sensitive to environmental conditions such as rainfall and solar radiation. Therefore, it is important to prolong the biological activity and fungicide resistance to the above-mentioned factors that can be achieved by adding a wetting agent to the working solution. Additionally, the quality and efficiency of preventive contact fungicides significantly depend on the application technique. Thus, it is important to make the right choice of the nozzles and adjust the working parameters of the treatment. The aim of this work is to determine the influence of a wetting agent and type of nozzle on copper hydroxide (2 L ha−2) deposits on sugar beet leaves. Experiments are set up under laboratory and field conditions. A pinole-based wetting agent is applied at three rates (0.3, 0.6, and 1.0 L ha−1) and two types of nozzles are used (standard with flat jet and modern turbo-drop twin-jet). A brilliant blue tracer is added to a working solution to enable the measurement of copper hydroxide deposits. The deposit amount is recorded before and after the rain simulation (15 L m−2) with a spectrophotometer light beam. In order to ensure the timeliness of the application of fungicides, remote sensing of vegetative indices is used as an indicator of disease occurrence. The results indicated an increase in copper hydroxide deposits with the increase in wetting agent rates for both types of nozzles and in both laboratory and field experiments. Moreover, when applying the copper hydroxide mixtures with modern turbo drop nozzles, the increase in copper hydroxide deposit is significant, compared to the standard nozzles.

The Multilateral Efficacy of Chitosan and Trichoderma on Sugar Beet

The majority of all fungal formulations contain Trichoderma spp., making them effective biological control agents for agriculture. Chitosan, one of the most effective natural biopolymers, was also reported as a plant resistance enhancer and as a biocide against a variety of plant pathogens. An in vitro three-way interaction assay of T. atroviride, chitosan, and important plant pathogens (such as Cercospora beticola and Fusarium oxysporum) revealed a synergistic effect on fungistasis. Furthermore, chitosan coating on Beta vulgaris ssp. vulgaris seeds positively affected the onset and efficiency of germination. We show that priming with T. atroviride spores or chitosan leads to the induced expression of a pathogenesis-related gene (PR-3), but only supplementation of chitosan led to significant upregulation of phytoalexin synthesis (PAL) and oxidative stress-related genes (GST) as a defense response. Repeated foliar application of either agent promoted growth, triggered defense reactions, and reduced incidence of Cercospora leaf spot (CLS) disease in B. vulgaris. Our data suggest that both agents are excellent candidates to replace or assist common fungicides in use. Chitosan triggered the systemic resistance and had a biocidal effect, while T. atroviride mainly induced stress-related defense genes in B. vulgaris. We assume that both agents act synergistically across different signaling pathways, which could be of high relevance for their combinatorial and thus beneficial application on field.

Characterization of the Molecular Mechanisms of Resistance against DMI Fungicides in Cercospora beticola Populations from the Czech Republic

Cercospora leaf spot (CLS), caused by the fungal pathogen Cercospora beticola, is the most important foliar pathogen of sugar beet worldwide. Extensive reliance on fungicides to manage CLS has resulted in the evolution of fungicide resistance in C. beticola worldwide, including populations in the Czech Republic. One important class of fungicides used to manage CLS is the sterol demethylation inhibitors (DMI). The aim of our study was to assess DMI resistance in C. beticola from the Czech Republic and elucidate the molecular basis of DMI resistance in this population. A total of 50 isolates were collected in 2018 and 2019 from the major sugar beet growing regions of the Czech Republic and assessed for in vitro sensitivity to the DMI fungicides propiconazole, prochloraz, and epoxiconazole. These analyses identified three strains that exhibited 50% effective concentration (EC50) values > 1.0 μg mL-1 against respective fungicides, which were therefore considered resistant. In contrast, strains that exhibited lowest EC50 values were considered sensitive. To explore the molecular basis of resistance in these three strains, the cytochrome P450-dependent sterol 14α-demethylase (Cyp51) gene was sequenced. Sequence analysis identified a Y464S mutation in all three resistant strains. To assess whether Cyp51 gene expression may play a role in DMI resistance, selected strains were grown in vitro with and without fungicide treatment. These analyses indicated that Cyp51 gene expression was significantly induced after fungicide treatment. Thus, we conclude that Y464S point mutation along with induced Cyp51 gene overexpression is likely responsible for resistance against DMI fungicides in C. beticola from the Czech Republic.

Weather-Based Predictive Modeling of Cercospora beticola Infection Events in Sugar Beet in Belgium

Cercospora leaf spot (CLS; caused by Cercospora beticola Sacc.) is the most widespread and damaging foliar disease of sugar beet. Early assessments of CLS risk are thus pivotal to the success of disease management and farm profitability. In this study, we propose a weather-based modelling approach for predicting infection by C. beticola in sugar beet fields in Belgium. Based on reported weather conditions favoring CLS epidemics and the climate patterns across Belgian sugar beet-growing regions during the critical infection period (June to August), optimum weather conditions conducive to CLS were first identified. Subsequently, 14 models differing according to the combined thresholds of air temperature (T), relative humidity (RH), and rainfall (R) being met simultaneously over uninterrupted hours were evaluated using data collected during the 2018 to 2020 cropping seasons at 13 different sites. Individual model performance was based on the probability of detection (POD), the critical success index (CSI), and the false alarm ratio (FAR). Three models (i.e., M1, M2 and M3) were outstanding in the testing phase of all models. They exhibited similar performance in predicting CLS infection events at the study sites in the independent validation phase; in most cases, the POD, CSI, and FAR values were ≥84%, ≥78%, and ≤15%, respectively. Thus, a combination of uninterrupted rainy conditions during the four hours preceding a likely start of an infection event, RH > 90% during the first four hours and RH > 60% during the following 9 h, daytime T > 16 °C and nighttime T > 10 °C, were the most conducive to CLS development. Integrating such weather-based models within a decision support tool determining fungicide spray application can be a sound basis to protect sugar beet plants against C. beticola, while ensuring fungicides are applied only when needed throughout the season.

Genome-Wide Association and Selective Sweep Studies Reveal the Complex Genetic Architecture of DMI Fungicide Resistance in Cercospora beticola

The rapid and widespread evolution of fungicide resistance remains a challenge for crop disease management. The demethylation inhibitor (DMI) class of fungicides is a widely used chemistry for managing disease, but there has been a gradual decline in efficacy in many crop pathosystems. Reliance on DMI fungicides has increased resistance in populations of the plant pathogenic fungus Cercospora beticola worldwide. To better understand the genetic and evolutionary basis for DMI resistance in C. beticola, a genome-wide association study (GWAS) and selective sweep analysis were conducted for the first time in this species. We performed whole-genome resequencing of 190 C. beticola isolates infecting sugar beet (Beta vulgaris ssp. vulgaris). All isolates were phenotyped for sensitivity to the DMI tetraconazole. Intragenic markers on chromosomes 1, 4, and 9 were significantly associated with DMI fungicide resistance, including a polyketide synthase gene and the gene encoding the DMI target CbCYP51. Haplotype analysis of CbCYP51 identified a synonymous mutation (E170) and nonsynonymous mutations (L144F, I387M, and Y464S) associated with DMI resistance. Genome-wide scans of selection showed that several of the GWAS mutations for fungicide resistance resided in regions that have recently undergone a selective sweep. Using radial plate growth on selected media as a fitness proxy, we did not find a trade-off associated with DMI fungicide resistance. Taken together, we show that population genomic data from a crop pathogen can allow the identification of mutations conferring fungicide resistance and inform about their origins in the pathogen population.

Evidence for the association of target-site resistance in cyp51 with reduced DMI sensitivity in European Cercospora beticola field isolates

BACKGROUND

Cercospora leaf spot caused by Cercospora beticola is the most relevant foliar disease in sugar beet cultivation. In the last decade a decreasing sensitivity of C. beticola towards demethylation inhibitors (DMIs) occurred. Different mechanisms mediating a reduced sensitivity towards DMIs have been identified in different plant pathogens to date, such as target site mutations, over-expression or active excretion of the fungicide.

RESULTS

A sequencing of the cytochrome P450-dependent sterol 14α-demethylase gene sequence (cyp51) of diverse C. beticola isolates collected in different European countries with reduced DMI sensitivity was performed in order to find a possible correlation of mutations with higher EC₅₀ values. The amino acid alterations Y464S, L144F and I309T combined with L144F were found to be associated with a reduced sensitivity. Furthermore, mutations I387M, M145W and M145W with E460Q were found uniquely. Additionally, constitutive and fungicide triggered expression of cyp51 was assayed by means of RT-qPCR. A very strong induction of cyp51 mRNA expression in sensitive isolates suggests that the fungal cells upregulate expression to maintain ergosterol biosynthesis in DMI presence. The less intensive cyp51 induction in isolates with higher EC₅₀ values underlines the possible correlation of the found target-site mutations with reduced sensitivity.

CONCLUSION

This study provides new results about possible alterations in the target gene mediating reduced sensitivity of C. beticola towards DMIs and hypothesized a fungicide induced over-expression of the target enzyme CYP51 as natural reaction of the fungus to fungicide application. © 2020 Society of Chemical Industry.

Spatial Variation and Temporal Dynamics of Fungicide Sensitivity in Venturia effusa Within a Pecan Orchard

An 18-ha commercial pecan orchard was sampled over 3 years to study the spatial and temporal variation in fungicide sensitivity of Venturia effusa, cause of pecan scab. The orchard was divided into a two-dimensional, 8 × 8 grid of 64 quadrats, each containing nine trees (unless there were missing trees), and samples were collected once per year from each quadrat to be tested for sensitivity to fentin hydroxide, propiconazole, and thiophanate-methyl. Averaged across the orchard, insensitivity to all three fungicides was significantly lower in 2016 compared with 2015, but significantly greater for fentin hydroxide and thiophanate-methyl in 2017. Although significant spatial autocorrelation was observed for sensitivity to propiconazole in 2017 and for thiophanate-methyl in 2015 and 2017, indicating clustering, all other fungicide-by-year combinations were not significant. Omnidirectional spatial dependence was observed for sensitivity to propiconazole and thiophanate-methyl in 2017. In both instances, the semivariance increased linearly with lag distance; however, the range of spatial dependence was >276.5 m and could not be estimated accurately. Additionally, a separate sampling was conducted in all 3 years to identify an appropriate sampling size and pattern for fungicide sensitivity screening. A leaflet sample size of 165 in 11 groups of 15 allowed for accurate sensitivity testing for the three fungicides in all 3 years; however, a sample size of 45 leaflets in three groups of 15 was sufficient for quantifying sensitivity for propiconazole and thiophanate-methyl, in most cases. These results indicate that considerable biological variation in fungicide sensitivity exists in orchard-scale populations of V. effusa and that the spatial characteristics of those populations may differ in two-dimensional space depending on the growing season.

Fungicide Resistance in Venturia effusa, Cause of Pecan Scab: Current Status and Practical Implications

Pecan scab, caused by Venturia effusa, is the most economically damaging disease of pecan in the southeastern United States, and annual epidemics are most effectively managed through multiple fungicide applications. The fungicide applications can be the single greatest operating cost for commercial growers and the return on that investment is impacted by fungicide resistance. V. effusa produces multiple generations of conidia per season, exhibits substantial genetic diversity, overwinters as stromata in the tree, and is under immense selection from the applied fungicides, all of which lead to a high risk for developing fungicide resistance. Since the mid-1970s, resistance or reduced sensitivity has been observed in isolates of V. effusa to the methyl benzimidazole carbamates, demethylation inhibitors, quinone outside inhibitors, organotin compounds, and the guanidines. Over the last 10 years, several studies have been conducted that have improved both scab management and fungicide resistance management in V. effusa. The aim of this review is to summarize recent developments in our understanding of fungicide resistance in V. effusa in the context of scab management in southeastern pecan orchards. The history, modes of action, general use of the labeled fungicides, and mechanisms and stability of fungicide resistance in V. effusa are discussed; conclusions and future research priorities are also presented.

Cross-resistance to the new fungicide mefentrifluconazole in DMI-resistant fungal pathogens

In the European Union (EU), regulation of sterol demethylation inhibiting (DMI) fungicides is tightened due to their suspected endocrine disrupting properties. However, the new DMI fungicide mefentrifluconazole was reported to have high fungicidal activity with minimal adverse side effects. In addition, some evidence suggests inconsistent cross resistance between mefentrifluconazole and other azoles. In this study, mefentrifluconazole and other triazoles were examined for activity to select pathogens sensitive or resistant to DMIs using mycelial growth tests on fungicide-treated culture medium or spray trials using cucumber plants. Cross-resistance was confirmed for all of the fungal species tested but activity levels varied. The sensitivity of Monilinia fructicola from peach to mefentrifluconazole was higher compared to other DMIs. In contrast, the inhibitory activity of mefentrifluconazole was equal or slightly inferior compared to difenoconazole, tebuconazole, propiconazole in Colletotrichum spp., Alternaria alternaria sp. complex and Cercospora beticola isolated from peach and sugar beet, respectively. Similar tendencies (i.e. equal or slightly inferior activity and cross-resistance) were observed for cucumber powdery mildew (Podosphaera xanthii) resistant to triflumizole, myclobutanil, and difenoconazole. Despite cross-resistance to other DMIs, mefentrifluconazole is a promising fungicide for fungal disease control on peach and other crops, with a reportedly more favorable toxicity profile.

Cercospora beticola: The intoxicating lifestyle of the leaf spot pathogen of sugar beet

Cercospora leaf spot, caused by the fungal pathogen Cercospora beticola, is the most destructive foliar disease of sugar beet worldwide. This review discusses C. beticola genetics, genomics, and biology and summarizes our current understanding of the molecular interactions that occur between C. beticola and its sugar beet host. We highlight the known virulence arsenal of C. beticola as well as its ability to overcome currently used disease management strategies. Finally, we discuss future prospects for the study and management of C. beticola infections in the context of newly employed molecular tools to uncover additional information regarding the biology of this pathogen.

TAXONOMY

Cercospora beticola Sacc.; Kingdom Fungi, Phylum Ascomycota, Class Dothideomycetes, Order Capnodiales, Family Mycosphaerellaceae, Genus Cercospora.

HOST RANGE

Well-known pathogen of sugar beet (Beta vulgaris subsp. vulgaris) and most species of the Beta genus. Reported as pathogenic on other members of the Chenopodiaceae (e.g., lamb's quarters, spinach) as well as members of the Acanthaceae (e.g., bear's breeches), Apiaceae (e.g., Apium), Asteraceae (e.g., chrysanthemum, lettuce, safflower), Brassicaceae (e.g., wild mustard), Malvaceae (e.g., Malva), Plumbaginaceae (e.g., Limonium), and Polygonaceae (e.g., broad-leaved dock) families.

DISEASE SYMPTOMS

Leaves infected with C. beticola exhibit circular lesions that are coloured tan to grey in the centre and are often delimited by tan-brown to reddish-purple rings. As disease progresses, spots can coalesce to form larger necrotic areas, causing severely infected leaves to wither and die. At the centre of these spots are black spore-bearing structures (pseudostromata). Older leaves often show symptoms first and younger leaves become infected as the disease progresses.

MANAGEMENT

Application of a mixture of fungicides with different modes of action is currently performed although elevated resistance has been documented in most employed fungicide classes. Breeding for high-yielding cultivars with improved host resistance is an ongoing effort and prudent cultural practices, such as crop rotation, weed host management, and cultivation to reduce infested residue levels, are widely used to manage disease. USEFUL WEBSITE: https://www.ncbi.nlm.nih.gov/genome/11237?genome_assembly_id=352037.

Optimizing Cercospora Leaf Spot Control in Table Beet Using Action Thresholds and Disease Forecasting

Cercospora leaf spot (CLS), caused by the fungus Cercospora beticola, is the dominant foliar disease affecting table-beet production in New York. CLS epidemics occur annually and, if uncontrolled, will rapidly lead to defoliation. In broad-acre production, season-long maintenance of healthy leaves is important to facilitate harvest by top-pulling. Fungicides are the dominant means of CLS control and applications are initiated at an action threshold of 1 CLS lesion/leaf. Regular fungicide application occurs thereafter without regard for scheduling based on weather-based risk. The current action threshold was evaluated with selected fungicides in two replicated field trials. Copper oxychloride + copper hydroxide and propiconazole significantly improved CLS control if initiated prior to infection. Pydiflumetofen + difenoconazole significantly reduced area under the disease progress stairs compared with other fungicides tested and was most efficacious when applications began at 1 CLS lesion/leaf. Six replicated field trials also evaluated the utility of scheduling fungicides on weather-based risk rather than a calendar approach. Two risk thresholds (moderate and high) integrating the accumulation of daily infection values based on temperature and relative humidity from a forecaster for CLS in sugar beet were evaluated. Applications of pydiflumetofen + difenoconazole were reduced from three to two by using the forecaster at either risk threshold compared with calendar applications without affecting CLS control. For propiconazole, the moderate risk threshold provided CLS control equivalent to calendar applications and saved one spray per season. Thus, there was substantial scope to reduce spray frequency by scheduling based on weather-based risk rather than calendar applications. The optimal risk thresholds for pydiflumetofen + difenoconazole and propiconazole were high and moderate, respectively. In these trials, periods of high risk occurred less frequently than moderate risk, increasing the reapplication intervals and, hence, represented a less conservative approach to disease management.

Rapid Detection of Cercospora beticola in Sugar Beet and Mutations Associated with Fungicide Resistance Using LAMP or Probe-Based qPCR

Cercospora leaf spot (CLS), caused by the fungal pathogen Cercospora beticola, is the most destructive disease of sugar beet worldwide. Although growing CLS-tolerant varieties is helpful, disease management currently requires timely application of fungicides. However, overreliance on fungicides has led to the emergence of fungicide resistance in many C. beticola populations, resulting in multiple epidemics in recent years. Therefore, this study focused on developing a fungicide resistance detection "toolbox" for early detection of C. beticola in sugar beet leaves and mutations associated with different fungicides in the pathogen population. A loop-mediated isothermal amplification (LAMP) method was developed for rapid detection of C. beticola in infected sugar beet leaves. The LAMP primers specific to C. beticola (Cb-LAMP) assay was able to detect C. beticola in inoculated sugar beet leaves as early as 1 day postinoculation. A quinone outside inhibitor (QoI)-LAMP assay was also developed to detect the G143A mutation in cytochrome b associated with QoI resistance in C. beticola. The assay detected the mutation in C. beticola both in vitro and in planta with 100% accuracy. We also developed a probe-based quantitative PCR (qPCR) assay for detecting an E198A mutation in β-tubulin associated with benzimidazole resistance and a probe-based qPCR assay for detection of mutations in cytochrome P450-dependent sterol 14α-demethylase (Cyp51) associated with resistance to sterol demethylation inhibitor fungicides. The primers and probes used in the assay were highly efficient and precise in differentiating the corresponding fungicide-resistant mutants from sensitive wild-type isolates.

Cytochrome b Mutations F129L and G143A Confer Resistance to Azoxystrobin in Cercospora nicotianae, the Frogeye Leaf Spot Pathogen of Tobacco

Azoxystrobin is the only synthetic, systemic fungicide labeled in the United States for management of frogeye leaf spot (FLS) of tobacco (Nicotiana tabacum L.), caused by Cercospora nicotianae. Though traditionally considered a minor disease in the United States, FLS has recently become yield and quality limiting. In 2016 and 2017, 100 C. nicotianae isolates were collected from symptomatic tobacco from eight counties in Kentucky, United States. Prior to azoxystrobin sensitivity testing, some C. nicotianae isolates were found to utilize the alternative oxidase pathway and, after assay comparisons, conidial germination was utilized to evaluate sensitivity in C. nicotianae as opposed to mycelial growth. Azoxystrobin sensitivity was determined by establishing the effective concentration to inhibit 50% conidial germination (EC50) for 47 (in 2016) and 53 (in 2017) C. nicotianae isolates. Distributions of C. nicotianae EC50 values indicated three qualitative levels of sensitivity to azoxystrobin. Partial cytochrome b sequence, encompassing the F129L and G143A mutation sites, indicated single-nucleotide polymorphisms (SNPs) conferring the F129L mutation in C. nicotianae of moderate resistance (azoxystrobin at 0.177 ≤ EC50 ≤ 0.535 µg/ml) and the G143A mutation in isolates with an azoxystrobin-resistant phenotype (azoxystrobin EC50 > 1.15 µg/ml). Higher frequencies of resistant isolates were identified from greenhouse transplant (4 of 17) and conventionally produced (58 of 62) tobacco samples, as compared with field-grown tobacco (<4 weeks prior to harvest; 4 of 62) or organically produced samples (1 of 7), respectively. Together, these results suggest that resistance to azoxystrobin in C. nicotianae occurs broadly in Kentucky, and generate new hypotheses about selection pressure affecting resistance mutation frequencies.

Assessing Fitness Costs and Phenotypic Instability of Fentin Hydroxide and Tebuconazole Resistance in Venturia effusa

Sensitivity monitoring of Venturia effusa, cause of pecan scab, has revealed insensitivity to fentin hydroxide and tebuconazole, but recent research indicates that the insensitivity to fentin hydroxide is not stable. A study was undertaken to determine if a fitness cost may be responsible for this instability. In this study, experiments were conducted to evaluate fitness components and phenotypic stability of insensitivity of V. effusa to fentin hydroxide and tebuconazole. Conidial production, conidial germination, microcolony growth, sensitivity to osmotic stress, and sensitivity to oxidative stress in the absence of fungicide were compared for isolates with differing sensitivities to both fungicides. Percent conidial germination decreased linearly with increasing fentin hydroxide insensitivity, and microcolony growth on 1.0 mM H₂O₂ decreased linearly with increasing tebuconazole insensitivity. Stability of resistance was assessed on concentrations of 1.0, 3.0, and 10 µg/ml of both fungicides prior to and after five transfers on non-fungicide-amended medium. Tebuconazole insensitivity was stable after transfers, but fentin hydroxide insensitivity on 1.0 and 3.0 µg/ml decreased significantly after transfers, indicating instability. Here we provide evidence that in V. effusa tebuconazole insensitivity is stable and fentin hydroxide insensitivity is not. These results suggest that fentin-hydroxide-resistant V. effusa isolates have reduced conidial viability compared with sensitive isolates, which may allow the population to regain sensitivity in the absence of this frequently used fungicide.

Fungicide Sensitivity Monitoring of Alternaria spp. Causing Leaf Spot of Sugarbeet (Beta vulgaris) in the Upper Great Lakes

Alternaria leaf spot (ALS), caused by Alternaria spp., can occur wherever sugarbeet is grown. Infection by Alternaria spp. and disease management has historically been considered a minor issue in sugarbeet production in the United States. An increase of both incidence and severity in 2016 of ALS high enough to cause yield loss has been observed in Michigan. With a renewed need to consider potential management of this disease, the sensitivity was determined for populations of Alternaria spp. to three classes of fungicides currently labeled for management of leaf spot on sugarbeet, including demethylase inhibitor (DMI), quinone outside inhibitor (QoI), and organo-tin fungicides. Leaves with symptoms of ALS were sampled from sugarbeet fields in east-central Michigan and southwestern Ontario, Canada. Monoconidial isolates were obtained to determine sensitivity to each fungicide class above. A spiral gradient dilution method was used to estimate the fungicide effective concentration (in milligrams per liter) that caused a 50% inhibition of fungal growth in vitro for all isolates. Significant temporal shifts were detected in the frequencies of sensitivity phenotypes to DMI and QoI but not organo-tin fungicides from 2016 through 2017. Individual isolates of Alternaria spp. were recovered with cross-resistance to DMI and multiple resistance to DMI, QoI, and triphenyltin hydroxide fungicides. To our knowledge, this is the first report of a fungus other than Cercospora beticola with resistance to organo-tin fungicides. Fungicide sensitivity monitoring indicates that an effective integrated disease management approach combining fungicide efficacy trials and monitoring pathogen biology is essential for developing effective resistance management recommendations.

Dynamics of Fungicide Sensitivity in Venturia effusa and Fungicide Efficacy under Field Conditions

Venturia effusa, which causes pecan scab, has developed resistance to fungicides that were once effective. Over 2 years, laboratory-based sensitivity of fentin hydroxide (TPTH) and tebuconazole in V. effusa and their efficacy under field conditions were compared. Leaf and nut scab were assessed on pecan trees receiving 10 applications of TPTH, tebuconazole, azoxystrobin, azoxystrobin plus tebuconazole, TPTH plus tebuconazole, or no fungicide (NTC) per year. Sensitivity of V. effusa on leaflets collected from treated and nontreated trees was assessed in June and September, respectively. The mean relative germination (RGe) on TPTH at 30 µg/ml was 10.9 and 40.9% in 2016 and 4.2 and 0.6% in 2017. Mean relative growth (RGr) on tebuconazole at 1 µg/ml was 45.5 and 34.6% in 2016 and 69.3 and 56.3% in 2017. In both years, leaf and nut scab were significantly lower on trees treated with azoxystrobin, azoxystrobin + tebuconazole, or TPTH + tebuconazole when compared with NTC and tebuconazole-treated trees. Compared with the NTC, tebuconazole did not significantly reduce leaf scab in 2017 or nut scab in either year, indicating that an RGr value between 34.6 and 69.3% is likely to result in a control failure on tebuconazole-treated trees. Although better activity was expected, TPTH reduced scab with RGe values between 0.6 and 40.9%. These results are valuable for developing fungicide sensitivity thresholds to better predict fungicide performance.

Gene cluster conservation provides insight into cercosporin biosynthesis and extends production to the genus Colletotrichum

Species in the fungal genus Cercospora cause diseases in many important crops worldwide. Their success as pathogens is largely due to the secretion of cercosporin during infection. We report that the cercosporin toxin biosynthesis (CTB) gene cluster is ancient and was horizontally transferred to diverse fungal plant pathogens. Because our analyses revealed genes adjacent to the established CTB cluster with similar evolutionary trajectories, we evaluated their role in Cercospora beticola to show that four are necessary for cercosporin biosynthesis. Lastly, we confirmed that the apple pathogen Colletotrichum fioriniae produces cercosporin, the first case outside the family Mycosphaerellaceae. Other Colletotrichum plant pathogens also harbor the CTB cluster, which points to a wider role that this toxin may play in virulence.

Species in the genus Cercospora cause economically devastating diseases in sugar beet, maize, rice, soy bean, and other major food crops. Here, we sequenced the genome of the sugar beet pathogen Cercospora beticola and found it encodes 63 putative secondary metabolite gene clusters, including the cercosporin toxin biosynthesis (CTB) cluster. We show that the CTB gene cluster has experienced multiple duplications and horizontal transfers across a spectrum of plant pathogenic fungi, including the wide-host range Colletotrichum genus as well as the rice pathogen Magnaporthe oryzae. Although cercosporin biosynthesis has been thought to rely on an eight-gene CTB cluster, our phylogenomic analysis revealed gene collinearity adjacent to the established cluster in all CTB cluster-harboring species. We demonstrate that the CTB cluster is larger than previously recognized and includes cercosporin facilitator protein, previously shown to be involved with cercosporin autoresistance, and four additional genes required for cercosporin biosynthesis, including the final pathway enzymes that install the unusual cercosporin methylenedioxy bridge. Lastly, we demonstrate production of cercosporin by Colletotrichum fioriniae, the first known cercosporin producer within this agriculturally important genus. Thus, our results provide insight into the intricate evolution and biology of a toxin critical to agriculture and broaden the production of cercosporin to another fungal genus containing many plant pathogens of important crops worldwide.

Reversing resistance: different routes and common themes across pathogens

Resistance spreads rapidly in pathogen or pest populations exposed to biocides, such as fungicides and antibiotics, and in many cases new biocides are in short supply. How can resistance be reversed in order to prolong the effectiveness of available treatments? Some key parameters affecting reversion of resistance are well known, such as the fitness cost of resistance. However, the population biological processes that actually cause resistance to persist or decline remain poorly characterized, and consequently our ability to manage reversion of resistance is limited. Where do susceptible genotypes that replace resistant lineages come from? What is the epidemiological scale of reversion? What information do we need to predict the mechanisms or likelihood of reversion? Here, we define some of the population biological processes that can drive reversion, using examples from a wide range of taxa and biocides. These processes differ primarily in the origin of revertant genotypes, but also in their sensitivity to factors such as coselection and compensatory evolution that can alter the rate of reversion, and the likelihood that resistance will re-emerge upon re-exposure to biocides. We therefore argue that discriminating among different types of reversion allows for better prediction of where resistance is most likely to persist.

Management of Cercospora Leaf Spot in Conventional and Organic Table Beet Production

Cercospora leaf spot (CLS; Cercospora beticola) is the most important foliar disease affecting table beet. Epidemics occur annually and fungicides extend the survival of foliage to enable mechanized harvest. However, a high frequency of strobilurin-resistant C. beticola isolates necessitates the identification of fungicides with different modes of action for tactical rotation. There is also substantial demand for organically produced table beet, for which synthetic fungicides are prohibited. Five small-plot, replicated field trials were conducted over two years to evaluate conventional and Organic Materials Review Institute (OMRI)-listed products for CLS control in table beet cv. Ruby Queen at Geneva and Ithaca, New York. Benzovindiflupyr + difenoconazole significantly reduced temporal disease progress (measured by the area under the disease progress stairs; AUDPS) by 86.7 to 97.3% compared with nontreated plots, and mean survival time of leaves was significantly extended. The demethylation inhibitor, propiconazole, also provided significant disease control in two trials in 2016. Disease severity in plots treated with succinate dehydrogenase inhibitors (boscalid, fluxapyroxad + pyraclostrobin, and penthiopyrad) was significantly decreased compared with nontreated plots but less than other fungicides. Efficacious fungicides significantly increased the dry weight of foliage but did not significantly affect the dry weight of roots, and root shoulder diameter. The enhanced longevity of leaves and increased dry weight of foliage may extend opportunities for mechanized harvesting without deleteriously affecting root yield parameters which are strictly regulated for the processing markets. In two trials, copper octanoate + Bacillus amyloliquefaciens strain D747 (as Cueva + Double Nickel LC) resulted in significantly improved disease control in comparison with application of either product alone and provided comparable and reproducible disease control equivalent to conventional fungicides at both locations. The implications of these findings for CLS control in conventional and organic table beet production systems are discussed.

RNA-sequencing of Cercospora beticola DMI-sensitive and -resistant isolates after treatment with tetraconazole identifies common and contrasting pathway induction

Cercospora beticola causes Cercospora leaf spot of sugar beet. Cercospora leaf spot management measures often include application of the sterol demethylation inhibitor (DMI) class of fungicides. The reliance on DMIs and the consequent selection pressures imposed by their widespread use has led to the emergence of resistance in C. beticola populations. Insight into the molecular basis of tetraconazole resistance may lead to molecular tools to identify DMI-resistant strains for fungicide resistance management programs. Previous work has shown that expression of the gene encoding the DMI target enzyme (CYP51) is generally higher and inducible in DMI-resistant C. beticola field strains. In this study, we extended the molecular basis of DMI resistance in this pathosystem by profiling the transcriptional response of two C. beticola strains contrasting for resistance to tetraconazole. A majority of the genes in the ergosterol biosynthesis pathway were induced to similar levels in both strains with the exception of CbCyp51, which was induced several-fold higher in the DMI-resistant strain. In contrast, a secondary metabolite gene cluster was induced in the resistance strain, but repressed in the sensitive strain. Genes encoding proteins with various cell membrane fortification processes were induced in the resistance strain. Site-directed and ectopic mutants of candidate DMI-resistance genes all resulted in significantly higher EC50 values than the wild-type strain, suggesting that the cell wall and/or membrane modified as a result of the transformation process increased resistance to tetraconazole. Taken together, this study identifies important cell membrane components and provides insight into the molecular events underlying DMI resistance in C. beticola.

Fungicide Resistance in Cercospora kikuchii, a Soybean Pathogen

Isolates of Cercospora kikuchii, a soybean (Glycine max) pathogen causing Cercospora leaf blight and purple seed stain, were tested to determine baseline sensitivities (n = 50) to selected quinone outside inhibitor (QoI) fungicides by conducting radial growth assays on fungicide-amended media. Baseline effective fungicide concentration to inhibit 50% of fungal radial growth (EC₅₀) values were compared with EC₅₀ values for isolates collected in 2011 (n = 50), 2012 (n = 50), and 2013 (n = 36) throughout soybean-producing areas in Louisiana. Median EC₅₀ values for isolates subjected to QoI fungicides were significantly (P = 0.05) higher across all 3 years. Cross-resistance to QoI fungicides was observed in resistant isolates collected in 2011 to 2013. Discriminatory doses were developed for QoI fungicides to distinguish between sensitive and resistant isolates. On average, 89% of all isolates screened in 2011 to 2013 were resistant to QoI fungicides. At a discriminatory dose of thiophanate methyl (TM), a methyl benzimidazole carbamate (MBC) fungicide, at 5 μg/ml, resistance was detected in the 2000, 2011, 2012, and 2013 collections at 23, 38, 29, and 36%, respectively. Isolates exhibiting multiple resistance to QoI fungicides and TM also were detected in 2011, 2012, and 2013 at frequencies of 34, 26, and 31%, respectively. Based on these results, Cercospora leaf blight management strategies in Louisiana using solo applications of QoI or MBC fungicides in soybean should be reconsidered.

Identification of the G143A mutation associated with QoI resistance in Cercospora beticola field isolates from Michigan, United States

BACKGROUND

Cercospora leaf spot (CLS), caused by the fungus Cercospora beticola, is the most serious foliar disease of sugar beet (Beta vulgaris L.) worldwide. Disease control is mainly achieved by timely fungicide applications. In 2011, CLS control failures were reported in spite of application of quinone outside inhibitor (QoI) fungicide in several counties in Michigan, United States. The purpose of this study was to confirm the resistant phenotype and identify the molecular basis for QoI resistance of Michigan C. beticola isolates.

RESULTS

Isolates collected in Michigan in 1998 and 1999 that had no previous exposure to the QoI fungicides trifloxystrobin or pyraclostrobin exhibited QoI EC(50) values of ≤ 0.006 µg mL(-1) . In contrast, all isolates obtained in 2011 exhibited EC(50) values of > 0.92 µg mL(-1) to both fungicides and harbored a mutation in cytochrome b (cytb) that led to an amino acid exchange from glycine to alanine at position 143 (G143A) compared with baseline QoI-sensitive isolates. Microsatellite analysis of the isolates suggested that QoI resistance emerged independently in multiple genotypic backgrounds at multiple locations. A real-time PCR assay utilizing dual-labeled fluorogenic probes was developed to detect and differentiate QoI-resistant isolates harboring the G143A mutation from sensitive isolates.

CONCLUSION

The G143A mutation in cytb is associated with QoI resistance in C. beticola. Accurate monitoring of this mutation will be essential for fungicide resistance management in this pathosystem.

Efficacy of Variable Tetraconazole Rates Against Cercospora beticola Isolates with Differing In Vitro Sensitivities to DMI Fungicides

Cercospora leaf spot (CLS) of sugar beet is caused by the fungus Cercospora beticola. CLS management practices include the application of the sterol demethylation inhibitor (DMI) fungicides tetraconazole, difenoconazole, and prothioconazole. Evaluating resistance to DMIs is a major focus for CLS fungicide resistance management. Isolates were collected in 1997 and 1998 (baseline sensitivity to tetraconazole, prothioconazole, or difenoconazole) and 2007 through 2010 from the major sugar-beet-growing regions of Minnesota and North Dakota and assessed for in vitro sensitivity to two or three DMI fungicides. Most (47%) isolates collected in 1997-98 exhibited 50% effective concentration (EC₅₀) values for tetraconazole of <0.01 μg ml^-1, whereas no isolates could be found in this EC₅₀ range in 2010. Since 2007, annual median and mean tetraconazole EC₅₀ values have generally been increasing, and the frequency of isolates with EC₅₀ values >0.11 μg ml^-1 increased from 2008 to 2010. In contrast, the frequency of isolates with EC₅₀ values for prothioconazole of >1.0 μg ml^-1 has been decreasing since 2007. Annual median difenoconazole EC₅₀ values appears to be stable, although annual mean EC₅₀ values generally have been increasing for this fungicide. Although EC₅₀ values are important for gauging fungicide sensitivity trends, a rigorous comparison of the relationship between in vitro EC₅₀ values and loss of fungicide efficacy in planta has not been conducted for C. beticola. To explore this, 12 isolates exhibiting a wide range of tetraconazole EC₅₀ values were inoculated to sugar beet but no tetraconazole was applied. No relationship was found between isolate EC₅₀ value and disease severity. To assess whether EC₅₀ values are related to fungicide efficacy in planta, sugar beet plants were sprayed with various dilutions of Eminent, the commercial formulation of tetraconazole, and subsequently inoculated with isolates that exhibited very low, medium, or high tetraconazole EC₅₀ values. The high EC₅₀ isolate caused significantly more disease than isolates with medium or very low EC₅₀ values at the field application rate and most reduced rates. Because in vitro sensitivity testing is typically carried out with the active ingredient of the commercial fungicide, we investigated whether loss of disease control was the same for tetraconazole as for the commercial product Eminent. The high EC₅₀ isolate caused more disease on plants treated with tetraconazole than Eminent but disease severity was not different between plants inoculated with the very low EC₅₀ isolate.

Characterization of CbCyp51 from field isolates of Cercospora beticola

The hemibiotrophic fungus Cercospora beticola causes leaf spot of sugar beet. Leaf spot control measures include the application of sterol demethylation inhibitor (DMI) fungicides. However, reduced sensitivity to DMIs has been reported recently in the Red River Valley sugar beet-growing region of North Dakota and Minnesota. Here, we report the cloning and molecular characterization of CbCyp51, which encodes the DMI target enzyme sterol P450 14α-demethylase in C. beticola. CbCyp51 is a 1,632-bp intron-free gene with obvious homology to other fungal Cyp51 genes and is present as a single copy in the C. beticola genome. Five nucleotide haplotypes were identified which encoded three amino acid sequences. Protein variant 1 composed 79% of the sequenced isolates, followed by protein variant 2 that composed 18% of the sequences and a single isolate representative of protein variant 3. Because resistance to DMIs can be related to polymorphism in promoter or coding sequences, sequence diversity was assessed by sequencing >2,440 nucleotides encompassing CbCyp51 coding and flanking regions from isolates with varying EC(50) values (effective concentration to reduce growth by 50%) to DMI fungicides. However, no mutations or haplotypes were associated with DMI resistance or sensitivity. No evidence for alternative splicing or differential methylation of CbCyp51 was found that might explain reduced sensitivity to DMIs. However, CbCyp51 was overexpressed in isolates with high EC(50) values compared with isolates with low EC(50) values. After exposure to tetraconazole, isolates with high EC(50) values responded with further induction of CbCyp51, with a positive correlation of CbCyp51 expression and tetraconazole concentration up to 2.5 μg ml(-1).

Evaluation of the potential for sexual reproduction in field populations of Cercospora beticola from USA

Cercospora leaf spot, caused by the hemibiotrophic fungal pathogen Cercospora beticola, is the most economically damaging foliar disease of sugarbeet worldwide. Although most C. beticola populations display characteristics reminiscent of sexual recombination, no teleomorph has been described. To assess whether populations in northern United States have characteristics consistent with sexual reproduction, 1024 isolates collected over a 3-y period were analyzed for frequency and distribution of mating type genes. After clone correction, an approximately equal distribution of mating types was found for each sampling year. Mating type frequency was also assessed in individual lesions. Lesions always consisted of isolates with a single mating type and microsatellite haplotype, but both mating types and up to five microsatellite haplotypes could be found on an individual leaf. The MAT1-1-1 and MAT1-2-1 genes were sequenced from 28 MAT1-1 and 28 MAT1-2 isolates, respectively. Three MAT1-1-1 nucleotide haplotypes were identified that encoded a single amino acid sequence. For MAT1-2-1, five nucleotide haplotypes were identified that encoded four protein variants. MAT1-1-1 and MAT1-2-1 gene expression analyses were conducted on plants inoculated with either or both mating types. MAT1-1-1 expression remained low, but MAT1-2-1 spiked during late stages of colonization. A segment of the MAT1-2-1 coding sequence was also found in MAT1-1 isolates. Taken together, these results suggest that C. beticola has the potential for sexual reproduction.

Fungicide resistance assays for fungal plant pathogens

Fungicide resistance assays are useful to determine if a fungal pathogen has developed resistance to a fungicide used to manage the disease it causes. Laboratory assays are used to determine loss of sensitivity, or resistance, to a fungicide and can explain fungicide failures and for developing successful fungicide recommendations in the field. Laboratory assays for fungicide resistance are conducted by measuring reductions in growth or spore germination of fungi in the presence of fungicide, or by molecular procedures. This chapter describes two techniques for measuring fungicide resistance, using the sugarbeet leaf spot fungus Cercospora beticola as a model for the protocol. Two procedures are described for fungicides from two different classes; growth reduction for triazole (sterol demethylation inhibitor; DMI) fungicides, and inhibition of spore germination for quinone outside inhibitor (QoI) fungicides.