Baseline and Differential Sensitivity to Two QoI Fungicides Among Isolates of Phytophthora cactorum That Cause Leather Rot and Crown Rot on Strawberry

Ametoctradin resistance risk and its resistance-related point mutation in PsCytb of Phytophthora sojae confirmed using ectopic overexpression

Ametoctradin is mainly used to treat plant oomycetes diseases, but the mechanism and resistance risk of ametoctradin in Phytophthora sojae remain unknown. This study determined the ametoctradin sensitivity of 106 P. sojae isolates and found that the frequency distribution of the median effective concentration (EC50) of ametoctradin was unimodal with a mean value of 0.1743 ± 0.0901 μg/mL. Furthermore, ametoctradin-resistant mutants had a substantially lower fitness index compared with that of wild-type isolates. Although ametoctradin did not show cross-resistance to other fungicides, negative cross-resistance to amisulbrom was found. In comparison to sensitive isolates, the control efficacy of ametoctradin to resistant mutants was lower, implying a low to moderate ametoctradin resistance risk in P. sojae. All ametoctradin-resistant mutants contained a S33L point mutation in PsCytb. A system with overexpression of PsCytb in the nucleus was established. When we ectopically overexpressed S33L-harboring PsCytb, P. sojae developed ametoctradin resistance. We hypothesized that the observed negative resistance between ametoctradin and amisulbrom could be attributed to conformational changes in the binding cavity of PsCytb at residues 33 and 220.

Identification of sequence mutations in Phytophthora cactorum genome associated with mefenoxam resistance and development of a molecular assay for the mutant detection in strawberry (F. × ananassa)

Phytophthora crown rot (PhCR) caused by Phytophthora cactorum is one of the most damaging diseases of strawberry worldwide. Mefenoxam is one of the major fungicides currently used to manage PhCR. However, the emergence and spread of resistant isolates have made controlling the pathogen in the field problematic. In the present study, using whole genome sequencing analysis, mutations associated with mefenoxam-resistant isolates were identified in six different genomic regions of P. cactorum. The 95.54% reads from a sensitive isolate pool and 95.65% from a resistant isolate pool were mapped to the reference genome of P. cactorum P414. Four point mutations were in coding regions while the other two were in noncoding regions. The genes harboring mutations were functionally unknown. All mutations present in resistant isolates were confirmed by sanger sequencing of PCR products. For the rapid diagnostic assay, SNP-based high-resolution melting (HRM) markers were developed to differentiate mefenoxam-resistant P. cactorum from sensitive isolates. The HRM markers R3-1F/R3-1R and R2-1F/R2-1R were suitable to differentiate both sensitive and resistant profiles using clean and crude DNA extraction. None of the mutations associated with mefenoxam resistance found in this study were in the RNA polymerase subunit genes, the hypothesized target of this compound in oomycetes. Our findings may contribute to a better understanding of the mechanisms of resistance of mefenoxam in oomycetes since serves as a foundation to validate the candidate genes as well as contribute to the monitoring of P. cactorum populations for the sustainable use of this product.

Resistance Risk and Resistance-Related Point Mutations in Target Protein Cyt b of the Quinone Inside Inhibitor Amisulbrom in Phytophthora litchii

Amisulbrom is a novel quinone inside inhibitor, which exhibits excellent inhibitory activity against phytopathogenic oomycetes. However, the resistance risk and mechanism of amisulbrom in Phytophthora litchii are rarely reported. In this study, the sensitivity of 147 P. litchii isolates to amisulbrom was determined, with an average EC₅₀ of 0.24 ± 0.11 μg/mL. The fitness of resistant mutants, obtained by fungicide adaption, was significantly lower than that of the parental isolates in vitro. Cross-resistance was detected between amisulbrom and cyazofamid. Amisulbrom could not inhibit the cytochrome bc1 complex activity with H15Y and G30E + F220L point mutations in cytochrome b (Cyt b) in vitro. Molecular docking indicated that the H15Y or G30E point mutation can decrease the binding energy between amisulbrom and P. litchii Cyt b. In conclusion, P. litchii might have a medium resistance risk to amisulbrom, and a novel point mutation H15Y or G30E in Cyt b could cause high amisulbrom resistance in P. litchii.

Resistance of Strawberry Cultivars and the Effects of Plant Ontogenesis on Phytophthora cactorum and P. nicotianae Causing Crown Rot

Phytophthora crown rot (PhCR) is an important disease of strawberry worldwide. Phytophthora cactorum is the most common causal agent, however, P. nicotianae was also recently reported causing PhCR in the U.S. Therefore, the goals of this study were to evaluate the resistance of strawberry cultivars from Florida and California, and to study the etiology of the two Phytophthora species causing PhCR. Sixteen strawberry cultivars were evaluated over three Florida seasons for susceptibility to P. cactorum, and P. nicotianae. Inoculations at different days after transplanting (DAT) were also carried out to evaluate the ability of both species to cause PhCR at different phenological stages of the plant. Plant wilting and mortality were assessed weekly, and disease incidence, and the area under the disease progress curve were calculated. Cultivars Sensation 'Florida127', 'Winterstar FL 05-107', and 'Florida Radiance' were susceptible, whereas 'Florida Elyana', 'Camarosa', 'Fronteras', 'Sweet Charlie', and 'Strawberry Festival' were highly resistant to both Phytophthora species. However, some cultivars exhibited stronger resistance to one species over the other. P. cactorum caused more PhCR when plants were inoculated at transplanting, 45, and 60 DAT, whereas P. nicotianae only caused disease when inoculated at transplanting. These results emphasize the importance of screening for disease resistance to guide management recommendations in commercial strawberry production as well as the need for proper pathogen identification since cultivar susceptibility might differ. Varying susceptibility to P. cactorum and P. nicotianae at different growth stages emphasizes the importance of considering both plant and pathogen biology when making management recommendations.

Analysis of resistance risk and resistance-related point mutations in Cyt b of QioI fungicide ametoctradin in Phytophthora litchii

BACKGROUND

Litchi downy blight, caused by Phytophthora litchii, is one of the most important diseases of litchi. Ametoctradin, as the only QioI (quinone inside and outside inhibitor) fungicide, has been registered in China in 2019. However, the ametoctradin-resistance risk and molecular basis in Phytophthora litchii have not been reported.

RESULTS

In this study, the sensitivity profile of 144 Phytophthora litchii strains to ametoctradin was determined, with a mean median effective concentration (EC₅₀ ) value of 0.1706 ± 0.091 μg mL^-1 . Nine stable resistant Phytophthora litchii mutants [resistance factor (RF) > 400] were derived from sensitive isolates using fungicide adaption. The compound fitness index of three resistant-mutants (HN10-1-1, HN10-1-2 and HN10-2-1) was similar or higher than that of their parental isolates in vitro. All these ametoctradin-resistant mutants were sensitive to metalaxyl, dimethomorph, oxathiapiprolin and cyazofamid. Two point mutations, leading to the S33L and D228N changes in PlCyt b (cytochrome b) were found in ametoctradin-resistant mutants. Eight ametoctradin-resistant mutants containing S33L showed increased sensitivity to azoxystrobin and amisulbrom, and one mutant containing D228N exhibited increased sensitivity to cyazofamid. In vitro enzyme activity test showed that ametoctradin could not inhibit the activity of cytochrome bc1 complex with S33L and D228N point mutation. AS-PCR primers were designed based on the S33L change to detect the ametoctradin-resistant strains in the future.

CONCLUSION

These results suggest that Phytophthora litchii has a medium to high resistance risk to ametoctradin in the laboratory. Two changes, S33L and D228N, in PlCyt b are likely to be associated with the observed ametoctradin resistance. © 2022 Society of Chemical Industry.

Natural plant products as effective alternatives to synthetic chemicals for postharvest fruit storage management

Fruits contain enormous source of vitamins that provides energy to the human body. These are also affluent in essential and vital vitamins, minerals, fiber, and health-promoting components, which has led to an increase in fruit consumption in recent years. Though fruit consumption has expanded considerably in recent years, the use of synthetic chemicals to ripen or store fruits has been steadily increasing, resulting in postharvest deterioration. Alternatives to synthetic chemicals should be considered to control this problem. Instead of utilizing synthetic chemicals, this study suggests using natural plant products to control postharvest decay. The aim of this study indicates how natural plant products can be useful and effective to eliminate postharvest diseases rather than using synthetic chemicals. Several electronic databases were investigated as information sources, including Google Scholar, PubMed, Web of Science, Scopus, ScienceDirect, SpringerLink, Semantic Scholar, MEDLINE, and CNKI Scholar. The current review focused on the postharvest of fruits has become more and more necessary because of these vast demands of fruits. Pathogen-induced diseases are the main component and so the vast portion of fruits get wasted after harvest. Besides, it may occur harmful during harvesting and subsequent handling, storage, and marketing and after consumer purchasing and also causes for numerous endogenous and exogenous diseases via activating ROS, oxidative stress, lipid peroxidation, etc. However, pathogenicity can be halted by using postharvest originating natural fruits containing bioactive elements that may be responsible for the management of nutritional deficiency, inflammation, cancer, and so on. However, issues arising during the postharvest diseases must be controlled and resolved before releasing the horticultural commodities for commercialization. Therefore, the control of postharvest pathogens still depends on the use of synthetic fungicides; however, due to the problem of the development of the fungicide-resistant strains there is a good demand of public to eradicate the use of pesticides with the arrival of numerous diseases that are expanded in their intensity by the specific chemical product. By using of the organic or natural products for controlling postharvest diseases of fruits has become a mandatory step to take. In addition, antimicrobial packaging may have a greater impact on long-term food security by lowering the risk of pathogenicity and increasing the longevity of fruit shelf life. Taken together, natural chemicals as acetaldehyde, hexanal, eugenol, linalool, jasmonates, glucosinolates, essential oils, and many plant bioactive are reported for combating of the postharvest illnesses and guide to way of storage of fruits in this review.

Cytochrome P450 and Glutathione S-Transferase Confer Metabolic Resistance to SYP-14288 and Multi-Drug Resistance in Rhizoctonia solani

SYP-14288 is a fungicide as an uncoupler of oxidative phosphorylation, which is effective in controlling fungal pathogens like Rhizoctonia solani. To determine whether R. solani can develop SYP-14288 resistance and possibly multi-drug resistance (MDR), an SYP-14288-resistant mutant of R. solani X19-7 was generated from wild-type strain X19, and the mechanism of resistance was studied through metabolic and genetic assays. From metabolites of R. solani treated with SYP-14288, three compounds including M1, M2, and M3 were identified according to UPLC-MS/MS analysis, and M1 accumulated faster than M2 and M3 in X19-7. When X19-7 was treated by glutathione-S-transferase (GST) inhibitor diethyl maleate (DEM) and SYP-14288 together, or by DEM plus one of tested fungicides that have different modes of action, a synergistic activity of resistance occurred, implying that GSTs promoted metabolic resistance against SYP-14288 and therefore led to MDR. By comparing RNA sequences between X19-7 and X19, six cytochrome P450s (P450s) and two GST genes were selected as a target, which showed a higher expression in X19-7 than X19 both before and after the exposure to SYP-14288. Furthermore, heterologous expression of P450 and GST genes in yeast was conducted to confirm genes involved in metabolic resistance. In results, the P450 gene AG1IA_05136 and GST gene AG1IA_07383 were related to fungal resistance to multiple fungicides including SYP-14288, fluazinam, chlorothalonil, and difenoconazole. It was the first report that metabolic resistance of R. solani to uncouplers was associated with P450 and GST genes.

Detection and Characterization of Quinone Outside Inhibitor-Resistant Phytophthora cactorum and P. nicotianae Causing Leather Rot in Florida Strawberry

Phytophthora cactorum and P. nicotianae cause leather rot (LR) of fruit and Phytophthora crown rot (PhCR) in strawberry. LR occurs sporadically but can cause up to 70% fruit loss when weather is conducive. In Florida's annual strawberry winter production system, PhCR can be severe, resulting in plant stunting, mortality, and severe yield loss. Azoxystrobin is labeled for control of LR but not for PhCR. The aims of this research were to determine the sensitivity of P. cactorum and P. nicotianae isolates from strawberry to azoxystrobin and to investigate mechanisms of quinone-outside-inhibitor resistance present in P. cactorum and P. nicotianae based on the known point mutations within the cytochrome b (cytb) gene. Isolates of both Phytophthora spp. causing LR and PhCR were collected from multiple strawberry fields in Florida between 1997 and 2020. Isolates were tested for sensitivity to azoxystrobin at 0, 0.01, 0.1, 1.0, 10, and 50 μg/ml on potato dextrose agar amended with salicylhydroxamic acid (100 μg/ml). Isolates were separated into two groups - sensitive isolates with the 50% effective concentration (EC₅₀) values <1.0 μg/ml, and resistant isolates having EC₅₀ values >50 μg/ml. P. cactorum and P. nicotianae resistance to azoxystrobin was found for isolates collected after 2010. The first 450 nucleotides of the mitochondrial cytb gene were sequenced from a selection of resistant and sensitive isolates of both species. The G143A mutation reported to confer resistance to azoxystrobin was found in all resistant P. cactorum isolates. However, in P. nicotianae, qualitative resistance was observed, but the isolates lacked all the known mutations in the cytb gene. This is the first report of resistance to azoxystrobin in P. cactorum and P. nicotianae.

Resistance to Mefenoxam of Phytophthora cactorum and Phytophthora nicotianae Causing Crown and Leather Rot in Florida Strawberry

Phytophthora cactorum and Phytophthora nicotianae cause leather rot on fruit and crown rot (PhCR) of strawberry plants. Leather rot is not a common disease in Florida; however, up to 50% yield loss has been reported in harvests after intense rainfall events. PhCR is an important disease worldwide and is characterized by a sudden wilting and collapse of plants. Mefenoxam is the most effective and widely used fungicide to control both diseases. P. cactorum and P. nicotianae isolates from leather rot and PhCR have been collected from multiple strawberry fields in Florida since 1997, and the sensitivity of 185 isolates was tested at 0, 0.05, 0.5, 5, and 100 µg/ml. The 50% effective concentration (EC⁵⁰) values of sensitive isolates ranged from 0.05 to 1 µg/ml. Resistance to mefenoxam (EC₅₀ values >100 µg/ml) was found among P. cactorum isolates collected after 2015, but no resistance was found in P. nicotianae isolates. During the 2015 to 2016, 2016 to 2017, 2017 to 2018, and 2018 to 2019 seasons, resistance was detected on 9, 10, 21, and 23% of the isolates collected, respectively. Mefenoxam-resistant isolates originated from 3 of the 24 strawberry nurseries monitored. This is the first report of the occurrence of P. cactorum resistance to mefenoxam in Florida, suggesting that alternative control strategies are needed to avoid the increase of mefenoxam-resistant populations of P. cactorum in Florida fields.

Phytophthora Crown Rot of Florida Strawberry: Inoculum Sources and Thermotherapy of Transplants for Disease Management

Phytophthora crown rot, caused mainly by Phytophthora cactorum but also by P. nicotianae, reported in 2018, is an important disease in the Florida strawberry annual production system. Mefenoxam is the most effective and widely used fungicide to manage this disease. However, because of pathogen resistance, alternatives to chemical control are needed. Phytophthora spp. were rarely recovered during the summer from soil of commercial farms where the disease was observed during the season. In a more detailed survey on research plots, neither of the two species was recovered 1 month after the crop was terminated and water was shut off. Therefore, Phytophthora spp. does not seem to survive in the soil over summer in Florida. In a field trial, asymptomatic nursery transplants harboring quiescent infections were confirmed as the major source of inoculum for these pathogens in Florida. Heat treatment of P. cactorum zoospores at 44°C for as little as 5 min was effective in inhibiting germination and colony formation; however, oospore germination was not inhibited by any of the tested temperatures in vitro. In the field, thermotherapy treatment of inoculated plants was shown to have great potential to serve as a nonchemical approach for managing Phytophthora crown rot in production fields and reducing mefenoxam-resistant populations in nursery transplants.

Sensitivity of Pythium spp. and Phytopythium spp. and tolerance mechanism of Pythium spp. to oxathiapiprolin

BACKGROUND

Oxathiapiprolin, developed by DuPont, is the only commercial oxysterol-binding protein inhibitor (OSBPI) of oomycete pathogens. Although the activity of oxathiapiprolin on some Pythium spp. and Phytopythium spp. has been reported, it has not been tested on many other species, and little is known about the mechanisms of Pythium spp. that are tolerant to it.

RESULTS

Oxathiapiprolin exhibited a strong inhibitory effect on mycelial growth of Phy. litorale, Phy. helicoides and Phy. chamaehyphon, with EC₅₀ values ranging from 0.002 to 0.013 μg mL^-1 . It also showed good effectiveness against Py. splendens and two Py. ultimum isolates, with EC₅₀ values ranging from 0.167 to 0.706 μg mL^-1 , but showed no activity against 14 other Pythium spp. Oxathiapiprolin provoked a slight upregulation of PuORP1 in Py. ultimum, but it did not lead to PaORP1-1 or PaORP1-2 overexpression in Py. aphanidermatum. Transformation and expression of PuORP1, PaORP1-1 or PaORP1-2 in the sensitive wild-type Phytophthora sojae isolate P6497 confirmed that either the PuORP1, PaORP1-1 or PaORP1-2 was responsible for the observed oxathiapiprolin tolerance.

CONCLUSION

This study showed that oxathiapiprolin had excellent activity against Phytopythium spp. but displayed a differentiated activity against different Pythium spp. ORP1s in Pythium spp. are positively related to the tolerance of Pythium species to oxathiapiprolin. © 2020 Society of Chemical Industry.

Decreased Sensitivity of Leptosphaeria maculans to Pyraclostrobin in Alberta, Canada

Leptosphaeria maculans, the causal agent of blackleg of canola (Brassica napus), can be managed with pyraclostrobin and other strobilurin fungicides. Their frequent application, however, poses a risk for the development of insensitivity in fungal populations. A collection of L. maculans single-spore isolates recovered from infected canola stubble in Alberta, Canada, in 2016 was evaluated for its pyraclostrobin sensitivity. In conventional growth plate assays, the concentration of pyraclostrobin required to inhibit fungal growth by 50% (EC₅₀) was determined to be 0.28 mg/liter in a subset of 38 isolates. This EC₅₀ was four times greater than the mean EC₅₀ (0.07 mg/liter) of baseline isolates collected in 2011. Two hundred sixty-three isolates were screened further with two discriminatory doses of 0.28 and 3.5 mg/liter of pyraclostrobin, resulting in growth inhibition values ranging from 16 to 82% and 41 to 100%, respectively. In microtiter plate assays with the same isolates, the mean EC₅₀ was determined to be 0.0049 mg/liter, almost 3.3 times greater than the mean EC₅₀ (0.0015 mg/liter) of the baseline isolates. The sensitivity of the isolates was also evaluated in microtiter plate assays with discriminatory doses of 0.006 and 0.075 mg/liter of pyraclostrobin, resulting in inhibition values ranging from 20 to 88% and 49 to 100%, respectively. This is the first report of isolates of L. maculans with increased insensitivity to pyraclostrobin in Canada, suggesting the need for improved fungicide stewardship.

Baseline Sensitivity and Control Efficacy of Pyraclostrobin Against Botryosphaeria dothidea Isolates in China

Botryosphaeria dothidea is an important fungal pathogen that causes apple ring rot, which can significantly reduce apple yield. Fungicide applications are the main control measure of apple ring rot worldwide. Pyraclostrobin is a quinone outside inhibitor (QoI) fungicide that has yet to be registered for control of B. dothidea in China. Baseline sensitivity of B. dothidea to pyraclostrobin (EC₅₀ of mycelial growth inhibition) was assessed for 97 isolates collected in Shandong Province. The EC₅₀ values ranged from 0.7010 to 7.1378 μg/ml with the mean value of 3.0870 μg/ml and displayed a unimodal frequency distribution. After cultured on fungicide-free PDA medium or on apples for multiple generations, the B. dothidea-resistant isolates (RS^T) remained resistant to pyraclostrobin, but exhibited similar virulence as the susceptible isolates (S^T). Cross-resistance investigation revealed that pyraclostrobin was not cross-resistant to tebuconazole, flusilazole, carbendazim, and iprodione. Field evolution showed that pyraclostrobin at 200 and 250 g a.i./ha provided greater than 80% control efficacy against apple ring rot disease when applied as a therapeutic or preventive fungicide. The efficacy was similar to fungicides that have been registered for apple.

Baseline Sensitivity and Control Efficacy of Tetramycin Against Phytophthora capsici Isolates in China

Tetramycin is a new biopesticide that combines high-level and broad-spectrum fungicidal activity, low toxicity, and environmental safety. In this study, 90 Phytophthora capsici isolates obtained from various regions in southern China were characterized for their baseline sensitivity to tetramycin. The protective and curative activities of tetramycin against P. capsici were determined on leaves of pepper, and the control efficacy of tetramycin in greenhouse experiments was also determined. Compared with mycelial growth, the formation of sporangia and the discharge of zoospores were inhibited by lower concentrations of tetramycin, approximately 5 µg ml^-1 on V8 media. The frequency distribution curves for the tetramycin sensitivity were unimodal, with mean values for the fungicide concentration that reduced mycelial growth, sporangia formation, and zoospore discharge by 50% compared with the control of 1.18 ± 0.91, 0.64 ± 0.42, and 0.63 ± 0.30 µg ml^-1, respectively. In addition, no correlation was observed between tetramycin and other fungicides tested, including mandipropamid, azoxystrobin, mefenoxam, fluazinam, fluopicolide, and famoxadone. Tetramycin exhibited both protective and curative effects against P. capsici in vitro, and its protective activity was better than its curative activity. In greenhouse experiments, tetramycin concentration of 60 and 90 µg ml^-1 provided a protective control efficacy of 47.1 to 56.4% and curative efficacy of 43.3 to 52.7%. These results demonstrated that tetramycin could serve as an excellent alternative fungicide to control Phytophthora blight of pepper.

Sensitivity of Mycosphaerella pinodes to Pyraclostrobin Fungicide

Mycosphaerella blight, caused by Mycosphaerella pinodes, is a destructive disease of field pea that is managed using foliar fungicides. Strobilurin fungicides have been used in western Canada for disease management since 2003. To assess the baseline sensitivities of M. pinodes isolates to the strobilurin fungicide pyraclostrobin, the effective concentration to reduce mycelial growth by 50% (EC₅₀) was determined for 70 isolates collected prior to 2003 from Alberta, Saskatchewan, North Dakota, and Washington State. Each of these isolates was sensitive to pyraclostrobin, with EC₅₀ values ranging from 0.03 to 0.29 mg liter^-1. The pyraclostrobin concentrations required to reduce conidia germination by 50% was lower, ranging from 0.008 to 0.041 mg liter^-1. In all, 324 isolates collected in 2010 and 2011 were tested for high levels of insensitivity by examining mycelial growth using a discriminatory dose of 5 mg liter^-1. Nineteen isolates were highly insensitive to pyraclostrobin, with EC₅₀ values of 80 to 216 mg liter^-1. Conidia of these isolates germinated when exposed to a discriminatory dose of 0.1 mg liter^-1. Insensitive isolates infected field pea plants treated with pyraclostrobin but sensitive isolates did not. The identification of insensitive isolates indicates that insensitivity may be emerging in the pathogen population.

Effects of the Novel Fungicide Benzothiostrobin on Sclerotinia sclerotiorum in the Laboratory and on Sclerotinia Stem Rot in Rape Fields

We determined the effects and efficacy of benzothiostrobin, a new strobilurin-derived fungicide, against the plant-pathogenic fungus Sclerotinia sclerotiorum (the causal agent of Sclerotinia stem rot). Mycelial growth and sclerotial germination in vitro were strongly inhibited by benzothiostrobin in the presence of salicylhydroxamic acid. On detached rapeseed leaves, benzothiostrobin at 40 μg/ml reduced lesion development by 87%. No cross-resistance was detected between benzothiostrobin and carbendazim, iprodione, fludioxonil, or boscalid. A formulated mixture of benzothiostrobin and fluazinam at 1:1 had synergistic activity against S. sclerotiorum in vitro. In field trials, benzothiostrobin alone or formulated with fluazinam at 1:1 (150 g a.i. ha^-1) was significantly (P < 0.05) superior to iprodione in controlling Sclerotinia stem rot of rapeseed. These results suggest that benzothiostrobin has substantial potential for the control of Sclerotinia stem rot.

Activity of a novel strobilurin fungicide benzothiostrobin against Sclerotinia sclerotiorum

Benzothiostrobin is a novel strobilurin fungicide. In this study, baseline sensitivity of Sclerotinia sclerotiorum (Lib.) de Bary to benzothiostrobin was determined using 100 strains collected during 2012 and 2013 from different geographical regions in Jiangsu Province of China, and the average EC50 value was 0.0218 (± 0.0111)μg/mL for mycelial growth. After benzothiostrobin treatment, hyphae were contorted with offshoot of top increasing and cell membrane permeability increased markedly, while sclerotial production and oxalic acid content significantly decreased. Benzothiostrobin strongly inhibited mycelial respiration within 12h and the oxygen consumption of the mycelia could not be inhibited after 24h. On detached rapeseed leaves, the protective and curative activity test of benzothiostrobin suggested that benzothiostrobin had good control efficiency against S. sclerotiorum, and protective activity was better than curative activity. These results will contribute to us evaluating the potential of the new strobilurin fungicide benzothiostrobin for management of diseases caused by S. sclerotiorum and understanding the mode of action of benzothiostrobin against S. sclerotiorum.

Evaluation of fungicides enestroburin and SYP1620 on their inhibitory activities to fungi and oomycetes and systemic translocation in plants

Enestroburin and SYP1620 are newly developed strobilurin chemicals carrying fungicidal activity and need to be fully characterized in activities of anti-oomycete or anti-fungi, disease prevention and systemic translocation in planta. Their inhibitory activities were examined by amending the chemical in agar media, on which selected plant pathogens were grown and mycelial growth were measured. Effective concentrations for 50% inhibition (EC50) of mycelial growth were calculated to determine the level of fungicide sensitivity of the pathogen. Azoxystrobin was used as control. To examine the prevention and systemic translocation in plants, the fungicides were either sprayed on wheat leaves or dipped on wheat roots, which then were detected using high performance liquid chromatography. All the three fungicides inhibited mycelial growth of Sphacelotheca reiliana, Phytophthora infestans, Peronophythora litchi, and Magnaporthe oryzae, with EC50 values ranging from 0.02 to 2.84μg/ml; EC50 of SYP1620 was significantly lower than that of azoxystrobin and enestroburin on Valsa mali, Gaeumannomyces graminis, Alternaria solani, and Colletotrichun orbiculare. The three QoI fungicides showed strong inhibitory activities on spore germination against the 13 pathogens tested and were highly effective on biotrophic pathogens tested. Enestroburin and SYP1620 penetrated and spread in wheat leaves, but the penetration and translocation levels were lower compared to azoxystrobin. The three fungicides were all rapidly taken up by wheat roots and transported upwards, with greater fungicide concentrations in roots than in stems and leaves. The results indicate that enestroburin and SYP1620 are systemic fungicides that inhibit a broad spectrum of fungi and oomycetes.

Distribution of QoI Resistance in Populations of Tangerine-Infecting Alternaria alternata in Florida

Chemical control, based on copper and quinone outside inhibitor (QoI) fungicides, has been essential for the management of brown spot of citrus, caused by Alternaria alternata. However, QoI control failures were detected recently in Florida. From 2008 to 2012, 817 monoconidial isolates of A. alternata from 46 citrus orchards were examined for sensitivity to azoxystrobin (AZ) and pyraclostrobin (PYR). Of the isolates, 57.6% were resistant to both fungicides, with effective concentration to inhibit 50% growth (EC₅₀) values greater than 5 μg/ml for AZ and 1 μg/ml for PYR. The mean EC₅₀ values for sensitive isolates were 0.139 and 0.020 μg/ml for AZ and PYR, respectively. The EC₅₀ values of both fungicides were highly correlated (P < 0.0001), indicating cross resistance. The proportion of resistant isolates differed significantly (P < 0.0001) among cultivars and with QoI application frequency (P < 0.0001). However, resistance was not significantly related (P = 0.364) to disease severity in the field (low, moderate, and high) or isolate virulence (P = 0.397). The molecular basis for QoI resistance was determined for a subset of 235 isolates using polymerase chain reaction restriction fragment length polymorphism of the cytochrome b gene. All resistant isolates showed the point mutation G143A. Based on the presence of one or two introns, isolates were classified as profile I and profile II, respectively. The resistance frequency was significantly higher (P < 0.0001) in isolate profile II, suggesting a higher selection pressure for resistant population profile II.

Reduced Sensitivity in Monilinia fructicola Field Isolates from South Carolina and Georgia to Respiration Inhibitor Fungicides

Quinone outside inhibitor (QoI) and succinate dehydrogenase inhibitor (SdhI) fungicides are respiration inhibitors (RIs) used for preharvest control of brown rot of stone fruit. Both chemical classes are site-specific and, thus, prone to resistance development. Between 2006 and 2008, 157 isolates of Monilinia fructicola collected from multiple peach and nectarine orchards with or without RI spray history in South Carolina and Georgia were characterized based upon conidial germination and mycelial growth inhibition for their sensitivity to QoI fungicides azoxystrobin and pyraclostrobin, SdhI fungicide boscalid, and a mixture of pyraclostrobin + boscalid. There was no significant difference (P = 0.05) between EC₅₀ values for inhibition of conidial germination versus mycelial growth. The mean EC₅₀ values based upon mycelial growth tests for 25 isolates from an orchard without RI-spray history were 0.15, 0.06, 2.23, and 0.09 μg/ml for azoxystrobin, pyraclostrobin, boscalid, and pyraclostrobin + boscalid, respectively. The respective mean EC₅₀ values for 76 isolates from RI-sprayed orchards in South Carolina were 0.9, 0.1, 10.7, and 0.13 μg/ml and for 56 isolates from RI-sprayed orchards in Georgia were 1.2, 0.1, 8.91, and 0.17 μg/ml. Overall, mean EC₅₀ values of populations from RI-sprayed orchards increased three-, two-, five-, and twofold between 2006 and 2008 for azoxystrobin, pyraclostrobin, boscalid, and pyraclostrobin + boscalid, respectively. A subset of 10 M. fructicola isolates representing low and high EC₅₀ values for azoxystrobin, boscalid, and boscalid + pyraclostrobin was selected for a detached fruit assay to determine disease incidence and severity following protective treatments of formulated RI fungicides at label rates. Brown rot incidence was greater than 50% when fruit were inoculated with isolates having EC₅₀ values of 2, 4, and 0.6 μg/ml for azoxystrobin, boscalid, and pyraclostrobin + boscalid, respectively. Pyraclostrobin failed to control any of the isolates tested in detached fruit assays. Based on minimum inhibitory concentration and brown rot incidence data, we recommend using 3 and 0.75 μg/ml as discriminatory doses to distinguish between sensitive isolates and those with reduced sensitivity to azoxystrobin and pyraclostrobin + boscalid, respectively. Results from our in vitro and in vivo assays indicate a shift toward reduced sensitivity in M. fructicola from the southeastern United States. No cross-resistance was observed between the QoI and the SdhI fungicides, which implies that rotation or tank mixtures of these two chemical classes can be used as a resistance management strategy.

Preventive Control of Pythium Root Dysfunction in Creeping Bentgrass Putting Greens and Sensitivity of Pythium volutum to Fungicides

Pythium root dysfunction (PRD), caused by Pythium volutum, has been observed on golf course putting greens established with creeping bentgrass in the southeastern United States since 2002. To evaluate preventative strategies for management of this disease, a 3-year field experiment was conducted in Pinehurst, NC on a 'G-2' creeping bentgrass putting green. Fungicide treatments were applied twice in the fall (September and October) and three times in the spring (March, April, and May) in each of the 3 years. Applications of pyraclostrobin provided superior preventative control compared with the other fungicides tested. Azoxystrobin and cyazofamid provided moderate control of PRD in two of three seasons. Experiments were conducted to determine whether the disease suppression provided by pyraclostrobin was due to fungicidal activity or physiological effects on the host. In vitro sensitivity to pyraclostrobin, azoxystrobin, fluoxastrobin, cyazofamid, mefenoxam, propamocarb, and fluopicolide was determined for 11 P. volutum isolates and 1 P. aphanidermatum isolate. Isolates of P. volutum were most sensitive to pyraclostrobin (50% effective concentration [EC₅₀] value = 0.005), cyazofamid (EC₅₀ = 0.004), and fluoxastrobin (EC₅₀= 0.010), followed by azoxystrobin (EC₅₀ = 0.052), and mefenoxam (EC₅₀ = 0.139). P. volutum isolates were not sensitive to fluopicolide or propamocarb. Applications of pyraclostrobin did not increase the foliar growth rate or visual quality of creeping bentgrass in growth-chamber experiments. This work demonstrates that fall and spring applications of pyraclostrobin, azoxystrobin, and cyazofamid suppress the expression of PRD symptoms during summer and that field efficacy is related to the sensitivity of P. volutum to these fungicides.

Resistance to QoI Fungicides in Ascochyta rabiei from Chickpea in the Northern Great Plains

Ascochyta blight, caused by Ascochyta rabiei (teleomorph: Didymella rabiei), is an important fungal disease of chickpea (Cicer arietinum). A monitoring program was established in 2005 to determine the sensitivity of A. rabiei isolates to the QoI (strobilurin) fungicides azoxystrobin and pyraclostrobin. A total of 403 isolates of A. rabiei from the Northern Great Plains and the Pacific Northwest were tested. Ninety-eight isolates collected between 2005 and 2007 were tested using an in vitro spore germination assay to determine the effective fungicide concentration at which 50% of conidial germination was inhibited (EC₅₀) for each isolate-fungicide combination. A discriminatory dose of 1 μg/ml azoxystrobin was established and used to test 305 isolates from 2006 and 2007 for in vitro QoI fungicide sensitivity. Sixty-five percent of isolates collected from North Dakota in 2005, 2006, and 2007 and from Montana in 2007 were found to exhibit a mean 100-fold decrease in sensitivity to both azoxystrobin and pyraclostrobin when compared to sensitive isolates, and were considered to be resistant to azoxystrobin and pyraclostrobin. Under greenhouse conditions, QoI-resistant isolates of A. rabiei caused significantly higher amounts of disease than sensitive isolates on azoxystrobin- or pyraclostrobin-amended plants. These results suggest that disease control may be inadequate at locations where resistant isolates are present.