Detection and Molecular Characterization of Benzimidazole Resistance Among Colletotrichum truncatum Isolates Infecting Bell Pepper in Trinidad

An Analysis of Postharvest Fungal Pathogens Reveals Temporal-Spatial and Host-Pathogen Associations with Fungicide Resistance-Related Mutations

Fungicides are the primary tools to control a wide range of postharvest fungal pathogens. Fungicide resistance is a widespread problem that has reduced the efficacy of fungicides. Resistance to FRAC-1 (Fungicide Resistance Action Committee-1) chemistries is associated with mutations in amino acid position 198 in the β-tubulin gene. In our study, we conducted a meta-analysis of β-tubulin sequences to infer temporal, spatial, plant host, and pathogen genus patterns of fungicide resistance in postharvest fungal pathogens. In total, data were acquired from 2,647 specimens from 12 genera of fungal phytopathogens residing in 53 countries on >200 hosts collected between 1926 and 2020. The specimens containing a position 198 mutation were globally distributed in a variety of pathosystems. Analyses showed that there are associations among the mutation and the year an isolate was collected, the pathogen genus, the pathogen host, and the collection region. Interestingly, fungicide-resistant β-tubulin genotypes have been in a decline since their peak between 2005 and 2009. FRAC-1 fungicide usage data followed a similar pattern in that applications have been in a decline since their peak between 1997 and 2003. The data show that, with the reduction of selection pressure, FRAC-1 fungicide resistance in fungal populations will decline within 5 to 10 years. Based on this line of evidence, we contend that a β-tubulin position 198 mutation has uncharacterized fitness cost(s) on fungi in nature. The compiled dataset can inform end users on the regions and hosts that are most prone to contain resistant pathogens and assist decisions concerning fungicide resistance management strategies.

Characterization of Bacillus velezensis AK-0 as a biocontrol agent against apple bitter rot caused by Colletotrichum gloeosporioides

Bacillus genus produces several secondary metabolites with biocontrol ability against various phytopathogens. Bacillus velezensis AK-0 (AK-0), an antagonistic strain isolated from Korean ginseng rhizospheric soil, was found to exhibit antagonistic activity against several phytopathogens. To further display the genetic mechanism of the biocontrol traits of AK-0, we report the complete genome sequence of AK-0 and compared it with complete genome sequences of closely related strains. We report the biocontrol activity of AK-0 against apple bitter rot caused by Colletotrichum gloeosporioides, which could lead to commercialization of this strain as a microbial biopesticide in Korea. To retain its biocontrol efficacy for a longer period, AK-0 has been formulated with ingredients for commercialization, named AK-0 product formulation (AK-0PF). AK-0PF played a role in the suppression of the mycelial growth of the fungicide-resistant pathogen C. gloeosporioides YCHH4 at a greater level than the non-treated control. Moreover, AK-0PF exhibited greater disease suppression of bitter rot in matured under field conditions. Here, we report the complete genome sequence of the AK-0 strain, which has a 3,969,429 bp circular chromosome with 3808 genes and a G+C content of 46.5%. The genome sequence of AK-0 provides a greater understanding of the Bacillus species, which displays biocontrol activity via secondary metabolites. The genome has eight potential secondary metabolite biosynthetic clusters, among which, ituD and bacD genes were expressed at a greater level than other genes. This work provides a better understanding of the strain AK-0, as an effective biocontrol agent (BCA) against phytopathogens, including bitter rot in apple.

Reduced sensitivity of azoxystrobin and thiophanate-methyl resistance in Lasiodiplodia theobromae from papaya

Stem-end rot caused by Lasiodiplodia theobromae is one of the most devastating diseases of papaya in northeastern Brazil. It is most effectively controlled by applications of fungicides, including site-specific fungicides at risk for resistance development. This study investigated the molecular mechanisms of reduced sensitivity to the QoI fungicide azoxystrobin and resistance to the MBC fungicide thiophanate-methyl in L. theobromae from Brazilian orchards. The EC₅₀ values for azoxystrobin in sixty-four isolates ranged from 0.36 μg/ml to 364.24 μg/ml and the frequency distribution of EC₅₀ values formed a multimodal curve, indicating reduced sensitivity to azoxystrobin. In detached fruit assays reduced sensitive isolates were not controlled as effectively as sensitive isolates at lowest label rate. Partial fragments were obtained from target genes β-tubulin (751 bp) and Cytb (687 bp) of isolates resistant to thiophanate-methyl and reduced sensitivity to azoxystrobin. Sequence analysis of the β-tubulin fragment revealed a mutation corresponding to E198K in all thiophanate-methyl-resistant isolates, while reduced sensitivity to axoxystrobin was not attributable to Cytb gene alterations. The target gene-based mechanism conferring resistance to thiophanate-methyl will likely be stable even if selection pressure subsides. However, the mechanism conferring reduced sensitivity to azoxystrobin is not based on target gene modifications and thus may not be as stable as other genotypes with mutations in Cytb gene.

Thiophanate-Methyl Resistance and Fitness Components of Colletotrichum musae Isolates from Banana in Brazil

Anthracnose, caused by Colletotrichum musae, is the most important postharvest disease of banana and is widely distributed among the banana production regions in Brazil. Although thiophanate-methyl is a fungicide frequently used in Brazilian banana orchards to control Sigatoka leaf spot, Collettotrichum populations are also exposed, resulting in the evolution of fungicide resistance and the inability to manage banana anthracnose. We investigated 139 Brazilian isolates of C. musae for thiophanate-methyl sensitivity in vitro. The 50% mycelial growth inhibition (EC₅₀) values varied between 0.003 and 48.73 μg/ml. One-hundred and thirty isolates were classified as sensitive, with EC₅₀ values ranging from 0.003 to 4.84 μg/ml, while the remaining nine isolates were considered moderately resistant, with EC₅₀ values ranging between 10.43 and 48.73 μg/ml. Resistant or highly resistant isolates (EC₅₀ > 100 μg/ml) were not found. A substitution of TAC for TTC at codon 200 in a coding region of the β-tubulin gene was associated with the moderately resistant phenotype. Applications of thiophanate-methyl formulation to detached banana fruit at the label rate (500 μg/ml) showed low efficacy in controlling the moderately resistant isolates on banana fruits. However, there is no indication of a reduction in fitness associated with fungicide resistance as sensitive and moderately resistant isolates do not differ with respect to mycelial growth rate (P = 0.098), spore production (P = 0.066), spore germination (P = 0.366), osmotic sensitivity (P = 0.051), and virulence (P = 0.057). Our results revealed absence of adaptability cost for the moderately resistant isolates, suggesting that they can be dominant in population if the fungicide continue to be applied.

Fungicide Sensitivity among Isolates of Colletotrichum truncatum and Fusarium incarnatum-equiseti Species Complex Infecting Bell Pepper in Trinidad

Bell pepper is an economically important crop worldwide; however, production is restricted by a number of fungal diseases that cause significant yield loss. Chemical control is the most common approach adopted by growers to manage a number of these diseases. Monitoring for the development to resistance to fungicides in pathogenic fungal populations is central to devising integrated pest management strategies. Two fungal species, Fusarium incarnatum-equiseti species complex (FIESC) and Colletotrichum truncatum are important pathogens of bell pepper in Trinidad. This study was carried out to determine the sensitivity of 71 isolates belonging to these two fungal species to fungicides with different modes of action based on in vitro bioassays. There was no significant difference in log effective concentration required to achieve 50% colony growth inhibition (LogEC50) values when field location and fungicide were considered for each species separately based on ANOVA analyses. However, the LogEC50 value for the Aranguez-Antracol location-fungicide combination was almost twice the value for the Maloney/Macoya-Antracol location-fungicide combination regardless of fungal species. LogEC50 values for Benomyl fungicide was also higher for C. truncatum isolates than for FIESC isolates and for any other fungicide. Cropping practices in these locations may explain the fungicide sensitivity data obtained.