97 Shades of Gray: Genetic Interactions of the Gray Mold, Botrytis cinerea, with Wild and Domesticated Tomato

Resistance to Boscalid in Botrytis cinerea From Greenhouse-Grown Tomato

Gray mold, caused by the fungus Botrytis cinerea Pers ex Fr., is one of the most destructive spoilage diseases, severely affecting tomato production in Henan Province, China. Spraying fungicides from the flowering to the harvest stage is a necessary measure to reduce losses associated with B. cinerea infection. However, B. cinerea has developed resistance to fungicides in many countries. Boscalid is a succinate dehydrogenase inhibitor (SDHI) fungicide and was registered for the control of gray mold. In this study, a total of 269 B. cinerea isolates were collected from tomato in commercial greenhouses in different locations of Henan Province in 2014 and 2015. The sensitivity and resistance of B. cinerea field isolates were determined based on mycelial growth. The effective concentration 50 ranged from 0.11 to 15.92 µg/ml and 0.16 to 8.54 µg/ml, in 2014 and 2015, respectively. The frequency of low resistance to boscalid was 12.6 and 7.6%, and moderate resistance was 2.7 and 1.3% in 2014 and 2015, respectively. No highly resistant isolates were found in Henan Province, China. Mycelial growth, mycelial dry weight, spore production, and pathogenicity were not significantly different between resistant and sensitive phenotypes of the B. cinerea isolates. The results of cross-resistance testing showed no correlation between boscalid and carbendazim, procymidone, pyrimethanil, fluazinam, or fluopyram. In this study, the succinate dehydrogenase genes B (sdhB), C (sdhC), and D (sdhD) were analyzed and compared in sensitive and low-resistance and moderately resistant B. cinerea isolates to boscalid. Results showed that point mutations occurred simultaneously at sdhC amino acid positions 85 (G85A), 93 (I93V), 158 (M158V), and 168 (V168I) in 4 out of 10 sensitive isolates and 23 of 26 low-resistance and 5 of 5 moderately resistant B. cinerea isolates to boscalid. No point mutations were found in the sdhB and sdhD genes of all isolates. Furthermore, no point mutations were found in sdhB, sdhC, and sdhD genes in 3 of 26 low-resistance B. cinerea isolates to boscalid. Therefore, we speculate that the simultaneous point mutations in the sdhC gene may not be related to the resistance of B. cinerea to boscalid. These results suggested that there might be a substitution mechanism for the resistance of B. cinerea to the SDHI fungicide boscalid.

The polyphagous plant pathogenic fungus Botrytis cinerea encompasses host-specialized and generalist populations

The host plant is often the main variable explaining population structure in fungal plant pathogens, because specialization contributes to reduce gene flow between populations associated with different hosts. Previous population genetic analysis revealed that French populations of the grey mould pathogen Botrytis cinerea were structured by hosts tomato and grapevine, suggesting host specialization in this highly polyphagous pathogen. However, these findings raised questions about the magnitude of this specialization and the possibility of specialization to other hosts. Here we report specialization of B. cinerea populations to tomato and grapevine hosts but not to other tested plants. Population genetic analysis revealed two pathogen clusters associated with tomato and grapevine, while the other clusters co-occurred on hydrangea, strawberry and bramble. Measurements of quantitative pathogenicity were consistent with host specialization of populations found on tomato, and to a lesser extent, populations found on grapevine. Pathogen populations from hydrangea and strawberry appeared to be generalist, while populations from bramble may be weakly specialized. Our results suggest that the polyphagous B. cinerea is more accurately described as a collection of generalist and specialist individuals in populations. This work opens new perspectives for grey mould management, while suggesting spatial optimization of crop organization within agricultural landscapes.