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First Report of Botrytis mali Causing Gray Mold on Strawberry in California

Cosseboom, S. D., Ivors, K. L., Schnabel, G., Holmes, G. J.
Plant disease 2018 v.102 no.3 pp. 679
Botryotinia, Botrytis cinerea, acreage, ambient temperature, boscalid, conidia, cyprodinil, fenhexamid, fludioxonil, fruits, fungal growth, genes, glyceraldehyde-3-phosphate dehydrogenase, gray mold, humidity, iprodione, mycelium, pathogens, planting, polymerase chain reaction, pyraclostrobin, strawberries, thiophanate-methyl, tubulin, California, Eastern United States
Gray mold of strawberry is caused by Botryotinia ricini, Botrytis caroliniana, Botrytis cinerea, Botrytis fragariae, and Botrytis mali, but only B. cinerea has been reported in California (Amiri et al. 2016; Dowling and Schnabel 2017). In 2016, spores of Botrytis spp. were collected from strawberry fruit symptomatic of gray mold from 40 fields distributed throughout California’s strawberry production acreage. A polymerase chain reaction–based assay of variability within the NEP2 gene was used to differentiate a proportion of these isolates between B. cinerea, B. fragariae, and B. mali (Dowling and Schnabel 2017). Out of 80 isolates, one isolate collected from Coachella Valley was B. mali. In vitro colony morphology was slightly different from B. cinerea, with more pronounced aerial mycelium. Conidia measured 10.5 to 13.5 × 8.7 to 10.2 µm (average 10.5 × 8.8 µm). This isolate was also characterized as resistant to fludioxonil, reduced sensitivity to polyoxin D, and sensitive to boscalid, cyprodinil, fenhexamid, iprodione, isofetamid, penthiopyrad, pyraclostrobin, and thiophanate-methyl. The strawberry planting containing B. mali had a history of applications of fludioxonil but not polyoxin D. The glyceraldehyde-3-phosphate-dehydrogenase and β-tubulin genes of this isolate were sequenced (O’Gorman et al. 2008) and deposited in GenBank with accession numbers MF975546 and MF975545, respectively. Search of sequences in GenBank revealed 100% similarity for both regions with B. mali discovered in the Eastern United States (Dowling and Schnabel 2017). Koch’s postulates were performed with this isolate and an isolate of B. cinerea by stab-inoculating six mature ‘Monterey’ strawberry fruit with a 21G needle to a depth of 1 cm and applying 30 µl of a conidial suspension (1 × 10⁶ conidia/ml) to the wound. The same numbers of fruit were stabbed and received 30 µl of sterile deionized water as a control. After incubation at high humidity and at room temperature for 6 days postinoculation, typical gray mold symptoms developed on both the B. cinerea and B. mali inoculated fruit. The pathogens recovered were morphologically identified as B. cinerea and B. mali, respectively. No fungal growth occurred on control fruit. B. mali has been previously described on strawberry in the Eastern United States (Dowling and Schnabel 2017) but not in California. This additional insight of the distribution of B. mali advances knowledge of this recently revived species.