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Poster: Biology & Disease Mgmt: Chemical Control


Resistance of Colletotrichum gloeosporioides from strawberry to azoxystrobin and reduced-sensitivity to thiophanate-methyl
M. OLIVEIRA (1), M. Chamorro (1), N. Peres (1) (1) University of Florida, U.S.A.

Colletotrichum gloeosporioides causes Colletotrichum crown rot (CCR) of strawberry in the southern United States. Symptoms start with wilting of the plants which consequently collapse. A discoloration can be observed when the crown is cut open. CCR is managed with the use of disease-free transplants and fungicide applications. Our objective was to evaluate the sensitivity of C. gloeosporioides to two common fungicides used to manage CCR in Florida strawberry fields. Sixty-three C. gloeosporioides isolated from strawberry crowns from 1995 to 2015 were evaluated in a conidial germination assay for sensitivity to thiophanate-methyl and azoxystrobin at discriminatory concentrations of 3 and 100 μg/ml. An isolate collected in 1999 was found resistant to azoxystrobin. After that, resistance was observed sporadically only in isolates collected after 2007. Most of the isolates recently collected during the 2015-16 FL strawberry season were resistant to azoxystrobin. A reduced sensitivity to thiophanate-methyl was observed for all isolates in both fungicide concentrations tested, and none of the isolates was completely sensitive. We conclude that C. gloeosporioides isolates resistant to azoxystrobin have been selected after the introduction of this fungicide in the market in 2001. However, it appears that C. gloeosporioides isolates from strawberry have a natural reduced sensitivity to thiophanate-methyl.