Managing Fungicide-Resistant
Gummy Stem BlightGummy stem blight (caused by Didymella bryoniae) is the most important fungal disease of watermelon in Florida and is very difficult to control during warm, rainy weather. Crown blight, leaf defoliation, and fruit rot can all occur when watermelon vines are infested with this fungus. Chlorothalonil products are fairly effective, but are limited near harvest, and ethylenebisdithiocarbamate (EBDC) products are less effective in rainy weather. Strobilurin fungicides were introduced a couple of years ago, and both azoxystrobin (Quadris®) and pyraclostrobin (Cabrio®) initially provided systemic protection against gummy stem blight.
Despite the recognition that resistance was an issue and labeling that limited the number and sequence of applications, resistance to this class of fungicide in watermelon has been observed, with less control of gummy stem blight each year. There is a new fungicide, boscalid, that has a unique mode of action. Like the strobilurins, the site of action is in the mitochondrion, but at a different point in the respiratory chain. Preliminary results in watermelon with this material in conjunction with pyraclostrobin and chlorothalonil demonstrate adequate gummy stem blight control. The addition of boscalid (plus the use of thiophanate-methyl in those areas not resistant to carbendazim fungicides) brings to a total of at least five effective chemistries that can be rotated to reduce the potential for gummy stem blight resistance. (Citrus & Vegetable Magazine, February 2004).
