Watermelon Gummy Stem Blight Fungicide Programs in 2010

Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; keverts@umd.edu

Our weather has not been highly conducive to gummy stem blight or anthracnose in the last two weeks. Therefore, under low disease pressure a good strategy is to apply Bravo on a 7-day schedule. Alternatively, our trials over many years have demonstrated that under low disease pressure the spray intervals can be lengthened. Following the weather forecaster ‘MELCAST’ http://mdvegdisease.umd.edu/forecasting/index.cfm can help determine the safe interval that can be used without the likelihood of risking disease increases.

There are several fungicides available for gummy stem blight management. Although several products are available, the usefulness of some of these products is limited by resistance development in the pathogen. On Delmarva we have confirmed the presence of resistance in Didymella bryoniae, the pathogen, to fungicides in the FRAC code group 11 (strobilurins, including Quadris and Cabrio) and FRAC code 3 (demethylation inhibitors or DMIs, including Topsin M). Resistance to Pristine exists in Georgia, and therefore Pristine is not recommended in that state. We have not yet detected resistance to Pristine here. However many of our transplants are grown in the south and it would not be surprising to find that resistance has been introduced here.

The following are fungicide programs that performed well in trials in Maryland and other areas in the southeast US in 2009:

● Switch 14 fl oz/A (1 day PHI; FRAC codes 9 and 12) alternated with Bravo

● Folicur 8 fl oz/A (7 day PHI; FRAC code 3) alternated with Bravo

● Inspire Super at 20 oz/A (7 day PHI) alternated with Bravo (Inspire Super is a new product that has two active ingredients. Although one component is in the FRAC code 3 group – Inspire Super performed very well in 2009.)

● Pristine 12.5–18.5 oz/A (0 day PHI; FRAC codes 11 and 7) alternated with Bravo (Pristine continues to perform very well in my trials -it usually ranks at the top, although it is not always significantly better than other products. However, because it performs very poorly in Georgia trials and because of the potential for resistance development, use caution and monitor disease levels carefully, if you choose to use Pristine.)

Watermelon is susceptible to other diseases as well. Scout for downy mildew, Phytophthora fruit rot and powdery mildew. The presence of these diseases will require additional fungicide applications with products with different modes of action.

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