Fungicide Spray Schedule for Cantaloupe and Watermelon

Foliar diseases of cantaloupe and watermelon can be a major negative impact on yield and fruit quality. This article will discuss management tools including cultural measures and fungicide schedules.

Know the foliar diseases that are likely to become a problem for Indiana production. The three most common foliar diseases of cantaloupe and watermelon are: Alternaria leaf blight, anthracnose and gummy stem blight. Other diseases of importance include Phytophthora blight and downy mildew.

A healthy field of cantaloupe or watermelon starts with healthy seed and transplants. Inspect seedlings as they are growing or at delivery for possible diseases. As suspicious symptoms arise, get an official diagnosis by calling me or sending the sample to the Purdue Plant and Pest Diagnostic Laboratory.

Remember, do not wait until symptoms of disease develop. Keep a regular schedule. To help watermelon growers apply fungicides according to the weather, Dr. Rick Latin at Purdue University developed MELCAST, a weather-based disease-forecasting system.

Contact fungicides should be the backbone of most cantaloupe and watermelon spray schedules. Two contact fungicides for this purpose have the active ingredient chlorothalonil or mancozeb. They may be used all season-long. Since both of these active ingredients have the FRAC code M, there is no need to alternate to a different mode of action.

Alternatively, contact fungicides may be used for most of the season with a few applications of relatively more expensive systemic fungicides included in the schedule. A contact fungicide is alternated with the systemic fungicides or systemic fungicides with different FRAC codes are used back-to-back. It is important not to use fungicides with the same FRAC codes back to back.

If the decision to integrate a few systemic fungicides into an application schedule is made, when should the systemic products be used? It is my belief that systemic products should be used at about lay-by. That is, at the point where the vines are piled on top of the plastic mulch-just after the last vine turning.  However, a trial to determine the best time to apply systemic fungicides has been funded by the Illiana Watermelon Association this year at the Southwest Purdue Ag Center.

The comments above are applicable to the common foliar diseases mentioned above (Alternaria leaf blight, anthracnose and gummy stem blight). Let me briefly discuss two other diseases-Phytophthora blight and downy mildew.

Phytophthora blight is caused by a soilborne fungus-like organism. It is favored by heavy rains and poorly drained soils. More information about Phytophthora blight can be found in issue 600 of the Vegetable Crops Hotline. The most effective fungicides are specialized systemic fungicides. Since Phytophthora blight does not affect watermelon vines, start fungicide applications when fruit are about softball size. Cantaloupe are less susceptible to Phytophthora blight compared to watermelon, however cantaloupe vines may become diseased. Therefore, specialized products should be started whenever conditions become conducive.

The fungus-like organism that causes downy mildew of cucurbits doesn’t overwinter in Indiana. Since the disease must ‘blow in’, downy mildew doesn’t usually appear in Indiana until August or September. It doesn’t make sense to apply specialized downy mildew products until the disease is nearby.   Watch for developments at this website or read the Vegetable Crops Hotline.

Some general comments. Contact fungicides, such as chlorothalonil and mancozeb, are designed so that if applied to foliage in a manner to get good coverage the foliage is protected. That is, the fungicides covers the foliage and inhibits spores that may be deposited on leaves. Contact fungicides do not move into the plant.

Systemic fungicides move into the plant. But how far the system fungicides moves within the plant depends on the product. Most do not move more than inch. Almost all systemic fungicides move toward the tip of the plant, not toward the base or roots. Systemic fungicides may have some efficacy against existing disease. Nevertheless, both contact and systemic fungicides are much more effective when applied before disease has become established.

I have mentioned a few fungicides in particular here, but just as examples. There may be other fungicides more effective for your purpose.  See the Midwest Vegetable Production Guide for Commercial Growers for more information.

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