Downy Mildew on Cucurbits
Dan Egel, Extension Vegetable Pathologist, SWPAC, Purdue University
Downy mildew has been observed on pumpkins in south central Indiana. The disease has the potential to affect all cucurbits including cucumber, cantaloupe squash, zucchini and watermelon. Growers in southern Indiana should consider treating for downy mildew as outlined below. Since downy mildew has been observed in southwestern Michigan on cucumber (Vegetable Crops Hotline Issue 572 - pdf file) growers in northern Indiana should also consider treating for this important disease of cucurbits.
The fungus that causes downy mildew of cucurbits does not overwinter in Indiana. Instead it must be blown into the state on winds or brought in on plant material such as transplants. Downy mildew is favored by cool temperatures (59 to 68 F) and as little as 6 hours of leaf wetness. An overnight dew is plenty of moisture. The recent cool, foggy weather over much of Indiana has been perfect for downy mildew. The disease does not affect fruit directly, but is can quickly devastate cucurbit foliage, causing loss of yield and fruit quality.
The symptoms of downy mildew vary somewhat by cucurbit crop. In general, leaf lesions are a bright yellow initially. In time, the center of lesions may become necrotic (brown). Often the lesions are limited by veins and thus may appear angular. On watermelon, the lesions may appear more diffuse and less angular. The bottom side of all cucurbit lesions will appear ‘fuzzy’ due to the growth of the downy mildew fungus. However, the fungal growth may only be obvious under moist conditions such as when due is present or after a rain. Downy mildew symptoms on pumpkin are shown here in Figures 1 and 2.
The spores of the downy mildew fungus that appear on the bottom of leaves are readily spread by wind. Under some circumstances, the spores can be blown great distances. Should the spores land on a cucurbit leaf, the appropriate temperature and leaf wetness will allow disease to become established.
Cucurbit growers can follow the spread of downy mildew in the Eastern U.S. through the website http://cdm.ipmpipe.org. It is possible to set up an alert system via this website so that one can get emails should the disease show up in your region. Articles on the presence of downy mildew will also be featured in the Vegetable Crops Hotline when appropriate. Finally, growers should scout their fields for symptoms of this disease. Plant samples suspected of having downy mildew may be submitted to the Plant Pest and Diagnostic Laboratory at Purdue University.
The decision regarding whether to apply a relatively expensive product for downy mildew may depend on the value of the crop and the time to harvest. Cucurbits within 2 weeks or so of harvest may not need special fungicides. Unfortunately, some watermelon growers have started harvest late this year and may need to take corrective action. Most pumpkin crops are just now starting to set fruit. In contrast, many cucumber and cantaloupes crops are close to harvest.
Although many products mentioned in the Midwest Vegetable Production Guide for Commercial Growers 2013 (ID-56) are listed as having ‘fair’ efficacy for several diseases including downy mildew, the products that are listed as good are often labeled for downy mildew and possibly Phytophthora blight only. The products that are listed as ‘good’ in the ID-56 include:
- Several products with the active ingredient phosphorus acid include Agri-Phos, Phostrol, Prophyte, Rampart. These are often tank mixed with the other products listed below. Also listed as having fair activity against gummy stem blight.
- Curzate-downy mildew only.
- Fontelis-labled for several diseases.
- Gavel-has performed well in the past. Not labeled for pumpkins.
- Presidio-one of our better products. Also listed for Phytophtora blight.
- Previcur Flex-downy mildew only. This product has performed well in my trials in the past.
- Ranman-also labeled for Phytophtora blight. Excellent product.
- Tanos-listed for a few other diseases as well.
- Zampro- a new product this year, has worked well in other parts of the U.S.
For homeowners, a product with the active ingredient chlorothanonil should help to slow the progress of the disease. Organic growers should apply a product with copper as an active ingredient after checking with your certifying agency.
All growers should remember that fungicides are applied to slow the progress of the disease. No product will cure the disease in the field.
Growers that have used the Purdue University MELCAST weather-based disease-forecasting program to schedule fungicide applications should instead apply fungicides on about a 7-day application schedule due to the presence of downy mildew.
More information about downy mildew on pumpkins can be found at: BP-140: Downy Mildew of Pumpkins (pdf file).
Please check the label before using any product. Contact Dan Egel for questions or comments (812-886-0198, firstname.lastname@example.org).