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2000 Guidelines for Managing Powdery Mildew and Other Diseases of Cucurbits

(Issued 6-19-00)

Margaret Tuttle McGrath
Associate Professor, Department of Plant Pathology,
Long Island Horticultural Research and Extension Center, Cornell University
3059 Sound Avenue, Riverhead, NY 11901


Thomas A. Zitter
Professor, Department of Plant Pathology,
Cornell University, 334 Plant Science Building
Ithaca, New York 14853

For current managment recommendations see Powdery Mildew Management Guidelines for 2010


Powdery mildew is the most important disease of cucurbit crops in New York because it occurs throughout the state every year. Other diseases occur more sporadically (gummy stem blight/black rot, downy mildew, and Phytophthora blight, to name a few). Resistant varieties are the ideal method to manage powdery mildew; however, they are not available for all crops (e.g. winter squash) or resistance does not exist for the diversity of varieties chosen by growers. Therefore, many susceptible open-pollinated and hybrid varieties continue to be grown and need to be protected with fungicide sprays.

For current managment recommendations see Powdery Mildew Management Guidelines for 2010

2000 Guidelines for Managing Powdery Mildew and Other Diseases of Cucurbits
(Issued 6-19-00)

Managing fungicide resistance is a critical aspect of managing powdery mildew with fungicides. Fungicides that are systemic or have translaminar activity are needed to obtain adequate protection of the underleaf surfaces, where conditions are more favorable for development of the pathogen than on upper surfaces. Unfortunately, systemic fungicides are generally at-risk for resistance development because they have a specific mode of action (single site of activity), and powdery mildew fungi have exhibited a high potential for resistance development. This has been especially true of the cucurbit powdery mildew fungus, which has developed resistance to Benlate (benomyl), Topsin M (thiophanate-methyl), and Bayleton (triadimefon) in a short period of time.

Current recommendations for managing powdery mildew and fungicide resistance include using a diversity of fungicides. These should include at least two at-risk fungicides with different mode of action (but with single site of activity) plus multi-site contact fungicides that have a low risk of inducing resistance. Fungicides with the same mode of action typically have correlated resistance; in other words, a fungal strain with resistance to one fungicide is also resistant or less sensitive to other similar fungicides. Fortunately, three new at-risk fungicides have been registered for use in the U.S. since March 1999 and two more are being reviewed by EPA. These three new fungicides are the strobilurin fungicides Quadris (azoxystrobin, Zeneca) and Flint (trifloxystrobin, Novartis) and the DMI fungicide Nova (myclobutanil, Rohm & Haas). Unfortunately, these new fungicides have a high risk for resistance development; thus it is imperative to use them in a fungicide program designed to manage resistance. A high level of resistance to strobilurin fungicides developed within two years in other countries where these fungicides were used exclusively. Although Nova is much more effective than Bayleton, these fungicides are in the same fungicide class. As a result, strains of the powdery mildew fungus that are fully resistant to Bayleton (e.g. cannot be controlled by Bayleton) are less sensitive to Nova than strains that are sensitive to Bayleton. This correlated resistance accounts for why Nova applied at 2.5 oz/A in 1999 (the rate under the 1999 Section 18 registration) where Bayleton-resistant strains were common was not as effective as Nova applied at higher rates nor as effective as Nova applied at the low rate in fungicide efficacy trials conducted several years ago when Bayleton-resistant strains were rare. Consequently, Nova has been registered at a higher rate (5 oz/A).

The fungicide program recommended for cucurbits in 2000 is a strobilurin fungicide (Quadris or Flint) applied in alternation with Nova tank mixed with a contact fungicide. One major change from the program recommended in 1999 is to begin with a strobilurin fungicide. There are two reasons for this change. Resistance to Nova was more effectively managed when Quadris was the first application in an alternation program than when Nova plus Bravo (chlorothalonil, Zeneca) was applied first in an experiment conducted in Riverhead in 1999. Also, the greatest value of strobilurin fungicides is their outstanding ability to inhibit spore germination, thus they should be most useful early in disease development. To manage resistance to strobilurin fungicides (currently Quadris and Flint), no more than half of the applications in a season should include these products. In other areas of the world a limit of one-third is recommended. Their use can be minimized by using multi-site contact fungicides late in the growing season when powdery mildew control is less critical if gummy stem blight is not a problem.

Several multi-site contact fungicides are registered for use on cucurbits. Chlorothalonil (Bravo, Echo, etc.) fungicides and copper fungicides (Basicop, Champ, Kocide, etc.) are also effective for diseases other than powdery mildew. Sulfur (Microthiol special, Micro Sulf, etc.), potassium bicarbonate (Armicarb 100), monopotassium phosphate (Nutrol), mineral oil (JMS Stylet-oil, SunSpray Ultra-Fine, etc.), and biofungicides (AQ10) are only effective for powdery mildew.

Strobilurin fungicides are effective for other diseases. Quadris is also labeled for the control of belly rot (Rhizoctonia), gummy stem blight and black rot (Didymella), and downy mildew (Pseudoperonospora), which are problematic in the state most seasons. Flint is also labeled for the suppression of downy mildew. As with powdery mildew, strobilurin fungicides should not be used exclusively for belly rot, gummy stem blight/black rot, and downy mildew. For belly rot, Quadris is effective if applied when immature fruit are in direct contact with moist soil. Similarly for gummy stem blight control, Quadris should be included in the spray program when immature fruit are developing and still green in color. Benlate (benomyl) is still effective for gummy stem blight control but should be tank mixed with a protectant like Bravo to prevent fungicide resistance from developing. Thiophanate methyl (Topsin M) is inherently less effective against gummy stem blight and is not recommended. Aliette and Ridomil fungicides (Ridomil/Bravo and Ridomil/copper) are also effective for downy mildew, and should be used in alternation with Quadris or chlorothalonil (Bravo).

For current managment recommendations see Powdery Mildew Management Guidelines for 2010