Understanding Fungicide Resistance Management For Cucurbit Downy Mildew Control

By Andy Wyenandt and Gerald Holmes

Downy mildew (caused by Pseudoperonospora cubensis) has become a serious problem in cucurbit production in the U.S. in recent years, particularly for cucumber. Downy mildew affects all members of the Cucurbitaceae family including cantaloupe, cucumber, pumpkin, squash and watermelon. Of all cucurbits, resistance to downy mildew infection has been successfully bred into all modern cucumber cultivars and a few cantaloupe cultivars. In cucumbers, the pathogen has overcome this resistance and has caused widespread epidemics since 2004.

On cucumber foliage, the pathogen produces angular, chlorotic lesions which are bound by leaf veins. During humid weather, masses of spores form a grayish-purple down on the undersides of leaves which will spread rapidly throughout a field. Cucurbit fields infected with downy mildew will 1.) produce reduced yields, 2.) produce a greater proportion of misshapen fruit (especially in cucumber) and 3.) predispose fruit to sunscald injury due to increased exposure to direct sunlight.

Downy mildew is an obligate parasite, meaning it requires living host tissue to survive and reproduce. Therefore, the pathogen must overwinter in an area that doesn’t experience a hard frost (i.e., southern Florida). In the U.S. each year, the pathogen overwinters and originates from southern Florida and moves up the east coast as cucurbit crops are planted in the spring. A forecasting system http://ces.ncsu/edu/depts/pp/cucurbit developed by North Carolina State University can be used by cucurbit growers to track downy mildew epidemics and help time preventative fungicide applications.

What makes some fungicide chemistries more effective than others for cucurbit downy mildew control?

Newer fungicide chemistries with specific modes-of-action have translaminar or systemic activity. This means the fungicide can move between the upper and lower surfaces of leaves protecting the foliage where protectant fungicides wouldn’t unless they were applied thoroughly. The benefits of these types of "mobile" fungicides is that they can move to and protect areas that are difficult to reach with sprays (i.e., underside of leaves) where the fungus is actively growing and producing spores.

Timing Is Everything
While it is difficult to over-emphasize the importance of using an effective fungicide, timing that fungicide’s application is just as important. As with most diseases - but especially with downy mildew - if fungicides are applied after symptoms appear they will be much less effective. Recent studies have shown over 50% reduction in cucumber yield when application was delayed 7 days beyond symptom appearance.

Fungicide resistance management of cucurbit downy mildew
Resistance in cucurbit downy mildew has been detected to the strobilurin (FRAC* code 11) fungicides (e.g., Quadris, Cabrio, and Flint) and to mefenoxam (FRAC code 4) (e.g., Ridomil Gold, Ultra Flourish). Strobilurin resistance is the same type of resistance detected in cucurbit powdery mildew. This type of resistance is governed by a single-site mutation in both pathogens and once resistance develops, all strobilurin fungicides will no longer effectively control the pathogen. Fortunately, in recent years, new fungicide chemistries (different FRAC codes) have been introduced for cucurbit downy mildew control. These fungicides include Previcur Flex (propamocarb HCL, FRAC code 28), Tanos (famoxadone + cymoxanil, FRAC codes 11 + 27), Ranman (cyazofamid, FRAC code 21), Curzate (cymoxanil, 27) and Gavel (zoxamide + mancozeb, FRAC codes 22 + M3). Mobile fungicides included from the above list include Previcur Flex and Tanos. Gavel, and Ranman have limited mobility and should be treated as protectant fungicides. In addition to all being effective in controlling downy mildew, all require that resistance management guidelines listed on the label be followed to reduce the chances for resistance development. Therefore, all should be tank mixed with a low-risk, protectant fungicide (FRAC code M, such as chlorothalonil (M5) or mancozeb/maneb (M3) and be rotated with fungicides with different FRAC codes.

*FRAC = Fungicide Resistance Action Committee http://www.frac.info/frac

Wyenandt is an Extension Specialist in Vegetable Pathology with the New Jersey Agricultural Experiment Station, Rutgers University and Holmes is an Extension Specialist in Vegetable Pathology with North Carolina State University.