TREC Plant Diagnostic Clinic: Downy mildew of cucurbits: identification and management By Dr. Shouan Zhang UF/IFAS/TREC

Downy mildew is one of the most economically important diseases of cucurbits, especially in areas with high humidity and rainfall such as South Florida. This disease occurs on cucurbits including cantaloupe, cucumber, pumpkin, squash and watermelon. Downy mildew is an annual disease problem on pumpkin and squash in eastern and central US states for many years. A tremendous breeding effort in the mid-twentieth century resulted in adequate control of this disease in cucumber production without fungicide application. However, downy mildew has become one of the most important diseases in cucumber since 2004. The resurgence of the devastating disease has caused substantial economic losses to cucumber growers, and it continues to be an important disease problem in other cucurbit crops that significantly impacts the cucurbit production. Because the production of cucurbits exists year-round throughout the state of Florida, downy mildew is endemic and occurs nearly every growing season to some degree.

Fig. 1 Damage to cucumber plants by downy mildew in the field.

Look out for the symptoms

Downy mildew begins as small water-soaked lesions on the underside of leaves. The lesions are usually limited by small leaf veins, giving the lesions an angular appearance. It is caused by an oomycetous pathogen called Pseudoperonospora cubensis. Under high humid conditions, a layer of gray-brown to purplish-black fluffy stuff appears on the underside of the infected leaves. Inspection with a microscope of the presence of the acutely and dichotomously branched sporangiophores (tree-liked structure) bearing lemon-shaped sporangia is characteristic of the downy mildew pathogen. Leaves will turn necrotic and curl upwards within days if the weather condition favors the pathogen. Therefore, scouting for the disease is critical.

Symptoms of downy mildew vary on watermelon and cantaloupe, and the lesions are not always angular and often associated with an upward curling of the leaves. The pathogen of downy mildew does not affect fruits of cucurbit crops. However, it can result in significantly reduced yields and deformation of fruits especially in cucumber. In addition, increased exposure to direct sunlight due to early defoliation causes more sun-scalded fruits in watermelon and winter squash.

Fig. 2. Symptoms of downy mildew on upside (left) and underside (right) of cucumber leaves. Note the angular and chlorotic lesions on infected leaves.

(http: www.cals.ncsu.edu/plantpath/extension/fact_sheets/Cucurbits_-_Downy_Mildew.htm)

The mystery behind downy mildew

Pseudoperonospora cubensis is an obligate parasite that requires live host tissues in order to survive and reproduce. The pathogen must overwinter in an area that has mild weather in winter season, e.g. South Florida, and where cucurbits are present. The spores are dispersed via wind or splash water to neighboring plants and to other fields by wind or irrigation water. Symptoms appear several days to nearly two weeks after infection.

The pathogen favors cool and moist weather. Therefore, downy mildew is common during winter growing seasons in South Florida. Optimum conditions for sporulation are 15-25°C (59-77°F) with 6 to 12 hours of water film on the leaves such as morning dew. Sometimes temperatures during daytime are not favorable for this pathogen, however, nighttime temperatures may be ideal.

Fig. 3. Symptoms of downy mildew on upside (left) and underside (right)

of butternut squash leaves. Note the gray-brown to purplish-black ‘down’ on the underside of the infected leaves.

Fig. 4. Symptoms of downy mildew on leaves of cantaloupe (left) and watermelon (right). Note irregular lesions and turn brown to black in color.

(http: www.cals.ncsu.edu/plantpath/extension/fact_sheets/Cucurbits_-_Downy_Mildew.htm)

Downy mildew management

Resistant Cultivars

Host resistance is most economically important and effective component in integrated disease management and should be utilized whenever possible. Resistant cultivars have been developed for cucumbers and cantaloupe, and to a lesser extent for squash and pumpkin. Though downy mildew has been severe on resistant cucumber cultivars in recent years, they are more effective than susceptible cultivars in delaying infection.

Early Detection

Early detection of downy mildew is critical for preventing cucurbits from the damage of this disease due to its rapid and destructive nature. Many growers lost huge values of cucurbits to downy mildew as they waited until symptoms of the disease were clearly seen before initiating sprays. Following early detection, immediately applying preventative fungicides is imperative for the control of this destructive foliar disease. A downy mildew forecasting system (http://cdm.ipmpipe.org/) has been established to assist cucurbit growers in timing their fungicide applications for maximum benefit. The system documents the disease outbreaks and provides a risk assessment for future outbreaks in regions where cucurbits are grown.

Chemical Control

It is unlikely to achieve satisfactory control of downy mildew without the use of fungicides. Many growers heavily rely on fungicides for downy mildew control due to the aggressive and destructive nature of this disease. Both protectant and systemic products should be applied. Fungicides are effective in downy mildew control when applied prior to infection and continually sprayed at 5- to 7-day intervals. Previcur Flex, Tanos, Ranman, Reason, Presidio, Aliette and Quadris are examples of fungicides for control of downy mildew in Florida. The products should be applied in a program to prevent resistance development in the pathogen population by rotating with fungicides of a different mode of action. Protective fungicides such as chlorothalonil and mancozeb should be incorporated as mixing partners. All fungicides should be applied according to the manufacturer's label.