Pseudoperonospora cubensis (downy mildew of cucurbits)
Cucurbits are members of the gourd family or Cucurbitaceae and include the popular crops cantaloupe, cucumber, pumpkin, squash and watermelon. Downy mildew, caused by Pseudoperonospora cubensis, is an important disease of all these crops, especially in areas with high humidity and rainfall (e.g., the eastern U.S.). In most years the disease is an annual, late-season problem on squash and pumpkin in the eastern and central US, however, since 2004 it has become one of the most important diseases in cucumber production. Considered a highly destructive foliar disease of cucurbits, successful breeding in the mid-twentieth century provided adequate control of downy mildew in cucumber without the use of fungicides. The resurgence in virulence has caused growers great concern and substantial economic losses, while downy mildew in other cucurbit crops continues to be a yearly hindrance.
Symptoms and Signs
The pathogen, Pseudoperonospora cubensis, causes angular, chlorotic lesions on the foliage. These lesions appear angular because they are bound by leaf veins. During humid conditions, inspection of the underside of the leaf reveals gray-brown to purplish-black ‘down’. This is the sporulation of the pathogen. Magnification of the sporulation reveals the acutely and dichotomously branched sporangiophores bearing lemon-shaped sporangia. Eventually, leaves will turn necrotic and curl upwards. The disease is sometimes called “wildfire” because of how rapidly it progresses, as if the crop were burned by fire.
Symptoms on watermelon and cantaloupe are different than on other cucurbits; leaf spots are typically not angular and turn brown to black in color. Often, an exaggerated upward leaf curling will occur. Regardless of which cucurbit is involved, only the leaves are infected (i.e., not fruit, flowers, stems or roots). However diseased leaves results in three major effects: 1) reduced yields, 2) a greater proportion of mishapen fruit (especially in cucumber) and 3) sunscalded fruit due to increased exposure to direct sunlight (especially in watermelon and winter squash).
P. cubensis is an obligate parasite or biotroph, meaning that it requires live host tissue in order to survive and reproduce. Because of this characteristic, the pathogen must overwinter in an area that doesn’t experience a hard frost (e.g., Southern Florida) and wild or cultivated cucurbits are present. The spores are dispersed via wind to neighboring plants and fields and often over long distances. Symptoms appear 4-12 days after infection. The pathogen thrives under cool and moist conditions, but can do well under a wide range of conditions. Optimum conditions for sporulation are 59F/15C with 6-12 hours of moisture present (usually in the form of morning dew). Remember that even when high daytime temperatures are not favorable for the pathogen (>95F/35C), nighttime temperatures may be very suitable. Oospores (thick-walled, resting spores) of P. cubensis are rare and their role in nature is unknown.
Host Specificity, Pathotypes
Within the cucurbit family, P. cubensis isolates will exhibit a specific host range, that is, it will infect certain cucurbits and not others. For example, we have observed cucumber and squash grown side-by-side and only the cucumber is diseased. When a pathogen exhibits this type of host specificity (within a plant family), it is referred to as a pathotype. At least five pathotypes of P. cubensis have been described in the U.S. More recent studies in the Czech Republic have revealed more pathotypes.
Controlling downy mildew requires the use of resistant cultivars, fungicide applications and early detection.
- Resistant Cultivars. Host resistance is an important tool in disease control and should be used whenever possible. Cultivars resistant to downy mildew have been developed for cucumber and cantaloupe and to a lesser extent for squash and pumpkin. Although cucumber downy mildew has been severe on resistant cultivars, these cultivars are more effective than susceptible cultivars in delaying infection.
- Chemical Control. Chemical control is highly recommended because downy mildew is an aggressive and destructive disease and satisfactory control without the use of fungicides is unlikely. Both protectant and systemic products should be applied. Fungicides are most effective when applied prior to infection and reapplied at 5- to 7-day intervals. The following products have proven to be the most effective fungicides in cucumber downy mildew control in North Carolina trials conducted in 2004, 2005 and 2006: Previcur Flex (propamocarb, Bayer), Tanos (fenamidone + cymoxanil, DuPont), Ranman (cyazofamid, FMC), and Gavel (zoxamide + mancozeb, Dow AgroSciences). These products should be applied in a program to prevent pathogen resistance (i.e., rotated with fungicides of a different mode of action). Protectant fungicides such as chlorothalonil and mancozeb should be used as mixing partners.
- Biological Control. Although many biological control products have been evaluated for their ability to control downy mildew, none have proven effective and are not recommended for use.
- Early Detection & Forecasting. Many growers have lost the battle against downy mildew by waiting until they could clearly see the disease before initiating sprays. Early detection of downy mildew and immediate or preventative fungicide application is imperative for the control of this disease. A forecasting system exists to assist growers in timing their fungicide applications for maximum benefit. The system tracks outbreaks of the disease and provides a forecast or risk assessment for future outbreaks.
- Holmes, G., Wehner, T. and Thornton, A. 2006. An old enemy re-emerges: downy mildew rears its ugly head on cucumber, impacting growers up and down the Eastern U.S. American Vegetable Grower, Feb. pp. 14-15.
- Thomas CE, Inaba T, and Cohen Y, 1987. Physiological specialization in Pseudoperonospora cubensis. Phytopathology, 77:1621-1624.
- Thomas, CE, 1996. Downy mildew. In: Zitter TA, Hopkins DL, and Thomas CE, eds. Compendium of cucurbit diseases. APS Press, St. Paul, Minnesota, 25-27.
- Lebeda A, Widrlechner MP, 2003. A set of Cucurbitaceae taxa for differentiation of Pseudoperonospora cubenbis pathotypes. Journal of Plant Diseases and Protection 110, 337-349.