Integrated Management of Grape Diseases

Powdery Mildew

Powdery mildew is an important fungal disease of grapes. If not controlled on susceptible cultivars, the disease can reduce vine growth, yield, quality, and winter hardiness. Cultivars of Vitis vinifera and its hybrids (French hybrids) are generally much more susceptible to powdery mildew than are native American varieties such as "Concord"(Table 6). On susceptible cultivars, the use of fungicides (primarily sulfur) to control powdery mildew is an important part of the disease management program. Failure to provide adequate control of powdery mildew early in the growing season can result in increased levels of other fruit rots such as Botrytis bunch rot and sour rot.

Powdery Mildew Disease Cycle (Figure 63)

Figure 63: Powdery mildew disease cycle. We wish to thank the New York State Agriculture Experiment Station for use of this figure. Figure taken from Grape IPM Diseases Identification Sheet No.2

The fungus can infect all green tissues of the grapevine. Disease loses due to fruit infection can be severe (Figure 64). It was previously thought that the fungus overwintered inside dormant buds of the grapevine. Research in New York has shown that almost all overwintering inoculum comes from cleistothecia, which are fungal fruiting bodies that overwinter primarily in bark crevices on the grapevine. In the spring, airborne spores (ascospores) released from the cleistothecia are the primary inoculum for powdery mildew infections.
NOTE: Ascospore discharge from cleistothecia is initiated if 0.10 inch of rain occurs at an average temperature of 50 ° F. Most mature ascospores are discharged within 4 to 8 hours. It is important to remember that these conditions can occur very early in the growing season. Thus, on highly susceptible cultivars, control needs to be initiated early in the growing season.

Figure 64: Powdery mildew on grape berries.

Ascospores are carried by wind. They germinate on any green surface on the developing vine, and enter the plant resulting in primary infections (Figure 63). The fungus grows and another type of spore (conidia) is formed over the infected area after 6 to 8 days. The conidia and fungal mycelia give a powdery or dusty appearance to infected plant parts (Figure 65 and 66). The conidia serve as "Secondary inoculum" for new infections throughout the remainder of the growing season. It is important to note that a primary infection caused by one ascospore will result in the production of hundreds of thousands of conidia, each of which is capable of causing secondary infections. Therefore, early season control of primary infections caused by ascospores is necessary. If primary infections are controlled until all the ascospores have been discharged, the amount of inoculum available for causing late season (secondary) infections is greatly reduced.

Figure 65: Powdery mildew primary infections on grape leaf.

(Figure 66) Powdery mildew covering grape leaf surface.

Most economic losses from powdery mildew result from fruit infection. Research in New York has shown that berries are susceptible to infection from bloom through a few weeks after bloom. Berries of Concord grapes are quite resistant within 2 to 3 weeks after bloom. Therefore, the most critical time to control fruit infection with fungicide is from immediately prior to bloom through 2 to 4 weeks after bloom. Even though the berries become resistant with age, cluster stems (rachis) and leaves remain susceptible throughout the season. Therefore, a full season fungicide program is generally required for powdery mildew control on susceptible varieties.

Conditions that favor disease development

Although infection can occur at temperatures from 59° to 90°F, temperatures of 68° to 77°F are optimal for infection and disease development. Temperatures above 95°F inhibit germination of conidia and above 104°F they are killed. High relative humidity is conducive to production of conidia. Atmospheric moisture in the 40 to 100% relative humidity range is sufficient for germination of conidia and infection. Free moisture, especially rainfall, is detrimental to the survival of conidia. This is in contrast to most other grape pathogens, such as black rot and downy mildew, that require free water on the plant surface before the spores can germinate and infect. Low, diffuse light seems to favor powdery mildew development. Under optimal conditions, the time from infection to production of conidia is only about 7 days.

It is important to remember that powdery mildew can be a serious problem during growing seasons when it is too dry for most other diseases, such as black rot or downy mildew, to develop. Thick canopies that retain high levels of relative humidity are highly conducive to infections in the center of the row canopy.

Cleistothecia are formed on the surface of infected plant parts in late fall. Many of them are washed into bark crevices on the vine trunk where they overwinter to initiate primary infections during the next growing season.