Keep an eye on downy mildew in grapes

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Recent rainy weather is conducive to downy mildew development in grapes. Downy mildew is caused by the oomycete (fungal-like) pathogen Plasmopara viticola and can attack all green parts of the vine, particularly the leaves. Young tissues are particularly susceptible to infection. Downy mildew can seriously damage leaves and clusters of susceptible cultivars. Leaf infections may lead to premature defoliation, which can reduce winter hardiness and sugar accumulation in the fruit in severe cases. Sometimes newly planted vines fall victim to this disease because people think that no sprays are necessary in the first year of establishment – heavy defoliation can set back vine growth and winter survival.

The first symptoms may be light green or yellow spots that may have a greasy appearance (oil spots). On older leaves, lesions are smaller and more angular as they are delimited by leaf veins. Infected shoot tips tend to be curled and have a shepherd’s crook appearance. Infected flower and fruit clusters may be covered with a downy white growth and later become necrotic. On leaves, sporulation usually occurs on the underside of the leaf. This is in contrast to powdery mildew, where sporulation mostly occurs on the upper surface.

Biology of the pathogen

The pathogen overwinters as thick-walled spores (oospores) in fallen infected leaves on the ground below the vine. As the leaves break down, the oospores are released into the soil where they can survive for a long time. Only oospores at or near the soil surface will germinate in the spring. Oospore germination is favored by moist soils and moderate temperatures (over 50°F), which were common in the past few months. Oospores develop a second spore type (sporangia), which are splashed by rain or carried by wind to young leaf and shoot tissues. Therefore initial infections are often found on the lower parts of the vine, unless the sporangia blow in from other vineyards, in which case lesions may be found in the upper canopy.

The optimum temperature for germination of sporangia is 72°F-75ºF. The sporangia release multiple zoospores (swimming spores) which need a film of water to infect the plant tissue. Zoospores infect the plant exclusively through the stomates, so only plant structures with functional stomates are susceptible to infection. Berries become less susceptible as they mature, but the rachis remains susceptible for a long time. Under optimal conditions, the time from germination until penetration is less than 90 minutes. Lesions appear 5-17 days after infection. The fungus will sporulate through the stomates of infected tissues under humid conditions (95-100 percent RH) at night. The optimal temperature for sporulation is 65°F-72ºF.

Rain is the principal factor promoting epidemics. Temperature plays a less important role by retarding or accelerating the development of the disease. The most serious epidemics occur when a wet winter is followed by a wet spring and a warm summer with intermittent rainstorms every 8-15 days. Recent research in Europe has indicated that oospores may germinate over a fairly long period of time (from early spring until mid- or even late summer) and secondary cycles of sporangia production may be less important in disease development than previously thought. This may explain why disease prediction models for downy mildew have not worked as well as expected.

Dr. Schilder's work is funded in part by MSU's AgBioResearch.

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