Click on any
image for a more
detailed view

Symptoms and Signs

Although all green parts of the grapevine are susceptible, the first symptoms of downy mildew of grapes, caused by Plasmopara viticola, are usually seen on the leaves as soon as 5 to 7 days after infection. Foliar symptoms appear as yellow circular spots with an oily appearance (oilspots) (Figure 2). Young oilspots on young leaves are surrounded by a brownish-yellow halo. This halo fades as the oilspot matures. The spots are yellow in white grape varieties and red in some red grape varieties (e.g., Ruby Red). Under favorable weather conditions, large numbers of oilspots may develop and coalesce to cover most of the leaf surface (Figure 3). After suitably warm, humid nights, a white downy fungal growth (sporangia) will appear on the underside of the leaves and other infected plant parts (Figure 4). The disease gets its name "downy mildew" from the presence of this downy growth.

Figure 2

Figure 3

Figure 4

In late summer and early fall, the diseased leaves take on a tapestry-like appearance when the growth of the pathogen is restricted by the veinlets (Figure 5). Confirmation of active downy mildew is made by the "bag test." To do this test, seal suspect diseased leaves and/or fruit bunches in a moistened (not wet) plastic bag and incubate in a warm (13-28ºC/ 55-82ºF), dark place overnight. Look for fresh, white downy sporulation beneath suspect oilspots or on shoots or fruit bunches (Figure 4). Note that mature berries, although they may be symptomatic and harbor the pathogen, may not support sporulation even when provided with ideal conditions. Infected parts of young fruit bunches turn brown, wither, and die rapidly. If infections occur on the young bunch stalk, the entire inflorescence may die (Figure 6). Developing young berries will either die or, if between 3 and 5 mm in diameter, become discolored (Figure 7). Berries become resistant to infection within 2-3 week after bloom, although all parts of the rachis may remain susceptible 2 months after bloom.

Figure 5

Figure 6

Figure 7

RETURN TO TOP