Identifying Downy mildew and Powdery mildew on Grapevine
Downy mildew symptoms:
Downy mildew affects all green parts of the plants like leaves, shoots, and fruits. The symptoms are the discolored lesions of regular or irregular shape, that are initially yellow and later turn to brown and dry. The underside of the discoloration will have white cottony fungal growth. Oil spot lesions are also seen. The following are the detailed symptoms seen in the grapevine for downy mildew disease. (The images are taken from the referred pieces of literature).
1. Yellowish-green lesions develop on the upper side of the leaf. As the lesion expands, the affected areas turn brown, necrotic, or mottled (Figure 1).
2. Severely infected leaves may curl and drop from the vine.
3. During late summer and autumn, the fungal infection produces a mosaic of small, angular, yellow to red-brown spots on the upper surface of the older leaf surface (Figure 2).
4. Lesions are commonly formed along leaf veins.
5. The underside of foliar lesions, is the white, downy fungal-like growth (Figure 3).
6. Infected green fruit turns light brown to purple, shrivels, and detaches easily. White, cottony sporulation is abundant on these berries during humid weather (Figure 4).
7. Later, infected berries turn dull green to reddish-purple, remain firm, and are easily distinguished from non-infected ripening berries in a cluster.
8. Diseased berries are easily detached from their pedicels leaving a dry stem scar.
Reference: http://plantpathology.ca.uky.edu/files/ppfs-fr-s-13.pdf
Powdery mildew symptoms:
All green parts of the plants are affected by powdery mildew. Dusty white-gray or greenish-white coating on leaf surfaces or other above-ground plant parts. It is commonly observed on the upper surfaces of leaves, but can also affect the lower leaf surface, young stems, buds, flowers, canes, and young fruit. The following are the detailed symptoms of powdery mildew
1. Dusty appearance or white powdery growth in patches on fruit (Figure 1), leaves (Figure 2), and vines (Figure 3).
2. When young expanding leaves are infected, they may become distorted and stunted. Severely infected leaves may exhibit mottling or deformity, including leaf curling and withering.
3. Severely affected leaves may curl upward during hot dry weather. Dark brown to black blotchy lesions form on diseased vines (Figure 3).
4. Infected berries often misshapen (not in original shape), turn grayish-white at first, and ultimately exhibit a brown rusted appearance. Infected fruit may crack, shrivel, or drop from clusters (Figure 3).
5. Severely diseased fruit may split open (Figure 4). When berries of purple or red cultivars become infected during ripening they fail to color properly, resulting in a blotchy appearance at harvest.
Figure 4. Split open berries due to powdery mildew infection ((University of Georgia Plant Pathology Archive, University of Georgia, Bugwood.org).
Reference:
1. Plant pathology fact sheet, Powdery mildew of grapes by University of Kentucky-College of Agriculture.
2. Bernd Maier and Natalie Goldberg, Grape powdery mildew Guide H-329. NM State University.
