Reports on Plant Diseases

Downy mildew, caused by the fungus Plasmopara viticola, is probably the most serious disease of unsprayed grape foliage in the Midwest. The disease occurs worldwide wherever the weather during the growing season is humid and rainy, and temperatures are cool to moderate. The optimum temperature for the development of an epidemic of downy mildew is 64° to 76°F (18° to 24°C), a minimum of 54° to 58°F (12° to 13°C), and a maximum of about 86°F (30°C).

Primary damage is through premature defoliation in the middle to late summer that may result in abnormal fruit development (Figure 1). The fruit from vines that are seriously diseased has a low sugar content. From 50 to 75 percent of the fruit crop may be destroyed where the vines are not sprayed, a highly susceptible cultivar is grown, and weather conditions are favorable for infection. Only a few of the older leaves may develop symptoms if the growing season has been hot and dry–unfavorable for the spread and growth of the causal fungus–or if the grape cultivar is a resistant one.

The downy mildew fungus attacks all common species of wild and cultivated grapes (Vitis species). The European grape (V. vinifera) is more susceptible than cultivated American grapes, which have derived from native species. Other hosts of the fungus include species of the closely related genus Parthe nocissus, which includes Virginia-creeper (P. quinquefolia) and Boston-ivy (P. tricuspidata).

Loss of leaves may not only affect that season's crop, it may also affect the next season's crop by retarding cane maturation and decreasing the amount of stored food reserves in the vine. In young vines, this effect may cause death of the plant.

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Symptoms

Leaves

Very young infections are minute, greenish yellow, and translucent "oily" spots that are difficult to see. The lesions enlarge, appearing on the upper surface as angular to irregular, pale yellow to greenish yellow spots with indefinite borders up to about 1/4 inch or more in diameter (Figure 2.)

Older leaves in the center of the vine are the first to become infected. Later, the disease spreads toward the young leaves near the ends of the canes. On highly susceptible grape cultivars (see Table 1), even the youngest leaves may die and drop prematurely by autumn.

On Fruit

Two periods of fruit infection may occur during a growing season. The first is when the berries are about the size of small peas. If infected at that stage, the young fruit turns light brown and soft, shatters easily, and in damp weather is frequently covered with the white downy growth of the fungus. During the hot summer months, little fruit infection normally occurs.

When the nights become cooler in the late summer and early autumn, a second infection period may develop. Berries infected at this time generally do not soften or form the downy mildew growth. Instead, they stop growth, become leathery, turn a dull green, then dark brown to brownish purple, wrinkle somewhat, and shatter easily. In severe cases, the entire fruit cluster may decay. The berries do NOT shrivel into "mummies" and turn black and hard as in black rot.

On Shoots and Tendrils

Early symptoms appear as water-soaked, shiny depressions on which a dense growth of whitish mildew appears. Young shoots are stunted, thickened, and may be quite distorted (Figure 6). Such infected shoots and tendrils turn brown and die.

In the Midwest, the downy mildew fungus overwinters in infected leaves on the ground, and occasionally in diseased fruit and shoots, as thick-walled resting spores (oospores). Weathering and decomposition of the grape tissues over one or two winters liberate the embedded oospores. During rainy periods in the spring, the oospores germinate at an optimum temperature of 68° to 77°F (20° to 25°C) to produce microscopic spores (sporangia or conidia). The sporangia are disseminated by wind, rain-splash, and by handling wet plants. In a film of moisture, a sporangium may rarely form a germ tube; more commonly its contents divide up to form up to 17 motile spores (zoospores or swarm spores). After swimming about for a few minutes to half an hour, zoospores come to rest and encyst; and in about 15 minutes, they produce germ tubes that penetrate the lower surface of young leaves, shoots, tendrils, and blossom clusters resulting in primary infections. The fungus spreads through grape tissue by growing between the cells. Food and water are obtained by globular structures (haustoria) that invade the host cells in large numbers. The fungus masses within the grape tissue; and when the nights are cool and damp, structures form on the underleaf surfaces and on stems that are delicate, branched, miniature, and treelike, called sporangiophores or conidiophores (Figure 7). They grow out singly or, more often, in clusters of 4 to 6 (but up to 20) through natural openings (stomates) or, rarely, by pushing directly through the epidermis. On the young fruit the sporangiophores emerge through lenticels. Each sporangiophore produces four to six branches at nearly right angles to the main stem, and each side branch produces two, sometimes three, secondary branches. Single, lemon-shaped sporangia (conidia) are formed at the tips of the secondary branches. The sporangia are easily disseminated by wind, rain, and by handling infected plants.

The sporangiophores, with their large numbers of terminally borne sporangia, make up the downy mildew growth. If water is present on grape tissue, the sporangia quickly germinate to produce large numbers of motile zoospores that can result in secondary infections throughout a vineyard and thus rapidly spread the disease. Young lesions can be seen 4 to 12 days after infection on susceptible grape cultivars while it takes up to 18 or 20 days on a resistant plant. Spore germination, penetration, and infection can occur in 30 minutes to 2 hours at an optimum temperature of 50° to 60°F (10° to 16°C) with a minimum of about 40°F (4°C). The sporangiophores and sporangia of the downy mildew fungus are usually formed at night and are most visible in early morning while the leaves, fruit, and other infected parts are still damp.

Toward the end of the growing season, oospores are formed in the old diseased leaves. To a lesser extent, oospores are also formed in infected fruit and shoots. The formation of the overwintering oospores completes the disease cycle.

Figure 6. Downy mildew on a developed cluster (R. Lafon).

Figure 7. Plasmopora viticola, the grape downy mildew pathogen, seen under a high-power microscope. Thick-walled oospore (lower left). Remember, branched sporangiophores bearing terminal, lemon-shaped sporangia (L.Gray).