Downy mildew of grape

Saugat Bhattacharjee, Abbs Department of Microbiology

•The disease is known to occur under humid conditions.

• In Europe and in eastern half of U.S.A , Asia, Australia, South America.

• In India the disease is common in Maharashtra and Karnataka.

• Downy mildew is a highly destructive disease of grapevines in all grape-growing areas of the world where there is spring and summer rainfall at temperatures above 10ºC (50ºF).

Occurrence

• Downy mildew affects the leaves, fruits and vine.

• Losses trough necrosis of leaf tissue and defoliation. Fruits are low quality or are destroyed. Dwarfing and killing of young shoots will occur.• 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.

SYMPTOMS

Leaves: Infected leaves develop yellowish-green, and translucent “oily” lesions on their upper surfaces. As lesions expand, affected areas turn brown, necrotic or mottled.

Downy mildew on upper side of grape leaf.

the corresponding underleaf surface sporulation of the pathogen, – a delicate, dense, white to grayish, cottony growth–occurs

Appearance of infection on underside of a leaf.

The lesions may be so numerous on the upper leaf surface that they merge to cover a large portion of the leaf. Infected tissue gradually becomes reddish brown to dark brown, irregular, and brittle

Symptoms on upper surface of a leaf

Affected leaves eventuallyturn brown, wither, curl, and drop early

Shoots and Tendrils: Symptoms appear as watersoaked, shiny depressions on which a dense growth of whitish mildew appears. When young shoots, petioles, tendrils, or cluster stems are infected, they often become distorted, thickened, or curled.

Downy mildew symptoms on a shoot

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 this 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 pathogen

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

Causal Organism

Plasmopara viticola.

Kingdom: Fungi

Class: Oomycetes

Order: Peronosporales

Family: Peronosporaceae

Genus: Plasmospara

The fungus overwinters in infected leaves on the ground and possibly in diseased shoots.

• The overwintering spore (oospore) germinates in the spring and produces a different type of spore (sporangium).

• These sporangia are spread by wind and splashing rain.

• When plant parts are covered with a film of moisture, the sporangia release small swimming spores, called zoospores.

• Zoospores, which also are spread by splashing rain, germinate by producing a germ tube that enters the leaf through stomates (tiny pores) on the lower leaf surface

• The optimum temperature for disease development is 64 to 76 degrees F (18 to 25 degrees C).

Disease cycle

The disease can tolerate a minimum temperature of 54 to 58 degrees F (12 C to 13 degrees C), and a maximum temperature of about 86 degrees F (30 degrees C).

• Once inside the plant, the fungus grows and spreads through tissues .• Infections are usually visible as lesions in about 7–12 days.

• At night during periods of high humidity and temperatures above 55 degrees F (13 degrees C), the fungus grows out through the stomates of infected tissue and produces microscopic, branched, tree-like structures (sporangiophores) on the lower leaf surface.

• More spores (sporangia) are produced on the tips of these tree-like structures.

• The small sporangiophores and sporangia make up the cottony, downy mildew growth. Sporangia cause secondary infections and are spread by rain.

Plasmopara viticola, the grapedowny mildew pathogen, seen under a highpower microscope.

Thick-walled oospore(lower left).

Remainder,branched sporangiophoresbearing terminal, lemon-shapedsporangia (L. Gray).

CONTROL

• Downy mildew is comparatively easy to control - foliage and fruit are kept protected by fungicide sprays

• Properly space vines, and choose a planting site where the vines will be exposed to all-day sun and good air circulation. Keep the vines off the ground and properly tied.

• Space vines properly in the row, if possible, orient the rows to maximize air movement down the row.

• To improve air circulation, control weeds and tall grasses in the vineyard and surrounding areas.

• Sanitation is important. Remove dead leaves and berries from vines and the ground after leaf drop.

• When pruning, select only strong, healthy, well-colored canes of the previous year’s growth.

• Grape varieties vary greatly in their susceptibility to downy mildew. In general, vinifera (Vitis vinifera) varieties are much more susceptible than American types, and the French hybrids are somewhat intermediate in susceptibility.

• A good fungicide spray program is extremely important.