Primary grape fungal diseases Powdery mildew Downy mildew

Primary grape fungal diseases

• Powdery mildew

• Downy mildew

Grape powdery mildew

• Caused by fungus Erysiphe necator

• Reduces plant yield

• Reduces berry quality for wine production

• Reduces vine and bud hardiness

Powdery mildew - Symptoms

• Powdery mycelium on shoots, leaves and berries

SymptomsSymptoms

Powdery mildew infects all green tissues of grapevinePowdery mildew infects all green tissues of grapevine

Symptoms - leavesSymptoms - leaves

Reduced photosynthesis in infected leavesReduced photosynthesis in infected leaves

Grayish-white powder on upper (sometimes lower) surface

Symptoms - rachisSymptoms - rachis

Grayish-white powder on surface

infections at bloom can cause poor fruit set

Symptoms - bunchSymptoms - bunchinfection may result in:infection may result in:

poor fruit setpoor fruit set crop losscrop loss reduced juice qualityreduced juice quality

Berries are susceptible to infection until the sugar Berries are susceptible to infection until the sugar content reaches 8%content reaches 8%

Symptoms - berriesSymptoms - berries

Symptoms - berriesSymptoms - berries

Infected purple or red cultivars may fail to color properlyInfected purple or red cultivars may fail to color properly resulting in off-flavored wineresulting in off-flavored wine

Powdery mildew – Disease factors

• Cultivar susceptibility

• Availability of newly grown tissue

• Temperature

• Humidity / rainfall

• Inoculum source

Control strategies

• Sulfurs (organic certification programs)• Inhibitors of Sterol Biosynthesis (IBS)• Strobilurins (new class of fungicides)• Oils (some are eradicative)• Potassium bicarbonate• Biological control agents (mildew pathogens,

Ampelomyces quisqualis)• Plant defense elicitors (chitosan)• Mycophagous mites

Chemical control strategies

Use sulfur on 7 – 10 day intervals (until veraison)

Downy mildew of grape

Plasmopara viticola

• The mycelium diameter varies from 1 to 60 micrometers because the hyphae take the shape of the intercellular spaces of the infected tissues; globose haustoria grow into the cells

• The mycelium produces sporangiophores on the underside of the leaves and on the stems through stomata and, in young fruit, through lenticels

The pathogen I

• Downy mildew is still most destructive in Europe and in the eastern half of the United States, where it may cause severe epidemics year after year and, in some years, in other humid parts of the world; dry areas are usually free of the disease

• Downy mildew affects the leaves, fruit and shoots of grapevines

• It causes losses through killing of leaf tissues and defoliation, through production of low-quality, unsightly or entirely destroyed grapes, and through weakening, dwarfing and killing of young shoots

• When the weather is favorable, downy mildew can easily destroy 50 to 75% of the crop in one season

The pathogen II

1. All young grapevine tissues are particularly susceptible to infection

2. At first, small, pale yellow, irregular spots appear on the upper surface of the leaves, and a white downy growth of the sporangiophores of the oomycete appears on the underside of the spots

3. Later, the infected leaf areas are killed and turn brown, while the sporangiophores of the oomycete turn gray, the spots often enlarge, coalesce to form large dead areas on the leaf and, frequently, result in premature defoliation

Symptoms I

Downy Mildew - leavesDowny Mildew - leaves

Symptoms - leavesSymptoms - leaves

1. Infected berries are quickly covered with the downy growth, may become distorted or thickened, and may die

2. If infection takes place after the berries are half-grown, the oomycete grows mostly internally, the berries become leathery and somewhat wrinkled and develop a reddish marbling to brown coloration

3. In late or localized infections of shoots, these usually are not killed, but show various degrees of distortion

Symptoms II

Symptoms - clusterSymptoms - cluster

cluster infectioncluster infection

DownyDowny mildewmildew

1. Several American grape varieties show considerable resistance to downy mildew, but most European (V. vinifera) varieties are quite susceptible; even the relatively resistant varieties, however, require protection through chemicals

2. The most effective fungicides for the control of downy mildew have been copper-based products such as the Bordeaux mixture, some broad spectrum protective fungicides, and several systemic fungicides

3. The applications begin before bloom and are continued at 7- to 10-day intervals or, depending on the frequency and duration of rainfall, during the growing season

Control

Grapevine fungal disease groupDipartimento di Produzione Vegetale

Università degli Studi di Milano, Italy Marcello Iriti(lead researcher)

Sara Vitalini(PhD)

Antonietta Ruggiero(PhD student)

Giulia Castorina(PhD student)

Chiara Lago(PhD student)