Grapevine downy mildew , Plasmopara viticola

Powdery mildew is the most serious disease on the vine, especially in areas with warm and humid climates. The first symptoms of the disease in varieties of the European vine (Vitis vinifera) were recognized in the Bordeaux region around 1878.

 

 

 

 

All varieties of Vitis vinifera are susceptible to downy mildew infestation. Hybrids of Vitis vinifera, Vitis aestivalis and Vitis labrusca are less sensitive, while Vitis rupestris, Vitis cordifolia and Vitis rodundifolia are generally resistant.

 

 

 

All varieties of Vitis vinifera are susceptible to downy mildew infestation. Hybrids of Vitis vinifera, Vitis aestivalis and Vitis labrusca are less sensitive, while Vitis rupestris, Vitis cordifolia and Vitis rodundifolia are generally resistant

Powdery mildew is an obligate pest. It needs living tissue to grow. So it can grow on all green parts of the plant.

Ovospores (wintering organs) are formed in late summer and autumn on the affected leaves, clematis or rails. With autumn defoliation they drift to the ground, where they overwinter on the infected leaves. There they can stay from 3 to 5 years (often up to 10 years).

Ovospores are the native organs of reproduction of powdery mildew. Their durable walls make them less susceptible to fungicides and extreme weather conditions than zoospores and spores.

At least 10 mm of rain (and/or irrigation), 10 oC or more, for at least 24 hours favor primary infections of the disease.

These conditions do not always ensure the progression of the disease, but they should mobilize the derivatives.

In particular, in order for oospores to germinate they require:

1. The soil should be wet for at least 16 hours. This is usually achieved with 3-5mm of rain (and/or irrigation).
2. The temperature should be maintained at the same time around 10 oC.

In these conditions the oospores germinate and release zoospores, which with raindrops or irrigation are transferred to the lower parts of the crown of the plant and are responsible for primary infections. Another 3-5 mm of rain (and/or irrigation) is required at this stage both for transport from soil to plant and for adequate wetting of the lower surface of the sexes which is a prerequisite for infection.

The foliage should remain moist for at least 2-3 hours at 20 oC (or 4-5 hours at 10 oC) for the spores to complete the primary infections.

The zoospores that settle on the lower surface of the leaf form the mycelium of the fungus, which grows inside the leaf, causing the known oily spots. These spots usually appear 5 to 17 days (more often 5 to 10 days) after infection. The appearance of spots is faster when the weather is warm (18-27 οC). At higher or lower temperatures the incubation period is longer.

Usually primary infections are low, with the appearance of 1-3 spots on every 50 m of planting line, but they are quite difficult to detect. These primary infections alone cannot cause serious losses in production. However, they are very important for the progression of the disease as, when favorable conditions prevail, they allow the disease to spread very quickly.

Secondary leaf infections on leaves, vines, inflorescences, rails and grapes require at least 98% relative humidity, 13 °C or more, at least 4 hours darkness and leaf moisture in the form of a drop for at least 2-3 hours.

A prerequisite for secondary infections is the existence of oily spots.

These spots and other infected tissues produce sporiagiagia during hot and wet nights. The production of seedlings requires at least 4 hours of darkness with the temperature being at 13 οC or more and the humidity at 68% or more.

The foliage should also be wet for at least 2-3 hours from the moment the seedlings are produced. This can happen with rain, excessive irrigation and occasionally when conditions of very high relative humidity prevail

Secondary infections can occur at any time during the growing season when there are oily spots and the prevailing conditions are favorable for the progression of the disease.

Sporiangia is the vegetative form of powdery mildew. They occur in the sporangiosphores, which have a tree form.

One to ten zoospores can be produced from each spore. Zoospores can move in water but are mostly dispersed by wind and raindrops.

Primary infections are the beginning of the biological cycle of the disease and result in the appearance of oily spots.

The spores produced in these spots can disperse in wind or rain and cause infections in new locations.

Secondary infections are the main source for the spread of the disease.