Grape plant fruits shriveled and covered with a white cottony growth have plasmopara viticol, commonly known as downy mildew.
What is it?
Grape downy mildew is caused by the fungus plasmopara viticola. Plasmopara viticola attacks grape foilage and fruit at the beginning of new growth prior to bloom all the way through the end of the grape harvest. As such it is a significant problem for grape plants.
What does it look like?
Downy mildew first appears on grape plants in the form of small yellow spots which will show up on the top side of the leaves while the lower sides of the leaves tend to be covered with a growth that is white and cottony. Grape fruits affected by downy mildew may also be covered with the white cottony growth, and the fruits on the grape plants may be small or shriveled. The grape fruits may also be discolored or turn brown, yellow or reddish. The grape leaves, shoots and tendrils are often affected as well as turn brown, becoming brittle over time when infected with downy mildew. This causes the leaves, shoots and tendrils to appear distorted and often discolored. Downy mildew usually attacks older leaves first, but it may also attack new young grape leaf growth.
How does it manifest?
The fungus plasmopora viticola is a fungus which causes leaf defolitation during the period of time from first bloom to last harvest. The plant leaf defoliation can prevent proper ripening of the fruits on the grape plant. When downy mildew become a severe disease in any grape plant, it can cause entire grape clusters to be covered with the white cottony material and they will die off. Poor growth may also occur because of reduced vine strength and vigor in seasons following the first infection of downy mildew. Downy mildew prefers cool and moist weather conditions and tends to be at its worst during the rainiest seasons and times of year.
What can you do about it?
As the plasmopara viticola species can survive over the winter on fallen plant debris and fruits, care should be taken to clear them carefully and destroy them at the end of harvest. During the growing season you will need to immediately remove and destroy all infected grape fruits and grape leaves. You should also spray the vines with a fungicide designed specifically for grape vines. Ask your local cooperative extension which the best one is to use in your area. You will need to repeat fungicide applications at ten day intervals for as long as signs of infection occur. You can do this safely up until one week prior to grape harvest. The following year you will need to spray the plants immediately before they have a chance to bloom. This will help kill off any downy mildew which over wintered and is emerging to reinfect as spring temperatures rise.
