What Does a Fungus Attack on a Grapevine Look Like?

Black Rot

• This is caused by a fungus called Guignardia bidwellii that all grapes are susceptible to. You will notice brown patches on the leaves, and then a few days later, slightly raised tiny black spots. These are the bodies of the fungi feeding off the plant matter. They then spread easily by wind or rain to other areas of the plant. Moist conditions are the prime cause of spread since vines in arid areas rarely are bothered by the fungus. Fruit infected with black rot shrivels and darkens as the fungus feeds off the grapes.

Downy Mildew

• Downy mildew is caused by the organism Plasmopara viticola. This fungus attacks the bottom sides of the grape leaves, leaving a greasy look. The leaves yellow, dry up and fall off the vines. Fruit or new growth covered in the fungus tend to die off. Damp conditions with cool nights set the stage for the fungus to spread rapidly through a grapevine, defoliating the vine of leaves, killing branches and destroying the fruit.

Powdery Mildew

• Powdery mildew was the reason for the loss of thousands of vineyards in Europe in the 1800s. It appears as a light powder on the surface of the green leaves. The fruit takes on a powdery look and then cracks or shrivels. Stems on the grapevine will have black or brown patches in them. The fungus up close appears as tiny black round bodies that fall off the plant tissues, surviving the winter in the bark of the tree.

Botrytis or Gray Mold

• Botrytis cinerea is a gray mold that attacks grapes as well as strawberries, rhubarb and bulbs. In grapevines, the flowers are infected with the fungi -- and then as the grapes ripen, the fungi grow in the sugar of the fruit, causing the whole bunch of grapes to shrivel and darken over time. In Europe, winemakers called this "noble rot," as it left the grapes more concentrated in flavor.

Eutypa

• Eutypa dieback happens when the fungi Eutypa feeds on the new shoots of vines 8 to 10 years old. They darken, and the leaves and shoots shrivel into a cup shape. The fungi stay around in the wood of plants and then infect them after pruning cuts open the vines to infection.

Phomopsis Viticola

• Phomopsis viticola is a fungus that affects grapevines usually in the spring. The fungus is spread by splashing rain. The resting bodies of the fungus, the pycnidia, overwinters in the bark of the vines and then explodes into growth after 10 hours of wet weather and 96-percent humidity. Black spots and lesions appear on the leaves and shoots of the grapevine. The spots do not enlarge, but increase in frequency.