What Causes Grape Plants to Turn Rusty?

Black Rot

• The fungal disease Guignardia bidwellii causes black rot. When it strikes a grape vine, it can cause the leaves to develop circular brown spots within 10 days of infection. You'll know if your grape vine has black rot if the brown spots develop small black spots. This fungal disease can also affect the fruit: grapes will appear with a light-colored circular spot that eventually discolors the whole grape. It often occurs after a long period of rainy weather.

Dead Arm

• The fungal disease known as dead arm also causes a rusty appearance of grape vines. The fungus Phomopsis viticola causes this disease, which will kill the shoots and branches of your grape vine, as well as the fruit and leaves. When lesions begin to form on the plant, they are dark colored. The lesions later grow to an elongated shape. Infection of the fruit is rare, according to West Virginia University, but when it occurs, it resembles a lighter-colored version of black rot.

Red Leaf

• Also called leafroll and white emperor disease, red leaf is a virus that sometimes afflicts grape vines. When it attacks, it causes the lower leaves of the grape vine to turn a reddish color, usually in September or October, according to Oregon State University. If the virus infects your vine, it might not begin to send out new growth in spring until later in the season, and the leaves might curl downward. The virus also causes bunches of fruit to be smaller, and generally reduces the vigor of the vine. Nematodes spread the red leaf virus. It is best to plant grape vines that are resistant to this disease, because no cure exists. If your area is subject to mealy bugs, keeping them under control will help to slow the spread of this disease. Remove and destroy infected vines.

Mosaic Virus

• Peach rosette mosaic virus can spread from peach trees to grape vines, according to Michigan State University. Nematodes and other infected vines can also spread it, causing red leaves to develop on red grape varieties and a mottled appearance of leaves. It can also affect canes and fruit. Prevention is the only cure: do not accidentally introduce this disease to your property. It's wise to purchase vines that have been tested for this virus.