If you are a commercial grape grower, you need to be aware of the disease called Phomopsis. This is a fungal disease of grapes caused by the fungus Phomopsis viticola. Prior to the mid-1970s, this disease was mistakenly called "dead-arm" disease. The Phomopsis fungus overwinters in the bark of dormant canes as fruiting bodies called pycnidia. When wet, spores exude from these pycnidia and are washed or splashed by rain to the newly developing leaves. The first leaf symptoms would normally be appearing now as small angular dead spots. However, our dry conditions have for the most part kept this fungus at bay. The leaves closest to the cane are the first to show symptoms. Later in the season, canes, tendrils, leaf petioles and even the rachis (or stem of the cluster) may show elongated, brownish or purplish lesions about 1/4 inch long. The berries are susceptible to infection by Phomopsis from bloom until the berries are pea size. These berry infections remain almost invisible until the berries start to ripen. Rachis infections can occur until just after the berries touch. Considerable crop loss due to berry or rachis infections then occurs with mechanical harvesting.

Niagara are very susceptible to this disease; while Concord tends to be less susceptible. However, where fungicidal control has been lacking or wet or rainy springs occur, significant losses can result.

A fungicide spray applied at bloom and another at pea size is needed for complete control. For bloom-time sprays, excellent control can be achieved by applying the fungicides Abound, Sovran, Flint, Dithane or Penncozeb or Manzate, and Pristine on some varieties, good control with Captan, while fair control is provided by ProPhyt, Nova and Elite.

That is all for today, this has been Al Gaus for Michigan State University Extension in Berrien County.