Vineyards in the Grand Traverse region are doing very well so far this year, with a good crop load and very healthy foliage. As the second half of the growing season approaches there are still a few possible pests and diseases that could appear, potentially causing significant problems for wine grape growers.
Powdery mildew is the most important disease threat from fruit set through ripening in our area. Powdery mildew is a fungal disease that infects the surfaces of leaves, shoots and grape berries. It thrives in warm and humid conditions, but even in dry weather it can spread rapidly in the shady, more humid interior of the vine canopy.
Canopy management, such as thinning the number of shoots, shoot positioning, removal of selected leaves and fungicidal sprays are used to help manage this disease.
As the ripening process softens and increases the sugar concentration of the flesh of the berries two other diseases can start to cause issues for growers. Botrytis bunch rot is a fungal disease that can spread very rapidly, consuming entire grape clusters in days. Infected berries first turn brown, then a coating of fuzzy gray spores cover the surface of the fruits. The infected fruits eventually dry out, leaving a shriveled “mummy.”
Sour rot is the other disease of ripening grapes, caused by a combination of bacteria, yeasts and fungi. Its incidence is greater on berries that have been previously infected with powdery mildew or damaged by insects.
Infected berries become very soft and juices may ooze from the cluster, attracting insects such as flies, wasps and bees. Botrytis bunch rot can be treated with sprays, but sour rot has no effective chemical controls.
A few insect pests will also appear in area vineyards soon. One is the Japanese beetle. Although much more widespread and numerous farther south in Michigan, this insect sometimes causes significant injury to grape leaves in our area. The adults are a little under one-half inch long, shiny metallic green and bronze in color and have a few tufts of white hair on their sides.