1) Black Rot
- Black rot is a fungal disease that causes brown, circular leaf spots and reduces many berries to black, shriveled, raisin-like mummies.
- The fungus pathogen overwinters in mummified berries from the previous season’s crop. Spores are released during wet periods before bloom when new shoots first emerge. Young leaves are infected first.
- The brown leaf lesions have many tiny, black, pimple-like structures in their centers that produce thousands of other spores that can infect the fruit. In general, the leaves are susceptible to infection for about 1 week until they are fully expanded; however, the fruit is susceptible until just before ripening when berry color is developing. As the skin thickens with ripening the fruit is much less vulnerable.
- There are no resistant cultivars.
- Minimize black rot problems by removing infected fruit, cleaning up mummies, and pruning out leaves that surround fruit clusters.
- Preventative fungicide sprays are a necessity to ensure a crop of edible fruit. If using organic sprays, be aware that sulfur may burn the foliage of certain varieties (e.g. Concord) and is not as effective as Bordeaux.
2) Crown Gall, Rhizobium radiobacter
- Crown gall is a bacterial disease that causes galls of various sizes to develop on the roots and canes. The pathogen is usually introduced to a new planting through infested planting stock and, once established, will persist in the soil for years.
- There are no effective chemical controls for crown gall. The appearance of galls on the canes is closely associated with cold temperature damage that injures but does not kill vines, triggering latent infections to become active and inducing the galls to form.
- Root galls can cause the vines to be weak and stunted, and can eventually kill them.
- Where the disease is known to occur, a regular effort should be made to promote the growth and trellising of renewal canes so that the old, galled cane can be removed.
3) Downy Mildew, Plasmopara viticola
- Downy mildew is a serious foliage and fruit disease that occurs erratically in Maryland.
- The fungus overwinters on the ground in infected leaves from the previous year’s crop. Spores infect the foliage and fruit throughout most of the season. Leaf lesions generally appear as bright yellow spots on the upper leaf surface, and the surface directly beneath these lesions appears almost fluffy with white mycelial growth.
- Fruit infections occur most commonly on the lower half of the cluster and appear first as a firm brown rot that causes the berry to shrivel and drop early.
- The disease is generally more severe in wet seasons and has the potential to cause total defoliation by the end of the summer, making the plants extremely susceptible to damage from cold temperatures in the winter.
4) Gray mold or bunch rot
- Gray mold or bunch rot is caused by a nearly ubiquitous fungus (botrytis) that infects flowers in bloom, as well as ripening fruit. Infections that occur during bloom may go unnoticed because the pathogen remains latent until the fruit’s sugar levels rise as ripening occurs.
- Once the fruit rot symptoms develop, the pathogen moves through the cluster stem infecting all or most of the berries in a cluster.
- Infections also can become established in ripening fruit when this fungus enters berries injured by the grape berry moth.
5) Powdery Mildew
- Powdery mildew is seldom a severe problem on American-type grapes but can be very destructive on French hybrid and vinifera types.
- Symptoms develop as a white, powdery fungus growth on the leaf and, later, the fruit surfaces.
- Where severe epidemics develop, early defoliation can occur that weakens the vines, making them subject to cold injury during the winter.
- Infected fruit clusters have a moldy or musty taste, which will persist in wine made from them.