A:   These questions come up every year about this time. If the grapes have been cultivated properly, i.e., pruned, fertilized and watered through the growing season, the most likely cause of crop failure is a disease called powdery mildew (PM). PM is a fungus which infects grapevines. Most cultivated varieties are susceptible. PM survives the winter on the canes of the vine. In spring when the vine begins to grow, the fungus also begins to grow and produce spores. These spores infect the young shoots and in time the young developing grape clusters. The skin of infected berries does not grow and develop normally. The result is that the berries desiccate, crack and dry up leaving you with brown, shriveled, useless fruit.

To prevent this scenario, the grapevine must be sprayed to control the PM fungus. The sprays must be applied early and regularly throughout the growing season until the grapes turn color (pale green in the case of white grapes) and begin to ripen. Sprays must be applied, usually on a 7- to 10-day schedule to keep the growing foliage and grape clusters protected against fungus infection.

There are many plant protection products available to control PM. There are several oils – JMS Stylet Oil, Saf-T-Side Spray Oil, Sunspray Ultra Fine Spray Oil, Neem Oil, and Jojoba Oil. Sulfur products are available and very effective. Bicarbonate compounds and biological fungicides plus several synthetic fungicides can also be used. Whichever product you choose, read the container label directions and use the product as directed. Finally, it is critically important to get complete coverage of the grapevine and especially the grape clusters if PM infection is to be prevented. For many of the products mentioned above, PM control will not be achieved on plant tissues that do not have a protective coating of sprayed fungicide. Also remember the 7- to 10-day spray schedule or whatever time interval is recommended on the label of the plant protection product you buy!