By Lynn Wunderlich
What a busy spring, heh? For those at the trough who are of the grape grower kind, this spring has brought questions concerning what the heck is going on with powdery mildew. To assist vineyard managers, I took a look at each of the 5 foothill mildew station's data and discuss it in a paper you will find linked here.
Recent rains, followed by fluctuating cool and warm temperatures, have got some managers spraying while others are waiting anxiously. Powdery mildew, considered by many the most severe disease of grapes grown worldwide, requires attention every year. The amount of management for any given season depends on a number of factors:
1.) Previous season's disease incidence in a particular block. The amount of overwintering mildew is proportional to the amount of disease in a given block the previous year. If you had mildew last year, carefully monitor that block and consider an early treatment.
2.) Grapevine growth-all green tissue (leaves, shoots) is susceptible to powdery mildew; and berries, only once they reach 8% sugar, are no longer susceptible to disease which works out nicely for harvest. Since all berries in a cluster aren't the same sugar content, consider 12-13% Brix a better number to use when ceasing treatment.
This powdery mildew lesion (yellow spot at the tip of the thumb)
3.) Spring rain and temperatures. Ascospores (sexual spore stage) held in overwintering chasmothecia on trunks and cordons are the first mildew spores released in spring. Ascospores are released at temperatures from 50°-80.5°F (optimum is 68°-75°F) after periods of leaf wetness due to rainfall, heavy fog, dew or overhead sprinklers- about 2mm or 0.08 in. is required for spore release. Whether mildew growth from those spores occurs will depend on the presence of susceptible (green) grape tissue and subsequent temperatures. If temperatures are mild, the mildew fungus will "wait" until it warms up to grow and then produce more spores.
4.) In-season temperatures, regardless of leaf wetness, drive mildew growth the rest of the season. After initial ascospore infection, mildew reproduces via conidia (asexual spores) that thrive at mild temperatures, about 70-85°F. A risk index (RI) model using the in-canopy temperatures collected by the foothill stations calculates the predicted mildew disease pressure based on temperature. The RI for the six foothill stations can be found at this UCIPM link.
5.) Grape variety. Chardonnay, Rousanne, Carignane, and Chenin blanc are all considered highly susceptible to powdery mildew and should be monitored closely. Managers should note which of their varieties have had mildew problems in the past and give those special attention.
6.) Choice of fungicide. Once the RI is started for the season, the spray interval will depend both on the temperatures and subsequent risk index, and the choice of fungicide used. Fungicides should be rotated to prevent resistance developing in the mildew fungus.
7.) Cultural practices and spray application techniques. Fungicides aren't the only management technique for mildew. Opening the canopy by pulling leaves and shoots where appropriate can bring air and sunshine in to prevent mildew growth and improve spray coverage. Sprayers should be properly calibrated to provide full coverage, especially important when contact fungicides are used.