Ahhh, spring in the Napa Valley! The grapevines are awake. The cordons are laden with tender green shoots, new leaves and the buds for the flowers that will soon open and bloom. The beginning of a new season for the grapevines! Rebirth! But wait, why are workers out in the vineyard spraying the vines, and what are they spraying on them? Well, what they are spraying is sulfur, and it is sprayed at this time of year to prevent the development of powdery mildew on the tender new growth. A fungus called causes this disease. Curiosity motivated me to learn more about this disease.
What does powdery mildew look like? It doesn’t always look the same; there are many different symptoms and they appear on different parts of a vine at different times during the growing season. On the brown dormant canes, red blotchy areas appear that turn brown or black during the growing season.
Late in the season black specks appear on the canes. On the leaves, the first sign will be areas that lose their green color and then appear to be covered with a white powdery substance.
As the disease develops on the leaves it turns into a webby mass of thread-like tubes.
Powdery mildew causes the stems and flowers to wither; if berries have already formed, they will drop off. On grape clusters, powdery mildew causes berries to be misshapen with rust-colored spots; sometimes entire clusters will be covered with the white powdery growth. Red grapes may have a blotchy appearance at harvest because they fail to color properly. Severely affected grapes may even split open.
How doespowdery mildew develop? The black specks on the canes at the end of the season are spores, and they are washed underneath the bark during the winter rains. They are released during the warm spring rains and are dispersed by the wind onto the emerging leaves, buds and flowers. These spores thrive in spring and summer temperatures and they infect both wet and dry tissue. However, the spores do not like free moisture, so it turns out that rainfall is actually detrimental to their survival. As the mildew grows, masses of secondary spores are released throughout the vineyard, causing a new cycle of infection to begin. This cycle of infection, spore production and spore release can repeat itself in as little as 5 to 7 days!
How do we control powdery mildew or prevent it from infecting the vines? It is best to use preventative methods when you first plant a vineyard. Selecting an open site with direct sunlight and good drainage helps a lot. Planting rows in the direction of the prevailing wind and with adequate sunshine promotes good air circulation and faster drying of foliage and fruit. Selecting grape varieties with a high resistance to powdery mildew can also help. Throughout the year, managing the leafy canopy to reduce shading also increases air circulation. In an established vineyard, applications of fungicides (like sulfur) must be timed to interrupt the repeating cycle of infections, and a spraying control program is usually initiated while the vines are still dormant and continued until the berries have reached 10-12 Brix sugar readings.
Powdery mildew is the most enduring and persistent disease problem faced by grape growers. Varieties of the Vitis Vinifera (wine grapes) and its hybrids are generally much more susceptible to powdery mildew than varieties indigenous to America. Powdery mildew reduces vine growth, yield and quality of the fruit as well as reducing the vine’s winter hardiness. Powdery mildew can potentially ruin whole crops if not controlled in a timely manner.
Photos - http://www.apsnet.org/online/feature/pmildew/gallery.htm