Powdery mildew can overwinter in cleistothecia lodged in bark. Cleistothecia become lodged in the bark when they wash off of leaves during fall rains. If cleistothecia fall to the soil instead of the bark, they do not usually remain viable. In the spring, cleistothecia in the bark release ascospores during mild prolonged wetness periods. These same temperatures (15 to 25 C) favor germination and infection of grape leaves. Cleistothecia do not remain viable for prolonged periods of 30 C. Canopies which do not have the majority of the leaves growing over the bark are less likely to have as many cleistothecia lodged in the bark. Most infections occur within 10 cm of the bark. Cleistothecia were less viable if they originated from the Central Valley or Southern California than from Coastal regions. This may be why most early infections in the southern areas are the result of infected dormant buds. However, prolonged temperatures over 30 C kill the fungus at any point in its life cycle. Cleistothecia can form as early as July and produce viable ascospores as early as August. Therefore, diligent disease control is important, including post harvest. Spring applications of wettable sulfur (5 lb / 100 gal / acre) were most effective in controlling early season mildew. Spraying should start when shoots are at the 0 to 2 inch stage. Applications should be repeated at ten day intervals when rains, soaking fogs and dews keep grape leaves wet for 12 hours or more.
/wp-content/uploads/2017/09/AFV-Header-Logo.png 0 0 AVF /wp-content/uploads/2017/09/AFV-Header-Logo.png AVF1991-11-17 08:03:302017-11-17 08:04:04Epidemiology and Control of Grapevine Powdery Mildew in California