The 90 day outlook for October, November, December calls for normal temperature and above normal precipitation. We have a number of good suggestions, but need additional ones. Downy and powdery mildews are common diseases that often build to epidemic levels on susceptible cultivars in fall. Heavy dews which form on cool nights in late summer and fall provide sufficient moisture for sporulation and spread of these fungi. A severe downy mildew outbreak can cause early defoliation which reduces winter hardiness.
Powdery mildew reduces photosynthesis and the plants ability to store adequate carbohydrates.
Allowing either disease to become established greatly increases the amount of inoculum that will be in the vineyard next spring. Thus, it is important to maintain some protection against these diseases throughout the fall until leaves drop naturally. Captan and mancozeb both provide excellent protection against spread of downy mildew. Copper fungicides also provide good control, but can cause phytotoxicity on certain varieties and under certain climatic conditions. Fixed coppers plus lime are the least likely to cause phytotoxicity.