Downy Mildew

Roots in Research - Yield of 2019 - Upper Marlboro CMREC

Timely Viticulture is an electronic newsletter designed to remind grape producers of seasonal vineyard needs and considerations throughout the year.

Many vineyards are approaching or already within 30 days of the anticipated harvest date for early wine grape varieties. During this window, grape berries are becoming very susceptible to a variety of late-season rots, such as ripe rot, sour rot, Botrytis bunch rot, and other “secondary-fungi” associated with bunch rots.

Downy mildew is now a year-long challenge for grape growers in the East. Downy mildew is often difficult to manage because it requires specific weather conditions to become a serious threat, but it can develop into an epidemic very quickly in favorable weather.

The growth period from 2 weeks pre-bloom until 3 to 4 weeks post-bloom is the most critical for protecting foliage, and especially flowers and newly set fruit from black rot (BR), Phomopsis (Ph), powdery mildew (PM), Downy mildew (DM), and Botrytis bunch rot infections.

The grape root borer (GRB), Vitacea polistiformis (Harris), is an insect pest native to the Eastern United States. Its range is from the central Midwest (west) to the Atlantic Ocean (east) and from Florida and Texas (south) to Vermont and Michigan (north). GRB is a pest of all commercial grape types and feeds on native Vitis. It has a long life cycle and takes many years for it to do significant damage to grapevines. However, once grape root borers are present in a vineyard, the unlimited susceptible food source can dramatically increase the infestation. The grape root borer effect may not be immediately apparent on the vines, but once the number of larvae reaches a sufficient level, damage can be significant. Because it is an ("invisible") root tunneling pest, many growers are unaware of the economic damage being caused by GRB.

Severe outbreaks of downy mildew in garden impatiens (Impatiens walleriana) were reported first reported in the fall of 2011 on landscape plantings in Europe as well as in the US (Northeast, Midwest). In Maryland, the Plant Diagnostic Lab received its first infected sample in early June 2012. If you grow garden impatiens, it is important to be on the lookout for this disease.

Difenoconazole is a sterol-inhibiting (SI) fungicide recently registered for use on grapes and other fruit. It belongs to FRAC Group 3, along with myclobutanil (Rally), tebuconazole (Elite, Tebuzol, Orius), and fenarimol (Vintage, formerly Rubigan). Difenoconazole protects vines and fruit from powdery mildew (PM), black rot (BR), and Phomopsis cane and leaf spot.

Downy Mildew of Lima Bean

Managing Downy Mildew of Cucurbits