POWDERY MILDEW
Powdery mildew or oidium, Uncinula necator, is a widespread and persistent fungal disease of grapes.
![Look inside bunches for young berries with grey-white mildew](images/problems/37.jpg)
Look inside bunches for young berries with grey-white mildew Photo credit: Peter Magarey
Look for in shoots- Flag shoots: stunted diseased shoots with ash-grey growth and distorted or upward curled leaves
- Oily-grey speckled blotches which turn red-brown to black as shoots harden off
Look for in leaves - Small, scattered, yellow-green blotches up to 10 mm diam. on upper surfaces of leaves
- Ash-grey to white powdery and web-like fungal growth on upper or lower surfaces of the older blotches (a 10´ hand lens is helpful)
- Brown veinlets on the underside of blotches are diagnostic on Chardonnay and Sultana
- Badly diseased leaves which are distorted or crinkled when young, may blacken after hot weather
Look for in bunches - A thin layer of ash-grey powder on young berries and bunch stalks. Later the whole bunch may be covered with this powdery spore deposit
- Web-like patterns on the skin of ripening berries. This is often hidden by the powdery growth. To check, wipe the berry with your thumb or finger
- Split berries when infection is severe
- Spreads from budburst particularly in shaded canopies during dry, mild weather
- Begin to monitor 3 weeks from budburst. If using the 2, 4, 6 spray strategy, begin at 8 weeks (ie before the canopy closes over).
- Flag shoots missed in earlier monitoring are often detected when new leaf spots develop nearby.
Flag shoots ‘flag’ where disease begins, developing from infected buds. They occur in low numbers. About one shoot in 1,000 will be affected. By mid-season, they are hidden in the canopy. A single grey spot (<1 cm diam.) on the undersides of leaves near vine wood may indicate ascospore infection. Powdery mildew spores occur on both upper and lower surfaces of leaves, while downy mildew spores appear only on the lower surface. Severely diseased canopies have a distinctive musty smell. Powdery mildew often goes undetected (because of poor monitoring) until it is well established, typically by mid-November in Australia and late December in New Zealand.
ACTION
Begin monitoring early season, looking especially inside the canopy on shaded leaves and bunches and near flag shoots.
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