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POWDERY MILDEW

Powdery mildew or oidium, Uncinula necator, is a widespread and persistent fungal disease of grapes.

From late summer, yellow and black resting bodies develop on mature mildew colonies
From late summer, yellow and black resting bodies develop on mature mildew colonies
Photo credit: Peter Magarey
Look For Look For Notes

   LOOK FOR

    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

   CONDITIONS

  • 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.

   NOTES

    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.

OTHER POWDERY MILDEW IMAGES

Powdery mildew mostly overwinters in buds Flag shoots develop from infected buds. Look for grey spores on stunted shoots. A flag shoot. Note the upward curled leaves amongst the foliage Flag shoots - often first noticed by the distorted leaves and the mildew on the shoot and petioles Leaves that are diseased when young are curled upward and distorted A severely diseased and stunted flag shoot with curled leaves and grey stems Greasy grey blotches develop on shoots. These blotches later turn red-brown and die Look for early signs of powdery mildew: irregular yellow-green blotches on the upper surface of leaves Later, powdery grey-white spores form on the blotches and cover nearly all the leaf Spores in x10 magnification Powdery mildew is often found too late for effective control, as in this photo Late in the season, leaf tissue turns black beneath the powdery growth Leaf with blotches Brown veinlets on the underside of a leaf blotch (Note for varieties like Chardonnay and Sultana, these veinlets turn brown When looking for mildew on leaves, angle the leaf toward the sun to see young white mildew colonies Ascospore infection from cleistothecia that overwinter on mature wood produces a single spot on the back of a leaf Look inside bunches for young berries with grey-white mildew Greasy, grey-black powdery mildew on berries often later covered by white spores Typical web-like patterns show where the fungus has grown Severe bunch infection - berries covered by mildew except where it has been wiped off Powdery mildew causes berries to split and break down Cleistothecia are seen as tiny specks, golden yellow when young They blacken as they mature Powdery mildew retarded by sunlight on exposed leaves has blackened the surface Berries split by powdery mildew

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