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DOWNY MILDEW

Downy mildew, Plasmopara viticola, is a major fungal disease of grapevines that occurs irregularly in most grapegrowing districts

Rapidly growing oilspots often merge to infect much of the leaf
Rapidly growing oilspots often merge to infect much of the leaf
Photo credit: Peter Magarey
Look For Look For Notes

   LOOK FOR


    Look for in leaves
  • Yellow oilspots which may enlarge and merge to cover most of the leaf
  • White downy growth which forms on the undersides of oilspots after a suitably warm humid night
  • Oilspots which have dried out after hot weather and become brown to red-brown with an outer ring of yellow
  • A tapestry-like patchwork of small, angular yellow-brown spots on older leaves in late summer and autumn

    Look for in bunches
  • Oily-brown flower clusters, bunches or berries. These rapidly wither and die
  • White downy growth on young bunches after a suitably warm humid night

   CONDITIONS

  • Spreads in warm moist weather
  • Downy Mildew begins with primary infection when spores are splashed from the soil to the leaves.
  • Downy mildew spreads in secondary infection when spores are splashed from leaf to leaf to bunch.

   NOTES

    10:10:24 is a guide to primary infection events. These occur any time during the season with at least 10 mm of rain and/or irrigation and temperature at least 10°C during a 24-hour period.

    Soil must be wet for the first 16 hours, followed by rain splash and then at least 2-3 hours of leaf wetness. So not all 10:10:24 events will lead to disease!

    Flowers and young berries are very susceptible to infection. Though stems remain susceptible, berries bigger than pea-size (5-6 mm diam.) are resistant.

    Ten tips for downy

    **Use 10:10:24 as a guide to primary infection
    **Look for a few oilspots to appear 5-17 days after primary infection
    **Expect to find only one oilspot, on lower leaves or inside the canopy, every 50 m vine row
    **Tag leaves with oilspots (See page 11)
    **Downy needs a 'warm wet night' to spread (secondary infection)
    **Watch for > 98% humidity and > 13°C at night with leaves wet in the morning
    **Check this by looking for fresh white down on the undersides of oilspots
    **Look for new oilspots to appear 5-17 days after secondary infection
    **Expect to find many oilspots clustered 50-100 cm around the primary oilspot
    **Tag leaves with new oilspots and watch their development (See page 11)
    **Repeat steps 6 to 10.

    ACTION

    Monitor more closely after oilspots are predicted by the incubation calendar, CropWatch (Horticulture hotlines) or AusVitTM , especially in spring and early summer.

    The 'bag test' indicates active downy mildew.

    Pour clean water into an empty, sealable plastic bag, shake the contents, then empty to leave the bag lightly moistened

    Collect fresh leaves or bunches suspected of having downy. Seal them in the bag and leave overnight in the dart at 20°C to 25°c

    Next morning, white down will have developed on the undersides of oilspots with active downy mildew or on young bunches (berries < 5 mm)

OTHER DOWNY MILDEW IMAGES

Young oil spot of downy mildew. Note the chocolate halo On Ruby Cabernet, oilspots are red The spot grows and the halo fades White down (spores) develops beneath oilspots after a suitably warm, humid night A single leaf spot has the potential to spread the disease rapidly by secondary infection Often downy is not detected until the disease spreads and more oilspots are visible Infection on old leaves will be yellow-brown and remain small, confined by the finest veins (tapestry pattern) Large oilspots browned and killed by well-timed sprays of a post-infection fungicide Large oilspots browned and killed by well-timed sprays of a post-infection fungicide Infected bunch stalks are oily-brown White down (spores) may develop after infection Berries infected soon after flowering develop a purple hue, shrivel and fall Infection near flowering kills bunches which then turn brown Later, diseased bunches blacken and fall. Some berries may survive Infection of berry stalks shrivels the berries Leaves begin to fall and the bunches are killed

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