Downy mildew in vineyards - Department of Agriculture and Food

PAGE 8DOWNY MILDEW IN VINEYARDSInfection on mature leaves will be yellowbrownand remain small, confi ned by thefi nest of veins.White down (spores) will also developbeneath infections on mature leaves after asuitably warm, humid night.Later in the growing season (i.e. latesummer and autumn) on mature leaves,leaf infections will appear as small,angular, yellow spots that are limitedin growth by the veins. These form atapestry-like (mosaic) pattern of spotsthat soon turn reddish brown. Defoliationcan occur in severely affected vines.Other causes of similar symptomson leavesYoung downy mildew infections may beconfused with powdery mildew, causedby Erysiphe necator. Downy mildewspots are oily with a chocolate halo anddevelop ‘white down’ on the undersideof the leaf. Powdery mildew spots areoften smaller and yellow-green whichthen develop a thin layer of ash-greypowdery spores that may eventuallycover both sides of the leaf (refer toBulletin 4575 Powdery Mildew in WineGrapes in Western Australia).Yellow spots on leaves may also be dueto spray drift damage from herbicidessuch as paraquat (e.g. Spray.Seed ® )or sucking insect damage. Paraquatdamage is distinguishable from downymildew as it does not grow and it tendsto develop a small brown spot in thecentre of the yellow area. Also, neitherform of damage will produce white downon the underside of the spots after thebag test (see ‘Monitoring’).White growth on the underside of a leafmay also be due to grape leaf blistermite damage. This white growth isdistinguishable from downy mildew as itforms within blister-like green galls thatbulge on the upper side of the leaf.White fungal growth on a leaf may alsobe due to other fungi such as Penicillium,Aspergillus or Rhizopus. These fungalinfections will eventually change togreen, blue, black or brown.ShootsInfection on young shoots, stems andtendrils are seen as oily brown areas.These oily patches may spread into leafstalks, which turn brown and may die.After suitable warm humid nights theseoily patches may also sporulate and becovered with white down.