Young downy mildew infections may be confused with powdery mildew, caused by Erysiphe necator. Downy mildew spots are oily with a chocolate halo and develop 'white down' on the underside of the leaf.
Powdery mildew spots are often smaller and yellow-green which then develop a thin layer of ash-grey powdery spores that may eventually cover both sides of the leaf (see Powdery mildew of grapevines in Western Australia).
Yellow spots on leaves may also be due to spray drift damage from herbicides such as paraquat (e.g. Spray.Seed®) or sucking insect damage. Paraquat damage is distinguishable from downy mildew as it does not grow and tends to develop a small brown spot in the centre of the yellow area (Figure 13).
Often downy mildew infection occurs at the bottom of leaf tips or at the junction of the petiole, where water has collected, and when the infection dies or is killed through management techniques this can be confused with botrytis leaf infection (Figure 14).
Frost damage to leaves can cause mosaic symptoms similar to those seen in mature leaf infection of downy mildew (Figure 15). Confirmation that it is downy mildew and not another form of damage requires a bag test where the downy mildew will produce white down on the underside of the oilspot whilst the others will not (see ‘Monitoring’).
White growth on the underside of a leaf may also be due to grape leaf blister mite damage. This white growth is distinguishable from downy mildew as it forms within blister-like green galls that bulge on the upper side of the leaf.
White fungal growth on a leaf may also be due to other fungi such as Penicillium, Aspergillus or Rhizopus. These infections will eventually change to green, blue, black or brown. Many can also cause grapevine bunch rots.
Plasmopara viticola is specific to grapevines (for example, Vitis vinifera), although not all Vitis spp. are susceptible. The American rootstock species and hybrids are less susceptible or resistant (refer to 'Varietal susceptibility').
Other species of downy mildew, such as those found on cucurbits and roses, do not attack grapevines. Although, weather conditions that favour the development and spread of grapevine downy mildew also may encourage the development and spread of cucurbit and rose downy mildews.
Damage and loss
Severe infection will cause leaves to fall prematurely, reducing yield and berry sugar content and will expose remaining bunches to sunburn. Total crop loss may occur if severe infection is not managed, especially near flowering. If partial bunch infection occurs this can cause significant aesthetic issues for table grape growers. Severe leaf fall also can cause yield loss in the following season due to the inability of the vine to store reserves.
All varieties of Vitis vinifera are highly susceptible to downy mildew infection. The strain of the disease in Western Australia does not infect or show symptoms on V. rupestris, V. cordifolia and V.rotundifolia and V. riparia vines. These are often used as rootstocks in WA.