Downy mildew of grapevines

Page last updated: Tuesday, 14 May 2019 - 1:09pm

Downy mildew, caused by Plasmopara viticola, is a major disease of grapevines that originates from North America. It was first detected in a commercial Western Australian vineyard in October 1998 and has since been found in all grape growing areas in the state.

It is characterised by the presence of oil spots on the surface of leaves and white down that can be seen on the underside of the leaves, canes and bunches in periods of high humidity. It can cause significant impact on yield if control measures are not implemented.

Symptoms

Downy mildew attacks all green parts of the grapevine.

Leaves

The symptoms vary with leaf age. On young leaves (in spring), the disease will appear on the upper surface as small yellow spots referred to as oil spots. They are about 10mm diameter often with a chocolate halo (Figures 1 and 2). These spots tend to grow to about 50mm diameter as they mature and the halo fades. As they enlarge they may appear to cover most of the leaf, especially if there is more than one spot on the leaf (Figure 3). On red varieties the oil spot can appear red (Figure 4).

A single oil spot about the size of a 20 cent coin on a grapevine leaf
Figure 1 A single oil spot on a grapevine leaf
A grapevine leaf with several downy mildew oilspots that have a chocolate coloured halo
Figure 2 Downy mildew oilspots on a leaf with a chocolate covered halo that fades as it ages
Grapevine showing multiple leaves infected with multiple downy mildew oilspots
Figure 3 Multiple infections can lead to several oil spots on a single leaf which can merge
A red oil spot
Figure 4 A red oil spot. On occasion these appear on red varieties

After warm humid nights, a dense, raised, white cottony growth develops on the underside of the yellow oil spots (Figure 5). This is commonly referred to as 'white down'. As the spots age naturally, or after a sporulation event (see later) or hot weather, their centres dry out and become a reddish brown leaving an outer ring of yellow (Figure 6).

The fungus in this yellow ring remains active and given favourable conditions at night, can produce a ring of 'white down' on this outer active edge (Figure 7).

Underside of a grape leaf with areas of raised white spores resembling down
Figure 5 White down (spores) develop beneath oil spots after warm, humid nights
A grapevine leaf with a downy mildew infection that is brown in the centre where it has died but remains active on the edge indicated by the yellow outer ring
Figure 6 An old oil spot that has died in the centre but remained active with an outer ring of yellow
The underside of a leaf that has a downy mildew lesion that has died in the centre but produced white down on the outer ring of yellow
Figure 7 After favourable conditions the active yellow ring surrounding the dead tissue is still capable of producing new spores

Later in the growing season (late summer and autumn) on mature leaves, leaf infections will appear as small, angular, yellow spots that are limited in growth by veins (Figure 8). These form a tapestry-like (mosaic) pattern that soon turn reddish brown. Defoliation can occur in severely affected vines.

Underside of a grapevine leaf infected with downy mildew, the infection is constricted by the leaf veins causing a mosaic appearance
Figure 8 Infection on mature leaves will be yellow-brown and remain small, confined by the leaf veins forming a mosaic pattern

Shoots

Infection on young shoots, stems and tendrils is seen as oily brown areas (Figure 9). These oily patches may spread into leaf stalks, which turn brown and may die. After warm humid nights these oily patches may also sporulate and be covered with white down.

Grape shoot appears oily black at the node where the leaf joins
Figure 9 Downy mildew leaf infection has spread to the shoot causing the oily black appearance

Inflorescences, bunches and berries

Infection on inflorescences, young berries and bunches are seen as oily brown areas. After suitable warm humid nights they may be covered with white down (Figure 10). Infected inflorescences and young bunches rapidly turn brown and wither (Figure 11). Infected young berries stop growing, harden and may later develop a purple hue. They turn dark brown, shrivel and fall from bunches.

Grape inflorescence covered in small white spores or down
Figure 10 Downy mildew infection (white spores) of a grapevine inflorescence
Infection with downy mildew at flowering resulting in the bunch shrivelling and turning brown
Figure 11 Infection near flowering can result in the death of the bunches which then turn brown

Berries become resistant to infection when they are pea size (5-6mm diameter). However, they may still be killed if the berry or bunch stems become infected. They may also sunburn and fail to ripen if defoliation occurs from leaf infection (Figure 12).

A grape bunch with several berries infected with downy mildew as evidenced by the white down on the berries
Figure 12 Downy mildew infection of a grape bunch showing infected berries with white down and resistant healthy green berries

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