Downy mildew of grapevines

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).

Figure 1 A single oil spot on a grapevine leaf

Figure 2 Downy mildew oilspots on a leaf with a chocolate covered halo that fades as it ages

Figure 3 Multiple infections can lead to several oil spots on a single leaf which can merge

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).

Figure 5 White down (spores) develop beneath oil spots after warm, humid nights

Figure 6 An old oil spot that has died in the centre but remained active with an 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.

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.

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.

Figure 10 Downy mildew infection (white spores) of a grapevine inflorescence

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).

Figure 12 Downy mildew infection of a grape bunch showing infected berries with white down and resistant healthy green berries