Background
Black spot or anthracnose of grapevines is caused by the fungus Elsinoe ampelina. The disease is found worldwide where grapes are grown, including Western Australia. It can be particularly damaging in regions that experience cool, wet springs and is often more of a problem for table grape varieties such as sultana and red globe. It can reduce fruit yield and quality.
The introduction of fungicides to prevent the disease has reduced its impact. In Western Australia, the disease can appear when spring rainfall occurs and control measures have not been used or have been used incorrectly.
Symptoms
The disease produces characteristic symptoms on leaves and shoots, particularly in the early stages of the growing season.
On leaves, small dark brown to black circular lesions appear after rain in the spring (Figure 1).
Several leaves or shoots can be infected in a localised area, with the more severely infected leaves showing the greater number of leaf spots. Small lesions expand with the leaf and eventually the centre of the lesion falls out, leaving the leaf with a 'shot hole' appearance (Figure 2).
Infected leaves often have a distorted appearance (Figure 3).
On shoots, infection is seen as small circular lesions with white to grey centres and a defined dark brown to black margin (Figure 4).
The centres of the lesions can appear flaky as the lesion ages (Figure 5).
Older lesions develop callused edges and the centre is sunken. This can occasionally be as deep as the middle of the shoot. Lesions may merge, creating several larger lesions on the stem (Figure 6).
Severely infected shoots can have lesions that girdle the shoot which is likely to either break off during the season or produce no bunches at all. Loss of shoots reduces the number of viable canes or spurs in subsequent seasons.
Small brown spots appear on berries that enlarge to be purple black in colour and occasionally have grey to white centres.
Life cycle
Black spot survives in the vineyard as sclerotia inside cane cankers caused by the previous season’s infection (Figure 7). These survival structures can remain in the soil for three to five years.
Spores are produced by the sclerotia after rainfall events in spring and are splashed onto the young, growing leaves where infection occurs. Foliage needs to be wet for some time (more than two hours) for infection to be successful. Once infection occurs on the leaves, new spores are produced within 4-13 days, depending on the air temperature.
If warm and dry conditions occur in early spring after infection growth of the disease is slowed due to its dependence on water for infection and spread.
Black spot is more localised than other grapevine diseases. It often has central infection foci, with the severity of infection reduced the further the leaves and stems are away from these points (Figure 8).
Management
Vines that have had the disease in the past should be tagged and monitored early in the season as this is where infection could occur.
Because rainfall and water are so crucial for the lifecycle of black spot, by the time bud burst has occurred spores will have been released into the environment. As young shoots and leaves are most susceptible to infection, early season application of fungicides is critical. Apply preventative fungicides at budburst and 10-14 days after. During rapid shoot growth, or if rainfall has occurred, this timeframe might need to be reduced.
Numerous fungicides of different activity groups are registered to prevent black spot in WA vineyards. A list of these can be found in the Viticulture Spray Guide.
No chemical products are available for post-infection control. Despite products containing the active component metalaxyl being registered for black spot control, these should only be used as part of a preventative regime.