Erisiphe necator (previously Uncinula necator)

The disease is characterised by white to ash-grey powdery fungal growth capable of infecting green vine tissue. Severe infection can lead to high crop losses and may also be detrimental to final wine quality. In Queensland, many of the commercial Vitis vinifera varieties are susceptible (particularly verdelho).

Downy mildew

Flag shoots
Two to six weeks after budburst distorted flag shoots may emerge from buds infected in the previous season before flowering. The leaves of these infected flag shoots often curl upwards (easily confused with mite damage).

Leaves
Yellow-green blotches (2-10 mm diameter) appear on leaves. Progresses to produce ash-grey to white powdery spores on upper and lower leaf surface. Often confused with downy mildew, which only produces spores on the lower surface of the leaf.

Berries
Produces ash-grey to white spores on immature berries and bunch stalks. Diseased berries may become distorted and split, causing them to shrivel or rot.

Severe infection of powdery mildew can depress vine vigour because damage to leaves and shoots limits photosynthesis and inhibits berry sugar development. Powdery mildew infection at harvest can cause considerable economic loss because diseased berries cause off-flavours in wine, so wineries may reject a consignment with more than three infected bunches per bin.

Monitoring is particularly important early in the season when flag shoots emerge between two and eight weeks after budburst. Monitoring should be focused on hot spots where the disease may have been present previously. This is most likely to be in sheltered or shaded areas of the vineyard, particularly at the base of contours. It is important to look inside the canopy where the shaded leaves and bunches are more prone to infection. Extra attention should be paid to highly susceptible varieties such as verdelho and chardonnay. Using a 10x hand lens to observe hyphae and conidiospore development on the leaves may help detect the disease early.

Monitoring allows you to detect disease earlier and treat it at the optimum time. Monitoring also enables you to assess how effective treatment is. Where new infections persist, the management program may need to be reviewed. You may need to reconsider how spray equipment is set up and calibrated as well as the product, application rate and spray volume.

Canopy management
Maintaining an open canopy will help to increase air circulation and reduce humidity and shading. Open canopies are exposed to sunlight and allow good spray penetration.

Chemical control
Early season control is critical to minimise disease build-up. It also prevents the disease spreading from emerging flag shoots and the previous season´s cleistothecia. While the vine is growing rapidly, a fungicide spray interval of 7-10 days may be needed.

Disease control is critical just before flowering and for five weeks post flowering because this is when the inflorescence and subsequent berries are most susceptible to infection.

Recommended application rates of sulphur to control powdery mildew in wine grapes have been increased as a result of Australian viticulture research. Field trials demonstrated the efficacy of wettable powder or dry flowable formulations was improved when the recommended application rate was increased by two or three times. The recommended rate for sulphur application is now between 200 and 600g/100L. The fungicidal activity of sulphur increases when temperatures are above 15 °C because the vapour activity is only effective above this temperature. Sulphur should therefore be applied when the temperature is above 15 °C. However sulphur should not be sprayed when the temperature exceeds 30 °C and when relative humidity is above 70% to avoid sulphur burn (phytotoxicity).

In vineyards with a history of powdery mildew infection, a vigorous spray program early in the season will reduce disease pressure later in the season.

A system for identifying the different growth stages in grapevines (known as E-L stages) was developed by Eichhorn and Lorenz (see the Australian Wine Research Institute website). It is recommended that sulphur sprays are applied at these growth stages:

• E-L stage 12-13 (5-6 leaves separated) approximately 2 weeks after bud burst
• E-L stage 15-16 (8-10 leaves separated) approximately 4 weeks after bud burst
• E-L stage 17-18 (12-14 leaves separated) approximately 6-7 weeks after bud burst
• E-L stage 27 (berry set, berries at 2 mm) approximately 8-10 weeks after bud burst

Where powdery mildew has been a problem or disease pressure is high, control is improved by including two demethylation inhibiting (DMI) fungicide sprays during flowering at 20% and 80% capfall. Alternatively two strobilurin sprays may be applied at these same times.

DPI&F´s wine grape spray guide gives a whole-of-season approach to powdery mildew control.