Potential for powdery mildew infection of grapevines in the coming season is high, especially if spring is mild, following good rain through winter.

The more susceptible varieties are Verdelho, Chardonnay, Riesling, Chenin Blanc, Muscadelle and Semillon; Shiraz and Grenache are amongst the least susceptible.

Failing to control powdery mildew means berries may become scarred and distorted, rot and shrivel and consequently lose yield, lowering fruit grade and reducing payment.

Significant infections may result in a winery rejecting the fruit to avoid “off” flavours in the wine and then it would be unlikely that other wineries would want it.

Typically, powdery mildew will survive in dormant buds infected before they turned brown in the previous season.

In spring these buds produce highly infected and deformed foliage known as “flag shoots” with about one in every one thousand being affected.

Alternatively the disease will start when an over-wintering form, that survives on bark and mummified bunch stalks from the previous season, is wet by light rain and then emerges to infect lower leaves on the vine when the air temperature is more than 10 degrees Celsius.

If the disease in either form is not controlled in these early stages then progressive infections will multiply rapidly.

Around 40 days from budburst a critical level is reached when infections are so serious that control becomes very difficult and may become ineffective.

Disease is favoured where vines have dense foliage, especially after mild cloudy weather with low to moderate light.

Leaf symptoms include yellow-green blotches ranging from 2-10 millimetres wide with web-like fungal growth that may eventually cover the whole leaf, plus the well recognised ash grey to white powdery spores.

Both upper and lower surfaces of the leaves can be infected.

Shoot symptoms include stunting, distortion, oily grey blotches and ash grey to white spore patches as seen on leaves and older infections show as dark red-brown patches.

Severely diseased canes mature irregularly and in winter may die back from the tip.

Many growers would be familiar with red-brown web-like patterns that develop on infected canes late in the growing season and early winter.

It is important to avoid excessive vigour in the vines and manage canopies to encourage air movement through the vine to enable effective application of protective sprays – open canopies also let in sunlight to help prevent disease.
The most common control agent is dispersible sulphur that will fume most effectively from about 18 degrees Celsius.

However, there is a big gap between 10 degrees when infection will commence and 18 degrees when sulphur will effectively fume.

This is where manufactured fungicides will be very effective, especially for susceptible varieties.

Five manufactured protective fungicides registered for control in addition to sulphur are listed in the Grapevine Management Guide.

There are some limitations to application of fungicides – please read the label to check, and consult your winery representative if you are unaware of their policy in relation to the products.

Contact Harry Creecy, (02) 6960 1315, 0427 201 941.

This story appears Agriculture Today.