The Problem - Powdery Mildew

Powdery mildew (Erysiphe necator) is a common and increasingly problematic disease for grape growers in New Zealand. While being unsightly, its major impact is financial.

Powdery Mildew Infections

Powdery mildew infections of bunches can severely impact quality causing reduced berry size, delaying maturity and result in splitting. Powdery mildew infected grapes can result in off flavours in wine and are usually downgraded at the winery, resulting in a financial penalty to the grower.

Powdery mildew is often described as a “hot” weather disease as it develops under relatively dry conditions and does not require free moisture for infection. Powdery mildew is an obligate parasite in that it only infects living plant tissue, including leaves, shoots and young berries.

The obvious symptoms of powdery mildew infection are seen as grey to white powdery growth on leaves (on both upper and lower surfaces) and on bunches (see photos below). Obvious symptoms are seen from mid-season onwards and once established are very difficult to eradicate. Initial infections start early in the season and are difficult to see. These infections spread slowly and then more rapidly until an epidemic is obvious.
Powdery Mildew

Powdery mildew infections can arise in the spring from two sources. The most important in New Zealand is from diseased shoots called “flag shoots” (see photos below). These appear from overwintering diseased buds that were infected the previous season.

These flag shoots are distorted and are stunted and produce spores which then infect nearby healthy foliage. An epidemic the previous season will result in higher levels of “flag shoots” infected and should prepare vineyard managers for increased vigilance regarding powdery mildew management for the next season.

A second source of infections can be through overwintering fruiting bodies (sexual stage) called cleistothecia. Cleistothecia are tiny yellow to black specks that are more clearly seen under magnification with a hand lens. In the spring with moisture, cleistothecia produce and release ascospores which can infect leaves.

Until recently this sexual stage has not been present in New Zealand vineyards and its importance as an inoculum source in New Zealand conditions is not yet understood. The presence of the sexual stage will increase the genetic diversity of the powdery mildew fungus in New Zealand. This increases the risks of possible resistance developing to fungicides and emphasizes the importance of fungicide resistance management strategies in grapes.

Once infections develop a new generation of spores can be produced every 5-10 days. Its spread is favoured by mild-cloudy weather (optimum 22-28°C) with relative humidity greater than 40%. If uncontrolled, powdery mildew can steadily and then more rapidly spread. 90 days post infection the severity of leaf and bunch infections develops rapidly.
As dense canopies favour disease development and restrict spray coverage, using cultural techniques can assist control. These include physical removal of flag shoots and management of crop canopies (shoot thinning and trimming). Alongside cultural practices, a seasonal spray programme is required to protect grape crops from economic loss from powdery mildew.