PLANT MILDEWS
These diseases cause young leaves and tip growth to become distorted and appear grey.
They commonly occur on azaleas, apples, cucurbits, and a wide range of other plants. There are three main types of mildew:
Powdery Mildew
Causes a white powdery growth on the leaf surface.
Fungus grows over the surface of a leaf, seldom killing the plant but shading the leaf and reducing photosynthesis.
It generally starts in humid conditions (but not if leaf surface is covered with water).
Once started, it grows in almost any conditions. It most commonly grows on upper leaf surfaces, but also occurs below, and on other above ground parts.
It is a common disease on a wide range of both crop and ornamental plants, causing probably more losses than any other type of disease.
Plants Affected:
Some plants which commonly suffer badly include: cucurbits, strawberries, clover, roses, begonias, azalea, delphinium, grape, lilac, apple, catalpa and oak.
Control:
Natural - Don't overcrowd plants, water from underneath in hot, humid or moist conditions.
Chemical - Benlate, systemic fungicides.
Black Mildew
Appearance is similar to powdery mildew, only a dark colour. Sometimes confused with "sooty mould", but this fungi does not grow on "honey dew" given off by insects (which sooty mould does).
Downy Mildew
Often occurs in damp conditions. The upper leaf surface goes yellow, while a grey mould begins to develop underneath. To spread it needs water on the leaf surface and relatively high humidity, together with cool or warm (but not hot) conditions. It affects mainly foliage, but can spread to any tender parts including stems and fruit. It develops very rapidly. It is also a problem on seedlings of some vegetables.
Plants Affected:
Many including grain crops, vegetables, ornamentals, shrubs and vines. Particularly significant on grapes.
Control:
In cool, wet weather control is very difficult without destroying plants. Heat & dryness can help control.
Chemical: Zineb, Mancozeb or Sulphur; also systemic fungicides.
COMMON MILDEWS
Powdery Mildew on Roses
Widespread, worldwide, it attacks tender parts including buds, growing tips and new foliage. Slight blister-like areas occur on leaves which quickly develop grey-white powdery growth. Leaves can become distorted as they grow. Similar growths can develop on tender shoot tips, and occasionally on buds. Affected buds may fail to open, or may develop distorted growth. More likely to occur under wet conditions and with temperatures which fluctuate greatly. Many new rose varieties are very resistant, but the resistance can change under different environmental conditions. Many popular older varieties are also quite susceptible to this disease and require routine spraying.
Control with sulphur, benomyl or dinocap.
Downy Mildew on Grape
Small pale yellow spots on leaves at first, developing into a white furry growth on the lower surface of the leaf under each spot. The leaves turn brown slightly and then fall off. The mildew may also appear on the grapes themselves, and production is greatly diminished. This problem is worse in warm humid conditions, particularly in mid- to late spring. The fungus can survive over winter amongst the dead leaves on the ground. Mancozeb may be used for control, spraying thoroughly when the new shoots are about 25cm long. Spray monthly in drier conditions, fortnightly in more humid conditions. Overhead watering
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