Anita Morton - Growing Gardens
Stop the rot
Our climate is ideal for all kinds of fungal diseases. Selection is the answer, of course; choose plants native to the tropics and subtropics to reduce fungal problems permanently. Unfortunately, many of us have a passion for growing hybrid roses, fruit trees, grape vines, pumpkins, melons and other fungus magnets.
With these susceptible plants we need to be proactive against disease. Powdery mildew is likely to strike when the atmosphere is humid, but the plant is water-stressed. These conditions are common in late spring, particularly in potted plants as it can be hard to keep the potting medium sufficiently moist in hot weather. The coating of greyish-white spores on the leaves gives this one away, and if its left unchecked the plant will suffer and even die.
By the time you notice the symptoms its usually too late for total eradication, so preventive sprays are a good idea. Milk sprays work by making the leaf surface inhospitable to mildew spores, so you need to coat the whole plant. Mix one part full-cream milk with 10 parts of water and spray in the cool of the day. Repeat about every two weeks, or more often if we get heavy rain. Wettable sulphur sprays are more effective on established mildew again, cover the whole plant, and only spray in the evening and in cooler weather.
Downy mildew will probably start to appear soon too it loves a warm, humid atmosphere, so after our recent rains we should take precautions. Copper oxychloride sprays will prevent this nasty fungus from taking hold, and will control it if you apply as soon as you see those pale blotches on the tops of leaves (the downy bit is on the underside). Be sure to read and follow the application advice and cautions on the packet.