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In general, sprays are most effective if applied with the nozzle pointing upwards through the plants, so that the liquid reaches the under-surface of each leaf. Spraying in windy weather is wasteful, and, if the material contains any ingredient that is poisonous to man, the danger is increased greatly. It is best to spray in the early morning, for there is usually less wind at that time. Many sprays will cause severe damage to foliage if used on hot days.

mildew (Plate 42)

Mildew, caused by the fungus Sphaerotheca pannosa, is a very common rose disease. It is entirely superficial, and most common on young growth. Frequent changes of temperature during the growing season predispose to mildew, making it a serious disease in some seasons in southern Victoria, Tasmania, and the South Island of New Zealand.

In the spring it appears as greyish-white areas on the leaves, stems, and buds of young shoots. Later it has a white, powdery appearance. It is least prevalent during midsummer, but reappears with the advent of autumn's warm days and cool nights. Spores are present on all roses throughout the year. Tea Roses and Pernetianas are fairly resistant to mildew.

Control is relatively easy, especially if early preventive measures are adopted. Heavy feeding with wood ashes or one of the inorganic compounds of potassium increases resistance to mildew, but can never cure infection. Sulphur became the time-honoured fungicide for mildew. The various wettable and colloidal sulphur preparations, such as Wetsul and Spersul, are great improvements on the flowers of sulphur formerly used. Shirlan and Clensel are useful and safe at any time. Bordeaux mixture, Bordinette, and lime-sulphur kill many mildew spores when used after winter pruning. The more recently introduced preparations of TMTD are better than any of the older sprays, and, in addition, they are very efficient in controlling black spot. There are several brands of TMTD sprays available; Thiotox is the one of which I have had most experience.

For established mildew some people have had considerable success with a quarter of an ounce of washing soda, plus a little soft soap, to each gallon of water. It is simple and safe. In the past, the best remedies in my hands have been white-oil emulsions. They should be mixed in slightly greater strength than advised by the manufacturers, and not used on hot days. If a mild infestation of mildew is sprayed twice at an interval of two days the fungus clears away quickly. These emulsions have the advantage, too, of not marking foliage. Never mix preparations of white-oil with sulphur in any form; the combination is unnecessary and very damaging to almost all types of plants.

Karathane is the most recently introduced fungicide. It was used first as a miticide and was very effective in controlling red spider, but it was found that it would eradicate established mildew. It must be applied to every affected part of any plant so that the fungus and the foliage are quite wet. Spraying should be done on good drying days, when it is expected that the temperature will not exceed 8o° F. Karathane will cause damage if the leaves remain wet for too long after spraying, or if the temperature is too high, or if the spray mixture is too concentrated. Both the powder and the water must be measured accurately. It renders prophylactic spraying for mildew unnecessary. Thus only a few plants need to be sprayed at all, and not many applications on each of those plants in the course of the year will be necessary. This marks a great reduction in labour.

Several new sprays, with zinc compounds as their basis, are promising too.