Pests and Diseases Affecting Roses
Unless you are a masochist, do not spray unless you have to. Choose varieties of rose that are known to be healthy, buy good strong plants, cultivate them well, and their resistance will be that much greater.
The only diseases the average grower has to worry about are blackspot, mildew, and in some districts, rust. Nobody knows .as yet how to cure any of them, but you can control them if you tackle them in time.
Round black spots on the leaves, generally seen first on the lower leaves in late summer. They will spread, the leaves will turn yellow. And eventually drop off. If neglected over several years, this lack of leaves will weaken and eventually kill the rose. Pick off and burn any infected leaves — one of those delightfully simple things to say, like; ‘Be sure to pick up all the confetti afterwards.’
A whitish powdery covering which, when bad, can cover and distort the leaves, new shoots and buds so that they do not flower properly. It will not kill, but it looks horrible. It can appear as early as May, but is not usually bad before July onwards.
Rust-coloured swellings on the under surfaces of the leaves in spring and later. This can be a killer and infected shoots should be cut off and burned.
Blackspot can be checked by spraying with captan, mildew by spraying with karathane, and rust with maneb, zineb or thiram, but fortunately there are good general rose fungicides on the market that deal with the whole lot at once, so it does not mean three separate sprayings. How often you spray depends on your locality, the weather, and how bad your attack of disease is. It can be as often as once every ten days, it can be no more than twice or thrice a year, or it could be not at all. In other words each grower must find his own minimum, but when and if you do spray, wet the whole plant. The under sides of the leaves are just as important as the tops. Do not spray in the hot sun or the leaves may scorch.
The worst insects you are likely to be troubled with are greenfly. These and most of the other lesser pests can be controlled for weeks on end by spraying with a systemic insecticide. This enters the sap of the plant and cannot be washed off by rain. Spray when attacks start. Some systemics can be mixed with the fungicides, which saves a separate spraying. If the manufacturer of both of them is the same it is quite easy to check this and it will probably say so on the bottle or packet, but do be quite sure they will mix or you may be left with no leaves. Use one of the plastic pressure sprays. They are a joy.