The common diseases can also be grouped according to their symptoms and treatment. So that the interested gardener can soon acquire a working knowledge of them.
All the powdery mildews make plants look as if they had been dusted with flour. Roses can suffer very badly, especially in dry summers, and so can Michaelmas daisies. There are mildews which attack peas, apples, vines, gooseberries and a great many other plants. All can be tackled with dinocap, but it is necessary to get the treatment started early as it is a means of prevention rather than of cure. Some of the systemic fungicides, such as benomyl and triforine, will actually kill mildew in the plant but in general these treatments are more expensive.
Then there are the equally numerous rusts, so called because they produce rust-coloured spots on the leaves, and some-times the stems, of plants attacked. There is a very troublesome rust disease of hollyhocks and another of antirrhinums. These and most other rusts can be prevented by occasional spraying in summer with thiram, or triforine may even kill rust after infection has occurred.
There are a great many spot diseases and they are rather diverse in their origin. One of the worst is black spot of roses, which produces roundish black spots on the leaves which fall prematurely. The remedy is either to spray frequently with captan or maneb, starting in spring and continuing every 7 to 10 days all summer. Or rather less frequently with a systemic fungicide such as benomyl or triforine.
Captan is also a good means of preventing scab disease of apples and pears which also starts by producing black spots or patches on leaves and fruits.
The very widespread disease of potatoes known as blight is another that starts by producing dark spots or patches on the leaves. These spread rapidly, so that all growth above ground withers and dies. By this time the disease has spread to the potatoes themselves, which develop brown patches of decay. The same disease attacks tomatoes and makes the fruits rot. The best preventive is a copper fungicide such as Bordeaux mixture, but it must be sprayed over all the leaves and stems before infection occurs.
One of the most widespread of diseases is grey mould, also known as botrytis. It attacks all manner of plants. Causing a soft, dark decay and a characteristic is the fluffy white outgrowth which quickly follows this decay. It is a disease that is very prevalent in damp, cool weather and it is very difficult to control. Thiram and captan are the most useful chemicals to check it.
Seedlings in the greenhouse will often collapse in hundreds, stricken by fungi in the soil which kill their roots or attack their stems near soil level. This is known as damping off and it is most likely to occur when seedlings are overcrowded or when they are kept too wet and insufficiently ventilated. Sterilized soil helps to prevent this disease. Seed can also be dusted before it is sown with captan or thiram as a preventive.
Lastly there is a whole class of diseases caused by various kinds of virus. These produce many different symptoms and are amongst the most difficult of plant diseases to diagnose with accuracy. Leaves may be mottled with yellow in a quite attractive way, or they may be deformed and become twisted or very narrow. Growth may be stunted or dry brown spots and streaks may appear on stems and leaves. There is no cure for virus diseases that can be applied in the garden and all infected plants must be destroyed, but before doing so it is wise to get the advice of an expert for sometimes symptoms very similar to those of virus disease may be caused by bad cultivation or by the careless use of a selective weedkiller.