As a general rule it is the frame-grown plants that suffer most from fungus diseases, but those growing out-of-doors are not immune from attacks. An instance has just come to the writer's notice of two collections suffering from attacks of Urocystis violae. This disease causes the leaves and leaf petioles to become swollen and eventually burst. At first sight the condition looks as if it were the result of a gall-forming insect, but when the rupture takes place the black spores can be seen easily, and the character of the malady is thereupon disclosed. This is such a deep-seated disease that spraying is of little use. The only thing to be done is to pick off and burn any diseased foliage before the spores are distributed; or in very bad attacks, to burn all the plants and get a fresh stock.
This disease is due to another parasitic fungus that grows in the tissues of the plant. Its presence is denoted by pustules of powdery uredospores which appear on the leaves. The disease is somewhat like, but not identical with, that which attacks the Hollyhock. On its first appearance all affected leaves should be removed from the plants and burnt.
The presence of this disease may be detected by pale spots appearing on the leaves. These spots eventually develop tufts of short, erect threads. It is not so serious as the diseases already mentioned; nevertheless the plants should be sprayed with the Bordeaux mixture directly the disease is detected. This preparation can be purchased from horticultural sundries-men.
This fungus is like that which attacks the potato haulm and tubers. It attacks Pansies as well as Sweet Violets, and causes a whitish, felt-like covering on the under surface of the leaves. Like all mildews, this disease spreads quickest in damp weather or in a stagnant atmosphere; it will be less likely to attack Violets in frames if careful attention is given to ventilating the frames; in severe cases the plants may be syringed with potassium sulphide, at the rate of 1 oz. of potassium sulphide (or liver of sulphur) to 2 1/2 gallons of water. Dissolve the potassium sulphide in a quart of hot water; then make it up to 2 1/2 gallons with cold water.
This is another disease that Violets have exhibited in this country when cultivated in frames. An attack may be identified by the presence of scorched-like patches on the leaves. From these patches numerous minute spores are produced, and these, falling from the leaves to the ground, are liable to perpetuate the disease. Where a bad attack is experienced, the most satisfactory plan is to burn the plants, sterilise the soil or remove it to an out-of-the-way part of the garden, and thoroughly disinfect the frame before planting fresh stock. When the plants are well established, the plant and soil may be sprayed at intervals of a fortnight with potassium sulphide, at the strength of 1 oz. to 3 gallons of water.