This section is from the book "Commercial Gardening Vol4", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
There are several diseases of the Tomato caused by various fungi, the most dangerous being: (1) Leaf Spot, (2) Black Stripe, (3) Sleepy Disease, and (4) the Black Blotch or bacterial disease of the fruit.
The Leaf Spot or Yellow Spot is caused by a fungus known as Cladosporium fulvum. It forms brown mildew-like patches on the under surface of the leaves, and as the mycelium absorbs and destroys the interior tissues the upper surface is spotted with yellow owing to the destruction of the green colouring matter (chlorophyll) in the cells.
This disease is generally caused by a hot, moist, and stuffy atmosphere. Care should therefore be taken to maintain a proper and equable temperature, and regulate the ventilation without causing cold draughts or currents of air through the houses. In damp cheerless weather in spring or autumn the fires should be kept going, and no more water than is absolutely necessary should be given to the plants, so that the atmosphere may be kept in a fairly dry condition. Diseased plants are best taken out carefully and burned at once in the furnaces.
Black Stripe or Black Rot is caused by Macrosporium Tomati, also known as M. Lycopersici and M. Solani. The plants are attacked when in full growth. The stems begin to shrivel and discolour, with blackish stripes, the leaves to look sickly, blotched, and yellow, and then the fruits to turn black at the eye. The disease is probably due to sour or badly aerated soil, and perhaps too much organic manure. A hot and moist atmosphere are also predisposing causes. As preventives the soil should be deeply turned up and well dressed with freshly slaked or even unslaked lime at the rate of 1 lb. to the square yard, or about 2 tons to the acre, some time before planting. Overwatering should be avoided, and the atmosphere should be kept buoyant by proper regulation of the ventilators in fine weather.
The Sleepy Disease of Tomatoes is caused by the spores of Fusarium Lycopersici entering the roots of the plants from the soil. Its presence is indicated by the dull or yellowish colour and drooping of the leaves, and later on by a delicate mildew "bloom" on the base of the stems. These soon become patched with dull orange and signs of decay. If cut through near the base a brownish ring just beneath the bark will be noticed. Although the plants may be attacked when quite young, some weeks elapse before the presence of the disease manifests itself in the ways indicated.
All diseased plants should be taken up and burned immediately. It is useless spraying them with anything, as the disease is within the plants. The soil should be turned over and sweetened and well dressed with fresh lime. Too much organic manure should be avoided; and basic slag would be the safest artificial to apply to the crops generally, either under glass or in the open air. Proper ventilation should always be given, and the soil should never be drenched and sodden with water. It is better to water once a day properly than to make the soil sodden once a week.
The Black Blotch or bacterial disease attacks the ripening fruit of Tomatoes, although often earlier. It is said that the disease is caused by insects visiting the flowers and depositing the bacteria on the stigma. Insect visits to Tomato flowers are few and far between, and it is questionable if that is the right cause. It may be more likely to arise from mulchings or topdressings of stable manure often given to save watering. In any case the diseased fruits should be picked off when noticed, and burned. Spraying with insecticides, as often recommended, when the plants are in flower would do more harm than good, as the pollen would be wetted and thus prevented from being blown on to the stigmas to fertilize the young fruits. The safer plan would be keeping the ground clean by occasional hoeing or freshening up, plenty of air on all favourable days, proper watering, and shaking the plants at midday to distribute the pollen. It would be advisable also to dust the soil occasionally with flowers of sulphur.