VEGETABLE CROPS. 70 I 



Experiments indicati that water supply is of the greatest importance in. 

 the production and the > untrol of Black ,^pot. The disease has been pro- 

 duced on vigorous plants by a sudden decrease in the available water supply. 

 Excessive water supply has also produced the disease more readily and uni- 

 formly than a scant or intermittent one. Plants receiving a moderate and 

 regular supply of water develop less rotten fruit than cither lightly or 

 heavily watered ones. Too much exposure to the sun also tends to an 

 increase of the disease, so that methods of growth that allow for the foliage 

 shading the fruit, assist in checking the appearance of rot. 



The effects of fertilisers on the production of the disease vary with the 

 nature of the soil and the amount of water supplied. Experiments by 

 Brooks in New Hampshire, U.S.A., gave the following general condu- 

 it : — Lime is of value in reducing the disease, especially if the plants are 

 well watered, but under drought conditions it has little tendency to decrease 

 the disease; potash has no tendency to increase the disease, but nitrogenous 

 fertilisers favour its development. Soil factors that decrease the disease 

 are all such as favour oxidation, while those that have increased it — aside 

 from water supply — are either of such a nature as to check oxidation, or 

 else to increase the organic compounds that would require oxidising, e.g., 

 nitrate of soda, being an oxidising agent, has less tendency to increase the 

 disease than ammonium sulphate and organic fertilisers having an equiva- 

 lent amount of available nitrogen. Heavy applications of stable manure 

 increase the disease out of proportion to the increased vigour of the plants. 



As the disease is not primarily due to fungi or bacteria, spraying will not 

 control it. 



Blights. 



The tomato is included in the family of plants k.'iown as the Solanaceae, 

 which also includes the potato and the tobacco. Many parasitic fungi attack 

 one or more species of plants in a definite family, hence it is natural to find 

 the same fungus diseases occur on the tomato, potato, and tobacco. This is 

 well shown by several of the fungi causing the diseases known as " Blights." - 



The tomato suffers from the following blights : — 



(1) Early Blight, due to Alternaria solani. 



(2) Late (or Irish) Blight, due to Phytophthora infestans. 



(3) Bacterial Blight, due to Bacterium solanacearum. 



(4) Leaf Blight, due to Septoria lycopersici. 



The first three are common to the tomato and potato, and will be only briefly 

 dealt with here, further details being niven in connection with potatoes on 

 page 493 of this Handbook. 



Early Blight. — The- fungus that causes this disease attacks plants 

 at any stage of their development. The first sign of attack is the presence 

 of small brown or black spots on the leaves, especially the lower ones. At 

 first these spots are scattered and inconspicuous, but later they increase in 

 size and number, and finally cause the death of the leaves. Spots may 

 develop on the stem, and are brown or black, more or less circular, and 

 slightly depressed. Fruit may become spotted either on the plant or after 

 picking. The disease may grow into the interior of the fruit and rot it. 



