TOBACCO DISEASES AND THEIR CONTROL. 29 



fire is more or less confined to the Southern States, whereas wild- 

 fire seems likely to be a serious disease in the Northern States as well 

 as in the South. In 1920 black-fire is said to have been especially 

 serious in Virginia and was found in 85 per cent of the fields in- 

 spected, causing an estimated loss of more than $5,000,000. 



In the seed bed the black-fire leaf spots are quite small, angular 

 in shape, and black or dark brown in color, with a narrow, clear 

 border especially distinct when held up to the light (PL VIII, fig. 1) . 

 The spots on seedling leaves always remain small and do not ap- 

 proach the size attained in the field. When a considerable number 

 of infections occur the leaf often becomes deformed and ragged. 



In the field young black-fire spots in the early stages are much 

 like those in the seed bed, but later they become as large as half an 

 inch in diameter. On thin leaves the spots are usually quite angular 

 in outline, whereas on thicker leaves they are more rounded, some- 

 times zonated, from tan to dark brown, sometimes almost black in 

 color (PI. VIII, fig. 2). The conspicuous bleached border or halo 

 so characteristic of wild-fire is absent, although some yellowing 

 occurs around the spot. 



Cause. — Black-fire is caused by a bacterium which has been named 

 Bacterium angulatum. This organism closely resembles the wild- 

 fire bacterium in appearance and behavior. So far as known the 

 overwintering and spread of this organism is practically the same as 

 that of wild-fire. 



Conditions favoring the disease. — As in the case of wild-fire, fre- 

 quent rainfall and especially rain with strong wind is favorable to 

 infection and spread of the disease. Good and well-balanced fertili- 

 zation appears to favor the occurrence of infection rather than poor 

 fertilization, which results in less rapid and succulent growth. Low 

 topping, as compared with high topping, has been found to favor 

 the disease. Low topping is, however, usually essential to the pro- 

 duction of proper size and body of leaf, especially in the dark export 

 types of tobacco. 



Control. — Seed-bed sanitation measures, as described earlier (p. -i), 

 and general precautions as recommended for wild-fire control (p. 28) 

 apply equally well to black-fire. There seems to be definite evidence 

 that in the case of black-fire the germs may live over from year to 

 year on seed and on cloth covers, and special attention should be paid 

 therefore to seed disinfection and the use of new cloth covers or to 

 the sterilization of old covers every season. As an aid to the pro- 

 duction of disease-free seed, it is advised that seed pods showing no 

 sign of infection should be selected. To cover flower stalks with 

 paper bags will aid in protecting pods against infection. Seed 2 

 or more years old is quite unlikely to carry infection, but in all 

 doubtful cases these precautions should be supplemented by seed 

 disinfection. The plowing under of stubble as soon as the tobacco 

 is harvested is recommended for infected fields, especially if tobacco 

 is to follow on the same land. 



THE WISCONSIN LEAF-SPOT. 



Description. — Two bacterial leaf -spot diseases have already been 

 described, namely, wild-fire and black-fire. The Wisconsin leaf-spot 

 is a third form of bacterial disease, causing similar but not identical 



