34 BULLETIN 1256, U. S. DEPARTMENT OF AGRICULTURE. 



From recent observational and experimental evidence it appears 

 that at least four main types of nonparasitic spotting exist on the 

 basis of causal agents, namely, those due to (1) a hereditary pre- 

 disposition toward spotting, (2) unbalanced nutrition, (3) absorp- 

 tion of toxic agents, (L) toxic agents applied externally. As illus- 

 trations of these groups the following may be cited: Under certain 

 conditions the Sumatra and Connecticut Broadleaf varieties com- 

 monly show spots when other varieties do not. Phosphorus deficiency 

 in the soil may result in spotting. Soils rendered toxic by steriliza- 

 tion or by the addition of certain salts may produce spotting of leaves. 

 Spraying with certain insecticides or fungicides produces spotting. 

 In all these cases, however, the weather conditions apparently affect 

 the occurrence of these spots to a greater or lesser extent. 



Attempts to classify the nonparasitic tobacco leaf-spots occurring 

 naturally on the basis of symptoms with some consideration of their 

 origin are not usually satisfactory. A few more or less distinctive 

 types have been named, and the predisposing weather conditions are 

 at least partially understood. 



Firing, as most properly used now, refers to a drying up of the 

 basal leaves, usually starting at the leaf margins and proceeding in- 

 ward between the veins, in blotches rather than spots (PL X, fig. 1). 

 This is conceded in most cases to result from dry weather, resulting 

 in the starvation of the plant as a whole for food and water and the 

 withdrawal of these from the basal leaves to supply the growing 

 point. A more or less sudden deficiency of food alone in the soil 

 in relatively wet weather may result similarly, especially on sandy 

 soils artificially fertilized and subject to leaching or otherwise losing 

 the reserve food supply. On the other hand, an excess of phosphoric 

 acid on certain soils is said to result in firing. 



The true forms of nonparasitic spotting are usually sharply de- 

 marcated, round to angular in shape, varying from a considerable 

 size to mere specks (PL X, fig. 2) and from almost black to white 

 in color (PL XI). Others exist more or less as continuous lines or 

 circles showing marked relation to the veins (PL XII). 



Dead blotches in tobacco leaves sometimes follow after distinct 

 symptoms of specific malnutrition have existed for some time, as in 

 the case of potash and magnesium starvation, but these will be given 

 separate consideration, as they are not regarded as typical leaf-spot 

 troubles. 



Leaf spotting sometimes results from physical causes, such as 

 bruising by rain, hail, or wind. 



The control of the main forms of nonparasitic spotting is for the 

 present limited to avoiding varieties or strains, where possible, which 

 are predisposed to this injury, maintaining a proper balance of fer- 

 tilizers in the soil, or avoiding the use of soils which are not amen- 

 able to fertilizer treatment. 



THE MOSAIC DISEASE. 



Description.— Mosaic is one of the most common and widespread 

 diseases of tobacco. The disease is often referred to by different 

 names in various districts, such terms as " calico." w * mottling," " gray- 

 top," and "mongrel" being often used. "Frenching" is another 

 term commonly applied to mosaic, but this term properly used ap- 

 plies to a distinct though somewhat similar disease. Mosaic is not 



