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



that some parasite must be concerned, which has so far escaped de- 

 tection either because of its extremely small size or because of other 

 peculiarities. Not only tobacco but a considerable number of other 

 plants are affected with mosaic identical with or closely similar to 

 that of tobacco. Tobacco mosaic can be transmitted to certain other 

 plants, particularly to some belonging to the same botanical family, 

 such as tomato and pepper. This fact is especially important, since 

 the ability of this disease to live on other plants may account 

 for overwintering in some cases. Experimental and observational 

 evidence in tobacco mosaic thus far indicates that the causal agent 

 does not live over winter in the soil or in the seed. In the case 

 of mosaic of other plants it has been shown that the disease per- 

 sists over winter in the living underground parts of perennial 

 plants. Such a general carrying-over agent for tobacco mosaic has 

 not yet been demonstrated. On this principle, however, weeds in and 

 around tobacco beds should not be tolerated on farms where mosaic 

 is to be reckoned with as a serious disease. 



In this connection it should be stated here that probably all mosaic 

 of tobacco occurring in the field originates from seed-bed infection. 

 The signs of the disease in the bed usually are only very slightly vis- 

 ible, if at all, and usually occur only on isolated plants. From these 

 plants, nowever, it is readily transmitted to many other plants in 

 the operations of pulling and transplanting or by other means. The 

 importance of preventing seed beds from becoming infected is evi- 

 dent. Aside from the overwintering of the parasite in perennial 

 host plants and its subsequent transfer to plant beds, it should also 

 be stated that under some conditions of storage of infected material 

 it may survive for a year or more, so that overwintering on tobacco 

 refuse, plant-bed covers, frames, or other equipments is not beyond 

 reason. 



When infection has once occurred in the spring, its spread from 

 plant to plant may usually be accounted for. Aside from transmis- 

 sion during the numerous times the individual tobacco plants are 

 handled, it is likely that insects, especially plant lice, may spread the 

 disease under field conditions, as can be readily demonstrated ex- 

 perimentally. 



Conditions favoring the disease. — Tobacco mosaic is dependent 

 upon specific infection from other mosaic plants. Once inoculated, 

 plants are almost certain to show the disease at one stage or an- 

 other in their growth. Aside from the fact that weather conditions 

 influence the length of the incubation period — that is, the length 

 of time between inoculation and the first signs of the disease — their 

 influence does not seem to be important as affecting the occurrence of 

 mosaic. It is quite evident that tobacco mosaic is favored in develop- 

 ment by relatively high temperatures (80° to 85° F.), although tem- 

 peratures higher than this tend to retard its development until at 

 about 100° F. it is no longer active, and diseased leaves tend to re- 

 cover. Closely associated is the fact that rapidly growing plants 

 tend to show the symptoms earlier and more markedly, though sub- 

 sequent growth may be retarded. Eainfall and humidity do not seem 

 to have any such direct relation to infection and progress of mosaic 

 as to certain other leaf diseases. Where insects are important car- 

 riers of the disease, conditions affecting their occurrence and numbers 

 appear to have a direct bearing on the occurrence of mosaic. 



