80 MASS. EXPERIMENT STATION BULLETIN 175. 



OCCUKRENCE. 



The mosaic disease has been known for years both in Europe and 

 America, and may be said to be present everywhere that tobacco is grown. 

 It apparently is a more serious disease in the tropics and in certain parts 

 of Europe than it is in this country. In New England it has been known 

 for some time, and, although present to a certain extent each year, is not 

 of such great economic importance as in some other locahties. In Massa- 

 chusetts it is found practically everywhere, and some years appears to be 

 much more prevalent over widespread areas than in others. As a rule, 

 however, the disease is not epidemic in character, and often only a com- 

 paratively few plants in a field will be found affected. 



On certain fields, however, — and these most often are such as have 

 been cropped to tobacco for many years without the practice of cover- 

 cropping or rotation, — mosaic disease is present year after year, and a 

 large percentage of the crop is always badly affected, the plants beginning 

 to show the trouble in from three to four weeks after planting in the field. 



The prevalence of the disease in the field, aside from the special cases 

 above noted, is apparently related in some way to conditions in the field 

 during the growing season, or during the time the plants are in the seed 

 bed. There is no question that a large percentage of the infection found 

 in the field, exclusive of that appearing on the sucker growth after topping, 

 or due to infection at the time of transplanting, is due to a primary infec- 

 tion from the seed bed. 



While the disease as a rule is first noticed in the field some time after 

 transplanting, very often the seedlings in the beds are affected. This is 

 particularly true in the case of old or carelessly treated beds. It is often 

 very difficult for the casual observer to identify the disease on the seedlings, 

 as the macroscopic or visible symptoms are either very slight or lacking. 

 In this way many plants are transplanted to the field by workmen without 

 their being aware that they are diseased, and the disease becoming more 

 pronounced in the later stages of growth, the infection is laid to the soil 

 in the field, when in reality the infected soil of the seed bed is responsible 

 and not the field soil. As has been stated, the closest examination of the 

 seedlings is necessary to identify the trouble in the seed bed, particularly 

 in mild cases of infection. 



From observations made repeatedly, not only on seed beds but also 

 experimentally under controlled conditions in the greenhouse with soils 

 from old beds, afterwards transplanting the seedlings to soil previously 

 not used for tobacco, and using as checks healthy plants from new soil, 

 the writer has come to the conclusion that at least 80 per cent, of our 

 field infections come from the seed bed and do not originate in the field as 

 is commonly supposed. 



