yo Journal of Agricultural Research voi. xvii, no. 2 



may overcome the disease. If the warm period is delayed until late 

 July or August, recovery may still be made and a late crop of good 

 yield produced, provided the balance has not swung to the other ex- 

 treme — that is, forced maturity. 



No condition is more commonly seen in infested tobacco fields than 

 that of plants budded out and ready for topping two to three weeks 

 before the normal date when the plants have obtained only one-fourth 

 to one-half their normal growth. This is a direct result of the starvation 

 of the plants caused by disease. A drouth may bring on the same 

 condition. The plants then must be topped when this stage is reached, 

 and although much spread of leaf may subsequently occur, owing to the 

 arrival of more favorable conditions for growth, yet the yield is almost 

 certain to be light. 



Several years of practical observation of infested fields have shown 

 that heavy infection almost always occurs in June. Every tobacco 

 grower of experience, at least in Wisconsin, can cite cases where during 

 the first two or three weeks after planting the crop prospects have been 

 excellent, followed by a like period of uncertainty, when the condition 

 of the crop has apparently made no progress or has gone slightly back- 

 ward, and finally, for no apparent reason, where the crop has taken on a 

 new lease of life, or, on the contrary, has remained to the end more or 

 less of a failure. In Wisconsin a large percentage of poor crops in the 

 years 1913, 1915, and 1917 was due either to poor yield or delayed 

 maturity directly traceable to the rootrot. In the years 191 4 and 191 6 

 fairly good yields were obtained, and not much root disease occurred 

 even on infested soils. 



It is believed that an examination of the summarized soil temperature 

 records for these years in Table VII, or a glance at the temperature 

 curv'es in Plates 6-8, will furnish in a large measure an explanation for 

 the results obtained with tobacco in 1915, 1916, 1917, and 1918. The 

 year 191 5 was an especially cold season; according to weather bureau 

 records at Madison it was the coldest on record, and also a comparatively 

 wet one. The studies of the writers on the influence of soil moisture, 

 however, have now convinced them that its importance as a controlling 

 factor under field conditions is small as compared with temperature. 

 In 1 91 5 the loss from the rootrot of tobacco was estimated at from 

 $10,000,000 to $20,000,000 in the United States alone. The year 191 8 

 showed very poor prospects of a good crop for a period of several weeks 

 in July and early August. In the latter half of August, however, the 

 Wisconsin crop made a remarkable growth even in the most heavily in- 

 fested fields; this growth was unquestionably a direct result of the in- 

 crfeased soil temperatures during this month. 



