jan. 3, 1921 Fusarium-Wilt of Tobacco 529 



not have been great, since the soils were repeatedly mixed and stirred. 

 Infection apparently occurred within a wide range of soil reaction, 

 although it was strikingly more pronounced at the higher acidities 

 (PI. 67, III). For this reason we can not agree with MacMillan (8) 

 that infection with Fusarium is favored by alkaline soils. The behavior 

 of Fusarium in the experiments described is also in line with the results 

 secured in the culture of F. oxysporum var. nicotianae in culture media 

 of varying reaction. 



The Fusarium-wilt organism was inoculated in tubes of beef broth 

 at reactions ranging from — 5 per cent to + 5 per cent. After 5 days 

 the best growth was at + 1 . After 1 2 days it was apparently growing 

 best at +3 and poorest at +5, but after about 40 days the fungus 

 growth seemed most profuse at +5. On potato agar, however, the 

 best growth was obtained at neutral to +0.7. After 8 days there was 

 decidedly poorer growth as alkalinity was increased as well as a retarded 

 growth at + 1 per cent. This fungus, in common with most forms, is not 

 favored by alkaline media, and there seems to be no good reason for 

 expecting it to be more virulent in alkaline soils. 



Other environmental conditions. — With respect to other environ- 

 mental conditions, we are able to say very little. Observation seems 

 to indicate that high soil moisture is not especially favorable to the 

 disease. Infection has been noted incidentally in both relatively dry 

 and moist soils, but the writer has been of the opinion that the soil 

 should be kept relatively dry to get good artificial infection. The disease 

 in Maryland occurred on high, sandy land, and the two years, 191 6 and 

 1919, in which the disease was called to the writer's attention were both 

 both notably hot and dry. 



A single trial with soils ranging from no organic matter to pure leaf 

 mold did not indicate any decided preference on the part of the disease 

 for the presence of organic matter in the soil. 



In the soil-inoculation experiments it has appeared that the highest 

 infection has always been secured by planting to tobacco soon after the 

 inoculation of the soil. Later plantings in the same soil usually resulted 

 in a lower percentage of infection. The parasite apparently does not 

 find the soil a very favorable medium for maintaining itself, even in 

 the presence of host plants, and in their entire absence it probably 

 gradually dies out completely. Nothing definite is known, however, 

 as to how long the fungus may persist in the soil. 



To summarize briefly, the conditions which seem most necessary for 

 good infection and progress of the disease are : 



1. Heavy soil infestation. 



2. Wounded host tissue, particularly of stems below the surface of 

 the soil. 



3. A relatively high temperature. 



4. A susceptible variety. 



