May 15, 1919 Infiiience of Soil Environment on Rooirot of Tobacco 77 



sand was considerably more favorable to infection of roots with T. 

 basicola than was clay. 



Another test with the Maryland broadleaf variety, started on October 

 i3» 19171 gave practically the same results; root examination showed 

 the greatest infection with sand, less with an admixture of one-fourth 

 clay, and almost none with one-half clay, and still less with larger 

 amounts of clay. However, in one pot heavy infection occurred at the 

 base of one plant, and the results were interfered with somewhat by 

 nematode injury. 



All the soils were then resterilized with steam and two pots of each 

 series heavily infested with a suspension of young endoconidia of T. 

 basicola from agar slants. White Burley was again replanted into all 

 the crocks. The results obtained in this case differed somewhat from 

 the preceding, owing most probably to heavier infestation, infection 

 seemed to occur most rapidly and severely in the one-half and three- 

 fourths clay mixtures, but in a few days the plants in the infested series 

 were practically identical in appearance, and after about four weeks, 

 nearly all were killed. 



These were now removed, and a more resistant variety, Connecticut 

 Havana, transplanted into the pots. After 18 days all these also were 

 practically equally diseased in the infested series, and were not quite 

 half the size of those in the uninfested series. After about six weeks' 

 growth these plants were cut and the air-dry weights determined as 

 shown in Table IX, Experiment IV. The pots were again planted to 

 tobacco with similar results (Table IX, Experiment V). The results of 

 the last experiments seem to indicate that in the presence of heavy infesta- 

 tion of the soil very little difference exists between clay and sand mix- 

 tures in the severity of infection of tobacco by T. basicola. 



The results obtained in the first experiments are believed to be due to 

 the fact that the parasite found clay soils unfavorable for growth and 

 multiplication, and especially for penetration of mycelium as compared 

 with the sand, and therefore less actual infection occurred. On the 

 other hand, with heavy infestation sufficient spores were in close prox- 

 imity to the roots to produce good infection at once. 



With respect to the persistence of T. basicola in soil, and its gradually 

 increasing infestation in spite of unfavorable conditions, it is believed 

 that clay soils may eventually be more injurious than sandy soils, but 

 the results seem to justify the conclusion that from the standpoint of 

 texture alone, the selection of loose sandy soils, or the use of clay soils 

 does not necessarily predetermine to any important degree what the 

 injury from T. basicola will be. It should be added that clay soils 

 draining more poorly and warming up more slowly, undoubtedly may 

 be considerably more haraiful than sandy soils, because of the influence 

 of saturated soils and low temperature upon the severity of the rootrot. 

 It is also believed that the tendency of the clay soils toward greater 

 compactness may also favor somewhat the occurrence of the disease. 



