CROPPINCx SYSTEMS FOR TOBACCO 11 



suggested that the root-rots have been the main cause of the diminished level of 

 yields of the past decade or two. The cause and remedy of black root-rot of to- 

 bacco are now pretty well understood. However, little is known about the cause 

 and remedy of the so-called brown root-rot. 



There are two schools of opinion regarding this disease or condition. The one 

 claims that it is caused by a specific parasite which attacks the root; the other is of 

 the opinion that a soil condition is the cause. The soil condition may be that of a 

 deficiency of a given nutrient or special form of it, or the presence of a plant to-xin. 



One of the great difficulties encountered in the study of the nature and cause of 

 brown root-rot is the lack of definition of the disorder and the Inability to detect 

 its presence with certainty. It appears likely that there may be several possible 

 causes of brown roots of tobacco. For example, Jones (1-1) has shown that soluble 

 aluminum, and Beaumont (5) that ammonium compounds, may cause the roots of 

 tobacco to turn brown or dark in solutions. Doran (8) has shown that timothy 

 infusions in certain stages of decomposition may cause a brown root-rot. More- 

 over, it appears that normal, healthy tobacco roots may become slightly brown 

 without deleterious effects on growth. A systematic examination of the roots of 

 tobacco grown in this experiment showed that what would ordinarily be diagnosed 

 as brown root-rot was nearly always present to some extent in the tobacco regard- 

 less of treatment. Tobacco grown in the animal-husbandry rotation showed the 

 highest percentage of infection, but it was relatively low for tobacco grown con- 

 tinuously with no cover. In the rotation including clover there was a higher 

 percentage of brown roots than in that with timothy. Brown roots were found on 

 48 per cent of the plants grown with red top and rye covers and on 60 per cent of 

 those with timothy cover. These figures should be considered as estimates only, 

 because of the difficulty of diagnosing the conditions. The results of root examin- 

 ations are summarized in Table 4. 



Table 4 — Percentage of roB.\cco Roots Showing PreseiNxe of Root-rots* 



(Average for 1925-1931) 



Black Brown 



root-rot root-rot 



Continuous Tobacco: 



No cover (check) 17 27 



Manure 19 29 



Timothy cover (old series) 5 60 



Red top cover 6 48 



Rye cover 11 52 



Rotation Tobacco: 



With corn and timothy 84 



With corn and clover 100 



With potatoes and onions 9 55 



*A11 roots showing any appreciable ninoiint of root-rot were counted. The above figures indicate 

 the average fiercentage of infected roots, but not degree of infection. 



In severe cases of brown root-rot, portions of the (obacco plant above the ground 

 show characteristic symptoms. From two to three weeks after growth starts 

 the plants begin to lag behind unaffected plants in growth. The leaves develop 

 a darker green color than is shown by normal plants. The lower leaves develop 

 slowly compared to the new growth, and the plant appears spindling or slender. 



