THE BROWN EOOT ROT OF TOBACCO AND OTHER PLANTS 



13 



readily do so on the basis of the examination of the root system. 

 Only very questionable root lesions have been found on corn. 



Cabbage, onions, beets, pumpkins, cucumbers, and lettuce appar- 

 ently are practically, if not entirely, resistant to any injurious action 

 from brown root-rot soil. 



There is room for considerably more experimental evidence along 

 the line of host range, but the work was discontinued largely be- 

 cause of the fact that this information was not leading to any im- 

 portant conclusion relative to either the cause or control of brown 

 root rot; that is, it has not been evident from subsequent experi- 

 mental work that the occurrence of root lesions bore any direct rela- 

 tion to the increase or overwintering of the disease in the soil. 

 It can not yet be concluded that the disease has its origin with any 

 particular natural or cultivated vegetation, although, as shown later, 

 it is apparently accentuated by some crops more than by others. 



Fig. 11. — Growth of corn (1), tobacco (2), tomatoes (3). and peas (4) in st^am- 

 sterilized (A) and unsterilized (B) brown root-rot soil. These soils are some- 

 times extremely detrimental to the growth of various crops, although the injurious 

 agent is not definitely known to be the same on all hosts. Steam sterilization has 

 also some beneficial effect on soil productiveness 



Brown root rot, however, appears to be associated with the character 

 of the soil itself as well as with its distant cropping history. 



VARIETY AND STRAIN TESTS WITH TOBACCO 



In the case of black root rot of tobacco it has been shown that very 

 decided differences in resistance to disease occur (10). Eight of the 

 more typically resistant and susceptible strains were planted on a 

 brown root rot field at Whately, Mass., in 1918. The results were 

 negative to a striking degree. (Fig. 12.) Apparently, the Thie- 

 lavia-resistant varieties succumbed as readily as the other varieties 

 to the brown root rot. Similar though less striking results were 

 secured on other plats in the same year, and where difference in re- 

 sistance occurred it was found to be attributable to soil infested with 

 Thielavia (black root rot). 



Brown root-rot fields, however, often show an unevenness of growth 

 suggesting individual differences in the resistance of plants. (Fig. 

 13.) Several of these outstanding plants were selected from " sick " 

 soils and the seed planted the following year in progeny rows, but 

 the results were at once discouraging. The writers are strongly in- 

 clined to believe, therefore, that no appreciable variation in resist- 

 ance to brown root rot occurs in varieties of tobacco (Nicotiana 



