6 



The extensive experiments of Garner and his associates show 

 that the tobacco plant is particularly sensitive to the effects of cer- 

 tain preceding crops used as winter soiling crops or in rotation with 

 tobacco. According to their results, the limited use of soil-im- 

 proving crops under favorable circumstances may prove profitable, 

 but conditions may easily arise where their use is positively detri- 

 mental to tobacco. Similar effects are shown to occur on potatoes 

 and corn following certain legumes and nonlegumes in rotation, 

 with the exception that corn and small grains have been generally 

 benefited by legumes in rotation. The evidence presented that these 

 results can not be attributed to any known soil-borne disease or to 

 plant-food relations is convincing. 



Mention should be made of other workers who have noted similar 

 results with tobacco and other crops. Among these, Bedford and 

 Pickering (i), Howard and Howard (5), and Hartwell and his 

 associates (tf), particularly, have shown cases of the harmful effect 

 of one crop upon another where the effect was presumably not patho- 

 logical in its nature. There is no general agreement among these 

 investigators, however, as to the nature of the deleterious agency 

 in these crop effects. 



ISOLATIONS FROM DISEASED ROOTS 



It is not proposed to present in detail the results of a large number 

 of isolations made from diseased roots and the inoculations per- 

 formed with the organisms obtained, since the results on the whole 

 are inconclusive, if not entirely negative. 



Most of the isolations have been made from plants grown in 

 the greenhouse in affected soil, although many of the specimens 

 used for that purpose came directly from the field. Small pieces 

 of the roots with lesions in varying stages of development were 

 washed in several changes of sterile water and plated out on potato 

 agar, usually acidified with lactic acid when fungous development 

 was desired. 



The platings have been made mainly from tobacco grown in soil 

 from two different fields in the Connecticut Valley and from one 

 field in Maryland. 



One series of records of these isolations shows that 505 frag- 

 ments were plated out and that growth of a species of Fusarium 

 was obtained from 329 fragments, or 65 per cent of the lesions. 

 The abundant occurrence of this fungus led to repeated attempts 

 to secure infection with this organism by reinoculating healthy soil 

 or by placing the fungus directly on roots growing in a special 

 moist chamber. In isolated cases some indication of infection was 

 found, but the negative evidence is so preponderant that it seems 

 highly improbable that Fusarium is the causal organism concerned 

 with the disease in question. 



Species of Fusarium are well-known soil saprophytes, and their 

 presence in decaying roots or even on healthy roots is to be expected. 

 This relationship seems to be well demonstrated in a series of isola- 

 tions made from roots of 7 different crop plants regarded as suscepti- 

 ble to brown root rot and 7 believed to be immune to it. Out of 

 132 fragments from susceptible roots, 126 yielded Fusarium, and 98 

 fragments from the immune roots yielded 60 with Fusarium. Since 



