22 



ence of spores of TMelavia basicola, whereas these are absent in 

 brown root-rot cases unless the two diseases occur together. The 

 stunting of crops as a result of brown root-rot is of course frequently 

 laid to unfavorable conditions of the soil or to weather conditions, 

 but repeated experiments have shown that some parasite is probably 

 concerned. 



Cause. — The cause of brown root-rot is not yet definitely estab- 

 lished. If soils producing this disease are sterilized with heat or 

 formaldehyde, the disease is completely controlled. This, together 

 with other facts, practically demonstrates that some organism is the 

 cause of this disease. Species of fungi, especially Fusarium, 

 Rhizoctonia, and Actinomyces, as well as other fungi and bacteria, 

 are commonly present in the diseased roots, but none of these has yet 

 been definitely shown to be primarily concerned with the produc- 

 tion of the disease. This same disease attacks a large number of 

 other plants, such as tomatoes, potatoes, and other closely related 

 plants, as well as many leguminous plants. The relation of the 

 disease to grasses and grains is not yet fully understood. The dis- 

 ease is common on sod land the first year to tobacco, however, indi- 

 cating that the- cause may have some connection with such crops. 

 The causal agent persists in the soil from year to year, although it 

 seems that under unfavorable conditions, such as excessive drying or 

 absence of host plants, it may die out with relative rapidity. 



Conditions favoring the disease. — Conditions favoring this dis- 

 ease are much the same as for black root-rot; that is, primarily, 

 relatively cool weather (PI. VI, fig. 2). Dry hot weather tends to 

 exaggerate the symptoms of the disease in the abovegrouncl parts 

 of the plant, but rains following such a period permit considerable 

 recovery. 



Control. — The losses from this disease are as far as known greatest 

 in the Connecticut Valley. The high cost of production of shade- 

 grown and other primed tobacco especially tends to make even low 

 percentages of reduction in }ueld an important consideration. On the 

 other hand, the tendency of crops to recover or land to " come back " 

 from this disease naturally induces growers to take risks more often 

 in planting infested fields. The experimental evidence on the rela- 

 tion of the crop-rotation system to* this disease is as yet incomplete. 

 Drying or aeration of the soil is favorable to its recovery from this 

 trouble. 



There seems to- be no difference in varietal resistance to brown 

 root-rot. 



ROOT-KNOT. 



Description. — The root-knot disease is sometimes known as root- 

 gall or big-root. This malady is quite common in the Southern 

 States on various plants, including tobacco, but it is practically 

 unknown in the Northern States. The disease proper is confined 

 to the root system. The aboveground symptoms of the disease are 

 stunting or dwarfing of the plant, accompanied by wilting, espe- 

 cially in dry hot weather. These signs of the disease are conse- 

 quently very similar to injury produced by other root diseases or 

 by excessively dry weather. If the plants are pulled out of the 

 around, however, (he roots will be found to be more or less enlarged 



