TOBACCO DISEASES AND THEIR CONTROL. 



23 



or swollen in sections, presenting a very irregular appearance in size 

 and shape (fig. 15). This deformity is produced by the activity of 

 the parasite concerned and results both in lessened feeding area of 

 the root system as well as interference with the proper transfer of 

 water and food to the plant. The number, size, and extent of the 

 development of the enlargements on the roots will usually be propor- 

 tional to the extent of infestation in the soil and other conditions 

 favorable to* the disease. 



Cause. — Boot-knot is caused by nematodes, or eelworms, which are 

 very small animals, often barely visible to the naked eye, fairly 

 abundant in nature. Fortunately, only a few species are commonly 

 parasitic on plants. The common root-knot disease is due to the 

 nematode Heterodera radicicola. The tissue of the roots attacked 

 by this parasite becomes stimulated to excessive growth in diameter, 









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-Si 





fihfe . 



\, 



s^i 





fccsLi, 





llai&&>,\ 



M 





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- a/ '^mSBBSBSt 





Y ^drSEl 













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f / 



Fig. 15. — Root-knot of tobacco. 



The enlarged roots produced by nematodes are typical 

 of this disease. 



thus causing the enlargements. This nematode also attacks a large 

 number of other plants, but corn, grains, and grasses as a class are 

 not susceptible; neither are peanuts, sorghum, velvetbeans, or re- 

 sistant cowpeas (Iron or Brabham). 



The organism lives over in the soil from j^ear to year and can 

 apparently survive for at least three years, even in the absence of its 

 host plants. It can, of course, be spread from field to field in the 

 same manner as any other soil-infesting parasite. 



Conditions favoring the disease. — Boot-knot is usually most serious 

 in light sandy soils. Heavy, clayey, or marshy soils are not favor- 

 able to its existence. High soil temperatures are favorable to its 

 development, and overwintering in soil seems to be dependent on 

 mild weather, such as exists in the more southern States. Moist 

 soils are more favorable for the development and parasitism of the 

 nematode than excessively dry or wet soils. Soils low in fertility 

 appear to be more seriously affected than soils properly fertilized. 



