CONDITIONS FAVORING ROOT-KNOT. 41 



tinction between the root-knot nematode and the true sugar-beet 

 nematode (Ileterodera schachtii), for the latter's presence not only 

 reduces the size of the affected beets, but also greatly reduces their 

 sugar content and usually lowers also the purity. 



The greatest depth at which Frank observed nematode galls was 

 33 centimeters (about 13 inches). On the other hand, the writer 

 finds that they may occur more than a yard below the surface of the 

 soil. To be sure, these are only scattering galls, for the great major- 

 ity of the nematode galls occur in the first foot of the soil. Indeed, 

 in practical culture it has been found that if trees can be forced to 

 root extensively at a depth of 16 inches or more they suffer but 

 little from root-knot^ 



CONDITIONS FAVORING ROOT-KNOT. 

 SOIL. 



Root-knot is essentially a disease of light soils. Wherever the 

 soil is sandy or contains a fairly large proportion of sand, other con- 

 ditions being favorable, the root-knot nematode may be expected 

 to thrive when once introduced. In heavy soils, on the other hand, 

 the disease seems never to be serious. In some of the writer's 

 experiments affected plants were planted in pots of stiff clay soils, 

 and not only was it almost impossible to obtain infection of sus- 

 ceptible plants placed in close proximity in the same pots, but even 

 on the diseased plants the new roots remained free from the trouble. 

 Similar experiences have been reported to the writer from various 

 parts of the country where diseased trees were set out in stiff' soil 

 and after a few years seemed to be entirely free from the trouble. 

 Contradictory statements sometimes find their way into print, but 

 they are explicable in most cases when one understands the great 

 popular confusion in the use of the words "heavy," "stiff," and 

 "light" as applied to soils. Thus, in parts of Florida and South 

 Carolina a very sandy, yellow soil containing only enough clay to 

 hold it together while moist, is called "clay" or "heavy soil." It 

 is clayey, to be sure, compared with some of the soils thereabouts, 

 for sometimes the latter are almost pure sand. "Light" and 

 "heavy" in the sense used in this bulletin have reference to those 

 soils containing, respectively, little and much clay. Soils that dry 

 out rather quickly, that do not cake hard on drying, and that are 

 easily crumbled to a fine granular mass are favorable to these nema- 

 todes, while the reverse is the case for the difficultly permeable, 

 hard-caking, clayey soils. This applies only to the root-knot nema- 

 todes, as the writer's investigations have not gone into this point 

 with reference to other sorts. It is known that the sugar-beet 

 nematode will thrive in some of the heavier as well as in light soils. 



217 



