CONTROL OF ROOT-KNOT. 61 



a mistake of this nature, performing the experiment as he did. Still 

 it is not certain that he had Heterodera radidcola unless he actually 

 had the mature nematodes, but on this point he says nothing. There 

 are some other nematodes besides this species that cause root galls, 

 and it is barely possible tJiat it may liave been one of these, not the 

 root-knot nematode that Prof. Rolfs liad, since this latter species is 

 rarely even partially external in the tomato. Yet with the confirma- 

 tion of these reports by Dr. Cobb's observations, it can hardly be 

 doubted that under some circumstances some of the root-knot 

 nematodes may survive drying out of the soil. 



Whether the drying out of the soil kills all the root-knot larvae or 

 not, there is no doubt that their activity ceases and there is no injury 

 by them in fairly dry soils. In a letter to the wTiter, C. P. Lounsbury, 

 entomologist of the Department of Agriculture of the Cape of Good 

 Hope, states that the nematode occurs only in loose soils well sup- 

 plied with moisture. Badly knotted grapevines set out in rather dry 

 soil not only recovered, at least in part, but the nematodes did not 

 spread to surrounding susceptible plants. Lavergne ^ in Chile, 

 Gandara ^ in Mexico, and Huergo ^ in Argentina also point out that 

 dry soils are unfavorable to the development of root-knot. The 

 writer has repeatedly sought for these nematodes in susceptible plants 

 in dry soil outside of but in close proximity to badly infested irrigated 

 fields in the semiarid parts of the country, but without success. 



In view of the foregoing facts, it is probable that deep plowing, so as 

 to loosen up the soil quite deeply without harrowing to pulverize it, 

 would permit it to dry out suiliciently in a dr}^ season to reduce 

 greatly the injury from the pest. Of course, this is possible only 

 where the climate is drv and the rainfall slight. In irrigated districts 

 it could probably be carried on, such fields not being irrigated for 

 some months after plowing. Of course this wiU not have much effect 

 if underground seepage or rains keep the soil moist. Unfortunately 

 the WTiter was unable to test the eflicacy of this proposed method by 

 direct experunent. It is a method that should be tested at the earliest 

 opportunity in those regions where it can be carried out. 



. TRAP CROPS. 



After Kiihn, the great German agriculturist, had demonstrated * 

 that the so-called Riibenmiicligkeit (beet tiredness) of sugar-beet 

 fields w^as due to a nematode, Heterodera scJiacJitii, he devised ^ a 

 method of reducing the injury based upon the principle of trapping the 

 nematodes in some susceptible plant and destroying the latter before 

 the larvae wiiich had entered the roots had reached maturity. For his 

 trap crop he used a sort of sunmier rape. This was sown closely and 



' Lavergne, 1901. s GAndara, 1906. 3 Huergo, 1902, 1906. * Kuhn and Liebscher, 1880. 



» Kiihn, 1881, 1882, 1886-1, 1886-2, 1891. 

 217 



