420 STATE BOARD OF AGRICULTURE. 



value in the case of some rapidly growinj^ crop, which could be matured 

 and renu)ved before the nematodes again became numerous enough to 

 materially injure the plants. In the case of a slow growing crop like 

 ginseng, which remains in the soil from four to six years, any treatment 

 which falls short of complete eradication is worthless, becau.se a few 

 nematodes left in the soil would completely restock it with their kind. 

 In the course of a year or two they would be as serious as they were be- 

 fore the treatment was applied. In the light of these experiments, no 

 chemical used could be recommended for the control of root-knot nema- 

 todes; furthermore, no exact explanation could be given as to just why 

 the treatments did not eradicate the pest. 



THEORIES AS TO THE FAILURE OF THE FIELD TREATMENTS. 



Various theories to explain the failure of the treatments were formu- 

 lated, but none of them were backed by experimental data. It was 

 thought possible that occasional nematodes more resistant than the aver- 

 age, were able to withstand the adverse conditions and that these resist- 

 ant ones in time succeeded in restocking the soil, when conditions again 

 became favorable for their growth and reproduction. A second theory 

 was that nematodes deep down in the soil, where the treatments ex- 

 erted little or no influence, came up and repopulated the treated soil 

 after the effects of the treatments had sufficiently diminished. Another 

 possibility was that the nematode eggs in the soil were of such a nature 

 that they were able to resist the adverse conditions and remain dormant 

 until favorable conditions allowed the young to come out into suitable 

 environment for their development. As the second of the above theories 

 did not appear to be of a nature to lend itself readily to investigation 

 in the laboratory, it was decided to turn attention to the other two in 

 an effort to determine what relation they might have to the failure of the 

 soil treatments. 



LABORATORY EXPERIMEiNTS AT MADISOX, WISCONSIN. 



Early in October 1918, the experiments at the field laboratory Avere 

 closed for the season, and laboratory work on various phases of the sub- 

 ject was begun in the greenhouses and laboratories of the Department 

 of Plant Pathology of the University of Wisconsin. For some little time 

 after arriving at Wiscon.sin, efforts were confined to fitting up experi- 

 ments in the greenhouses, and to getting susceptible plants started so 

 that there might be an abundant supply of nematodes to draw from for 

 laboratory work. Considerable time was spent in various forms of 

 laboratory experiments before suitable technique was worked out. An 

 effort was made to get larvae free from the plant tissues by externally 

 sterilizing knots from lettuce roots, and then placing them on plates of 

 sterile agar. In no case did the writer succeed in obtaining any larvae 

 free from the knots in this way. Failing in the above method, attempts 

 were made to get the larvae by dissecting quantities of root-knots from 

 lettuce and other plants. It was impossible in this way to get the larvae 

 in sufficient numbers for the treatments it was desired to make. 



Next Avas tried removing the roundish, pearly white female nematodes 

 from small knots on the roots of various plants. It was thought that 

 these females might produce eggs and lan^ae after being transferred to 



