CONTROL OF ROOT-KNOT, 63 



were permitted and egg laying were started the number of nematodes 

 would be increased instead of diminished. Probably such large num- 

 bers were present that only a part entered the trap plants and were 

 destroyed, enough remaining in the soil to infest badly the next year's 

 crop. It is possible that some other crop would have done better, but 

 it could not have been clover, as Frank suggested, for that did 

 not do well where the experiments were being carried on. The 

 requisites of a good trap plant are fairly cheap seed, great susceptibility 

 to nematode attacks, a wide-spreading root system, and rapid growth. 

 All these are possessed by the cowpea to a greater or less extent. 



STEAM . 



It has been seriously proposed to use steam to destroy nematodes 

 in the field in view of the success with its use in the greenhouse, cold 

 frame, and seed bed. The writer has made no experiments along this 

 line, owing to the expense of the undertaking. It is seriously to be 

 doubted whether a large field, producing a crop selling at $25 to $50 

 or even $100 net per acre, could be profitably piped for steam sterili- 

 zation. Small fields isolated from danger of reinfection by deep 

 ditches, water, stiff soil, or other obstacles and devoted to the inten- 

 siv(! culture of some very remunerative crop might be so treated with 

 profit. For a large field a very large boiler and many hundred feet 

 of perforated pipe would be necessary to steam the soil by the green- 

 house method. 



Several schemes for sterilizing the soil in a field by means of mov- 

 able apparatus have been devised, some of which have proved 

 effective under certain conditions. Thus, for combating the Thielavia 

 root-rot of tobacco, Gilbert ^ recommends the inverted-pan method 

 of steam sterilization. This was devised by Mr. A. D. Shamel, of 

 the Bureau of Plant Industry, for sterilizing nematode-infested soils 

 in Florida. The following description is taken from Gilbert's account : 



The apparatus consists of a galvanized-iron pan, 6 by 10 feet and 6 inches deep, 

 which is inverted over the soil to be sterilized and the steam admitted under pressure. 

 The pan is supplied with steam hose connections, has sharp edges, which are forced 

 into the soil on all sides to prevent the escape of steam, and is fitted with handles for 

 moving it from place to place, the weight of the entire pan being not over 400 pounds. 

 The soil is prepared as in the greenhouse method, a few potatoes being buried at a 

 depth of a foot to gauge the degree of heat attained. A soil thermometer may also be 

 used if desired. The steam should be kept at as high a degree of pressure as possible, 

 80 to 100 pounds being best, and the treatment should continue for one to two hours, 

 depending on the pressure maintained. In experiments conducted in the spring of 

 1907, one hour's steaming at 80° C. under 100 pounds pressure gave best results in 

 killing both the fungus and the weed seeds. WTien one section of the bed is treated, 

 the pan is lifted and carried to an unsterilized portion and the operation repeated 

 until the entire bed is steamed. 



1 Gilbert, 1909, pp. 35-36. 

 91294°— Bul. 217—11 5 



