44 ROOT-KNOT AND ITS CONTROL. 



tions occur more freely the warmer the weather, except where lack of 

 rain permits the soil to dry out, in which case both plants and 

 nematodes cease to thrive. 



CONTROL OF ROOT-KNOT. 



The problem of the control of root-knot is one that varies much 

 according to the place infested, the kind of plants grown, the methods 

 of culture followed, etc. We may distinguish between small, inten- 

 sively cultivated lots of soil, such as we find in greenhouses, hotbeds, 

 and seed beds, and field culture. Each group may be subdivided in 

 accordance with the answer to the question whether the crops are 

 annual or long lived. For the first great division, owing to the value 

 of the crops raised and the amount of capital invested, methods of 

 combating a disease may be used that would be barred from field 

 crops or other crops on larger areas of land, because the expense would 

 not be justified in view of the comparatively low earning power of 

 the land. Furthermore, the actual monetary loss to the crop due to 

 a given disease may be far greater in the restricted areas of intensive 

 culture than in large fields where each plant is of relatively less 

 value. So, for example, root-knot may affect a field of cowpeas 

 and actually reduce the crop one-half, but unless the field were very 

 large that might not equal the loss sustained by a grower of cucumbers, 

 lettuce, or tomatoes whose whole greenhouse crop has been totally 

 destroyed by the same pest. 



GREENHOUSES, SEED BEDS, ETC. 



LIVE STEAM. 



Probably the most satisfactory method for destroying the root- 

 knot in greenhouses and seed beds is the use of live steam under 

 considerable pressure. Tliis has been advocated by various persons, 

 viz. May, Galloway, Selby, and Rudd,^ but it was as a result of care- 

 ful experiments by Stone and Smith - that it became generally used. 

 The method recommended by them is a modification of that recom- 

 mended by Galloway and others. The scheme is essentially as fol- 

 lows: At the bottom of the bench or bed are laid either iron pipes 

 perforated with ^-inch holes every few inches or drain tiles. Live 

 steam is passed into these and escaping from the holes of the iron pipes 

 or between the ends of adjacent tiles heats the soil to such a degree 

 that all animals and most plants (except, of course, bacterial spores) 

 are killed. The pipes must be placed at intervals short enough to 

 permit the spaces between the rows of piping to be thoroughly per- 

 meated by the steam. This distance varies with the soil, but 12 



» May, 1896; Galloway, 1897; Rudd, 1893; Selby, 1896. » Stone and Smith, 1898. 



217 



