64 ROOT-KNOT AND ITS CONTROL. 



The great objection to this method, and one that makes it imprac- 

 ticable except for use on small spots, is the smallness of the area that 

 can be treated at one time. Even with a pan of twice the area of that 

 described, and allowing only one hour's sterilization each time, it 

 would require more than 15 days, working day and night, to sterilize 

 the soil on one acre of land. Furthermore, for deep soils, where, as 

 already explained, the nematode sometimes is present at a depth of 

 more than a yard, it is extremely doubtful whether the steam would 

 penetrate deeply enough to destroy all the nematodes. This last 

 objection applies to all methods of sterilization where an attempt is 

 made to kill the nematode by heat or poisons. 



FALLOW. 



It is self-evident that if a field be kept free from all vegetation for a 

 long enough period all the plant-parasitic nematodes within the soil 

 will die from starvation. This is the principle involved in the use of 

 the bare fallow. The field is plowed and kept free from weeds and 

 other plants by frequent cultivation. In those localities where the 

 winter is cold enough to prevent the further development of the 

 nematodes during that period, it does no harm if grass or weeds grow 

 up after the weather has become decidedly cool. This date might 

 safely be put at November 1 for North Carolina, South Carolina, 

 northern Georgia, Alabama, Mississippi, northern Louisiana, and 

 northern Texas. In central and southern Florida and probably the 

 southern portion of Texas and Louisiana, however, the nematode is 

 active the year around, so that it would be necessary to keep the 

 ground bare the whole time until the nematodes had died. In the 

 early spring, where vegetation was allowed to grow in the winter, 

 the cultivating to keep down the weeds must be taken up again before 

 the soil begins to warm up. The length of time necessary to remain 

 in fallow is not certainly known. Mr. A. D. Jackson, of Denison, 

 Tex., found that 15 months in fallow was not sufficient to rid a field 

 of root-knot nematodes entirely, although the number was greatly 

 diminished. On the other hand, two whole years seem to be amply 

 sufficient. 



This method has some objections which make it impossible to use 

 in some localities. The land is idle and not only not productive, but 

 requires the expenditure of time and labor to keep the vegetation 

 down. Furthermore, the light soils where the nematodes abound 

 are easily leached out when there is not a covering of vegetation. 

 Then, such soils are subject to bad washing during heavy rains when 

 they have no plant roots to bind them in place. A further objection 

 is the destruction of humus in the soil exposed directly to the action 

 of the fierce summer sun. The use of this method therefore can not 

 be universal, although it is successful where it can be put into effect. 



217 



