42 ROOT-KNOT AND ITS CONTROL. 



MOISTURE. 



A certain degree of moisture is necessary for the maintenance of the 

 life of the nematode in the soil. Experiments by the writer, Frank/ 

 and others have shown that the larvae of the root-knot nematode, 

 unlike those of many other nematodes, are destroyed by being dried 

 in the laboratory. Observations by the writer in New Mexico, Ari- 

 zona, and California confirm this abundantly, for in those communi- 

 ties the root-knot is practically confined to the irrigated land. This 

 does not mean that the soil must be wet, for that is not necessary. 

 The soil, however, must have sufficient moisture in it to be properly 

 called a moist soil, though not enough to fill the air spaces and inter- 

 fere with proper aeration. Thus, we have reports from South Africa,^ 

 Argentina,^ and Chile * which state that the nematodes grow only in 

 wet soils. This, in the light of conditions existing in America, evi- 

 dently means not what we would call wet, but merely moist, in the 

 eastern and southern part of the United States, but what many people 

 in irrigated districts would not hesitate to call w^et in contradistinc- 

 tion to the dry, unirrigated soils. Prof. P. H, Rolfs,^ Dr. N. A. Cobb, 

 and others report experiments which would seem to prove that dry- 

 ing of nematode-containing soil does not entirely kill out the Hetero- 

 dera radicicola. This will be discussed more in detail later. 



On the other hand, soils that are water-logged for a considerable 

 part of each year are usually free from the trouble. Some observa- 

 tions on the effects of floods on nematodes led the writer to believe 

 that flooding for a few days would destroy them, but field experiments 

 in Arizona and California showed that keeping the soil submerged for 

 five days was not sufficient to kill out the nematodes, at least not 

 those inclosed within the root galls of the trees and vines growing in 

 the fields. Yet it is certain that very wet soils are free where this is 

 long continued, and long periods of flooding kill out the nematodes. 

 Thus, in the Everglades of southern Florida there occur islands, parts 

 of which are never flooded and parts of which are out of the water 

 ordinarily, but submerged for two to six months of the ye&r. Truck 

 growers occupy some of these islands and find that the root-knot 

 nematode is abundant above the high-water level — i. e., where the 

 land is never flooded, but absent in the zone that is flooded every year. 



TEMPERATURE. 



As long as the soil is not too dry, the higher the temperature the 

 more actively the nematodes seem to develop. On the other hand, 

 they seem to become practically inactive when the soil temperature 

 falls below 50° F. Yet they are capable of remaining alive when 



1 Frank, 188.->. 2 Loiinsbury, 1904. « Huergo, 1902, 1906. < Lavergne, 1901. ' Rolfs, 1894. 

 217 



