22 ROOT-KNOT AND ITS CONTROL. 



ditions they might have shown root-knot. However, it is probable 

 that the above-named phmts will show themselves nematode resistant 

 in most cases. 



CROSS-INOCULATION EXPERIMENTS. 



It has been suggested by several investigators that Heterodera 

 radicicola, like Tylenchus dipsaci, may show the development of 

 strains preferring certain hosts and exhibiting a reluctance to attack 

 others, although these different strains are morphologically indis- 

 tinguishable.^ This explanation has been suggested for the fact 

 recorded by Stone and Smith ^ that lettuce often is not attacked in 

 beds in greenhouses where other crops suffer great injury. The 

 writer accordingly made a number of cross-inoculation experiments 

 to determine, if possible, to what extent the nematodes of certain 

 generally grown crops were interchangeable. The experiments 

 were performed as follows : Pots of soil were sterilized in an autoclave 

 for about an hour and a half, sometimes longer, at a temperature of 

 125° C. While this was perhaps not long enough to kill all bacterial 

 spores in the center of the pots, the temperature attained showed 

 itself to have been high enough to kill all nematode larvae or eggs. 

 In the sterilized soil were placed affected roots of the i)lant used as a 

 source of the nematodes. These roots were first carefidly washed 

 (sometimes in water containing a small amount of formaldehyde) to 

 remove all adhering dirt in wliich conceivably larvte or eggs of other 

 strains of nematodes might be present. These pots were planted with 

 seeds of plants to be tested as possible hosts of the nematode, either 

 at the same time or a few days after the roots were put into the pots. 

 Except when it was certain that the water was nematode free, it was 

 boiled and cooled before using it to water the pots. Experiments 

 made in this manner showed that the root-knot nematodes were 

 mutually interchangeable in the following plants: Red clover {Tri- 

 folium pratense; PI. Ill, fig. 2), white clover (J", repens), crimson 

 clover {T. incarnatum), cowj^ea (Vigna unguiculata) , strawberry 

 (Fragaria chiloensis), tree morning-glory (Ipomoea syringaefolia), 

 sunflower (Ilelianthus delilis), horse bean (Vicia faha), ginseng 

 (Panaxquinquefolium), purslane (Portulacaoleracea), fig (Ficus carica), 

 papaya {Carica papaya), catalpa (Catalpa speciosa), tomato {Lyco- 

 persicon esculentum) , and Old World grape (Vitis vinifera). These 

 all also affect the following, for which the reverse inoculation experi- 

 ments were not made: Lettuce (Vaduca sativa), green gram (PTiase- 

 olus radiatus), tobacco (Nicotiana tahacum), squash (Cucurhita 

 moschata) , cucumber (Cucumis sativus), and muskmelon (C. melo). 



1 Prof. J. Ritzema Bos (1900) reports that Tylenchus dipsaci becomes so adapted to a host plant after 

 growing on that species only for several generations that it will not attack with any severity the species 

 upon which it grew before until several generations have passed. 



2 Stone and Smith, 1898, p. 30. 



217 



