4 MISC. PUBLICATION 4 6, U. S. DEPT. OF AGRICULTURE 



only an occasional trace of infestation; moderately resistant plants 

 usually support some nematodes, though seldom great numbers of 

 them. However, under conditions either unfavorable to the growth 

 of the plant or else unduly favorable to the nematodes, especially in 

 the greenhouse, where much experimental work is done, resistance 

 may break down entirely. There is nothing in the conception of 

 resistance to imply that the plant will then escape the usual injuries of 

 a heavy infestation. 



The causes of resistance are imperfectly understood. They may 

 be physical, chemical, or physiological. They probably differ with 

 the type of plant. The theory of resistance based on lack of attrac- 

 tiveness is disproved for certain plants by the recent work of Linford 

 (14®)- The theory that larvae are unable to penetrate resistant roots 

 is disputed by the recent work of Barrons (13), at least for the condi- 

 tions of his experiment; preliminary investigations by the Division of 

 Nematology may have a similar implication. .AH of these studies 

 challenge further fundamental investigations on the nature of 

 resistance. 



Lack of gall formation following nematode invasion is no criterion of 

 resistance. Although certain grasses and other resistant plants form 

 only very small swellings and harbor only a few nematodes, it cannot 

 be said that they are successfully obstructing the invasion when those 

 nematodes are able to develop and reproduce. Moreover, lack of gall 

 formation may be observed also in certain susceptible plants. The 

 roots of freesia and of cyclamen, for example {221), do not react to this 

 parasite with the usual hypertrophy of cortical tissue and, perhaps 

 even for this reason, both of these plants suffer severely from a very 

 few nematode parasites. The opposite case of large galls serving as 

 sap reservoirs for plants in the Sahara Desert (245) has been quoted 

 many times. This is not resistance, though it might be called a local 

 and accidental sort of tolerance. 



Immunity is freedom or exemption from disease — in this case, 

 complete freedom from nematode infestation. It is possible that in 

 the future this conception may be modified to include plants in which 

 the nematode, though it may occasionally penetrate a rootlet, is 

 never under any conditions able to complete its development. The 

 word immunity has often been used loosely, meaning sometimes 

 resistance and sometimes only tolerance or profitable growth, but 

 promising too much in either sense. Complete immunity is exceedingly 

 rare. 



The above distinctions in meaning may seem unnecessary to a 

 grower whose needs can be satisfied by any plant that will thrive in 

 his infested ground. However, they become exceedingly important 

 when resistant plants are to be selected for the purpose of controlling 

 root knot by starvation. Unless marked as quotations, these words 

 are used in accordance with the definitions, except that it is necessary 

 to quote an author's exact words when he gives no explanation of his 

 meaning, even though it seems probable that his meaning was not 

 that of the definitions. 



Basis for Selection of Material 



Because of the number and diversity of reports on resistance and 

 other relationships of plants to root knot infestation, it is necessary 



