In order to supply background material for evaluating the most 

 varied interpretations of resistance and also to be useful in the greatest 

 possible number of practical problems, the subject is interpreted 

 rather broadly. A number of definitely susceptible crop plants and 

 ornamentals are included because they have at some time been called 

 "resistant"; it is quite possible that a useful degree of tolerance may 

 be found in some of these plants. A few weeds and certain other 

 plants of no agricultural value are also listed; one or another of these 

 may ultimately give a clue to some phase of the fundamental problem 

 of resistance. The fact that resistant varieties may lack other im- 

 portant qualities, such as market value or congeniality as rootstocks, 

 cannot be considered here; horticulturists are making progress on 

 these problems. 



Preliminary Discussion of the Problems Involved in Studying 



Resistance 



Resistance may depend to some extent on plant vigor, which in 

 turn depends on climate, plant nutrition, and other soil conditions. 

 It is therefore obvious that no absolute listing of resistant plants or 

 evaluation of degree of resistance can be made for all conditions; 

 neither is any one writer or organization able to take responsibility 

 for such statements. 



Information on plant resistance is based largely on negative evidence 

 and is thus considerably more controversial than are the positive 

 records of infestation. It requires a complete root and a most careful 

 search to be certain that even one plant is entirely free from infesta- 

 tion; it requires careful examination of the roots of many plants, 

 grown for a sufficient length of time in soil known to contain an 

 abundant and well-distributed nematode population, preferably also 

 in several different regions, to pronounce the species or variety resistant. 

 To evaluate any report, the reader should know how carefully this 

 examination was made. The following factors, not always sufficiently 

 taken into account, are suggested as responsible for some of the 

 negative results that have been reported: 



Field infestations are far from uniform as they occur in nature; in 

 addition, a large proportion of the existing infestation is sometimes 

 removed from the plot, on the trap-crop principle, when the indicator 

 plants of one nematode experiment are examined before starting 

 another. Not all experimenters state that their plots were heavily 

 infested, but all do imply that they considered the infestation adequate 

 for the conclusions drawn; for this reason their statements on this 

 point are not usually quoted here. 



The egg or larval stages necessary to carry on an infestation may 

 at certain seasons be lacking from what appears to be an abundant 

 inoculum of detached galls. 



Small amounts of inoculum, unless absolutely fresh, may have 

 been subject to desiccation or, if kept too wet, to putrefaction or to 

 an unbalanced activity of nematode enemies. 



Almost any plant may at some time escape infestation {188) 2 under 

 conditions of reduced nematode activity and be reported resistant. 

 Therefore a single report of resistance requires confirmation, though the 

 plant may merit further investigation. The fact that such a plant has 



2 Italic numbers in parentheses refer to Literature Cited, p. 78. 



