ROOT KNOT NEMATODE INFESTATION 6 



never been reported as a host may possibly indicate resistance, though 

 this cannot be assumed, especially for trees, weeds, uncommon plants, 

 or plants usually grown in northern regions. 



The question of host preferences by the nematode population under 

 test is not sufficiently understood, but it must be taken into considera- 

 tion as a possible factor in all negative results. Rootstocks that have 

 seemed highly resistant in many trials have been attacked when tested 

 in other plots (see Amygdalus, pp. 13-15), or for an extended time. 

 The recent report of Sherbakoff (see Gossypium, p. 36), though far 

 from conclusive, opens for further investigation this problem, which 

 was explained by Steiner (220) in 1925. 



Reports based on crop growth, on yield, or on superficial healthy 

 appearance should be questioned unless adequately compared with 

 control plants, because plant growth can be greatly influenced by soil 

 fertility and by other cultural and environmental conditions, minimiz- 

 ing for a time the injuries caused by root knot infestation. 



Definitions 



Because terms relating to the resistance or susceptibility of a plant 

 to its parasites have not yet become standardized, a few commonly 

 used words are again defined according to the best usage that can 

 be determined, but with special reference to the root knot nematode. 

 These definitions are based in part on the Report of the Committee on 

 Technical Words (197a). Some minor differences may be noted: (1) 

 The Committee's definition of resistance combines two distinct condi- 

 tions; the first is substantially the same as in the definition given here, 

 but the second, to " overcome the effects" of the pathogen, seems to 

 fit more accurately into the definition of tolerance on the same page 

 of the report. (2) Symptom expression and damage are often closely 

 connected in fungus diseases, whereas root knot damage may be more or 

 less independent of the symptom of gall formation. (3) Wilbrink's 

 definitions draw a pertinent distinction between susceptibility and 

 sensitivity. 



Susceptibility in a plant is its condition of being a suitable host for 

 a given parasite. In the case of root knot even resistant plants, under 

 certain conditions, may be more or less susceptible. 



The word affected has been used more or less indiscriminately in 

 the literature on root knot, without explanation of the meaning in- 

 tended. It may refer either to the mere fact of infestation or to the 

 effect of the infestation on the plant. It should be dropped in favor 

 of the more exact words. It is used in this compilation only as a 

 quotation, when the meaning of the author cited is in doubt. 



Tolerance is a capacity to endure without injury or, as applied to 

 root knot, the ability of a plant to continue productive growth even 

 while it is subject to a heavy and increasing infestation— year after 

 year in the case of a perennial. This productiveness or absence of 

 above-ground symptoms (but not absence of galls) has sometimes been 

 taken for resistance; the practical difference is that tolerant plants 

 contribute to the unlimited multiplication of the parasite, whereas 

 resistant plants may be grown to reduce the infestation of the soil. 



Resistance in plants is the ability to obstruct the invasion of para- 

 sites. The term implies a considerable but not necessarily an absolute 

 freedom from infestation in infested soil. Highly resistant plants show 



