6 MISC. PUBLICATION 406, U. S. DEPT. OF AGRICULTURE 



5. Reports of "no apparent injury" to infested plants, unless it is clear that the 

 plants were infested heavily and might thus have evidenced tolerance. 



6. Certain reports of light infestation. A degree of infestation called light by 

 one observer might be overlooked by another, who would then report as light what 

 the first would call moderate. For example, Neal (176) listed host plants as 

 "slightly affected" and as "badly affected." He explained the first heading on 

 page 12: "The 'tap-root,' descending deeply is rarely affected, and the plants 

 seem slightly affected till the sub-cortical layer is filled with worms in all stages of 

 growth. This checks growth . . . the food supply is cut off before decay is 

 visible . . . the plant dies." Another observer could have called such an infes- 

 tation heavy or severe. 



7. Weeds reported only lightly infested and weeds not specifically identified. 

 The growth of weeds is usually so vigorous as to show no apparent injury, even 

 though the roots harbor a sufficient nematode population to be a definite menace 

 to later crop plants. 



8. Generalized statements on the resistance of certain plants, e. g., cotton, of 

 which only certain varieties are conceded to be resistant, others susceptible. 



9. A number of publications difficult of translation. For example, Ustinov {241) 

 discusses in Russian the usual series of resistant field crops; it is assumed, perhaps 

 without justification, that no new experience on the reactions of these plants is 

 reported. 



10. Early reports by certain authors if later altered by them. 



Possible Standardization of Tests and Reports 



The conditions of plant growth and the many factors affecting plant 

 parasites are obviously not amenable to absolute standardization. 

 It is too early even to suggest a uniform method for testing the resist- 

 ance or tolerance of particular plants. Probably every test will re- 

 quire a number of hosts of known susceptibility and of several degrees 

 of resistance for comparison. A reasonable moderation in claiming 

 resistance will increase the value of all statements. These remarks 

 must not, however, be allowed to discourage the reporting of observa- 

 tions that might in any possible way be of use to either growers or 

 investigators. The possible errors mentioned above can be mini- 

 mized by well-planned controls. 



On the other hand, such a compilation as this shows very clearly 

 the need for standardization, a need that was emphasized at the Nash- 

 ville 3 and Atlanta 4 conferences on root knot. For the present it is 

 merely urged that all possible controls be included; that all conditions 

 of growth be described very fully; that the terms resistance, tolerance, 

 etc., be used in accordance with some specified definitions; that the 

 basis of the conclusions be explained (how carefully roots were ex- 

 amined, or whether plants were judged mainly by their above-ground 

 growth) ; and that the horticultural variety as wxll as the species of 

 every plant be reported. 



If variety names were more usually included in plant-disease records, 

 the assembled data would undoubtedly give much information. Where 

 reports concerning susceptibility appear contradictory, the trouble 

 may be merely a lack of information on the reactions of particular 

 varieties of the plant in question. A report of resistance in a notably 

 susceptible species may be discredited if the resistant variety is not 

 named; when the variety is named, additional reports on the same 

 variety are needed for comparison Examples in this publication 

 are corn, dahlia, fig, peony, rose, soybean, and tomato. 



3 U. S. Bureau of Plant Industry, proceedings of the root-knot nematode conference held 

 at Nashville, Tennessee, feb. 2 and 3, 1937. U. S. Bur. Plant Indus., Plant Dis. Rptr. Sup. 102, pp. 97- 

 122, illus. 1937. [Mimeographed.] 



* Tyler, Jocelyn, Ed. proceedings of the root-knot-nematode conference held at Atlanta, 

 Georgia, February 4, 1938. U. S. Bur. Plant Indus., Plant Dis. Rptr. Sup. 109: 133-142. 1938. 

 [Mimeographed.] 



