72 ROOT-KNOT AND ITS CONTEOL. 



formed by men who can devote considerable time to it, hardly pays 

 for the time and expense required. 



In cariying out simple selection we must remember that no new 

 characters are originated by this method. We simply select and 

 strive to fix in one strain certain characters that are present as 

 variations in the plants we are working with. Thus, if we find in a 

 field badly infested with nematodes that a certain proportion of the 

 plants are free from root-knot while the rest succumb, it would 

 probably pay to begin selecting seed from the unaffected plants. It 

 is better still if we can inbreed or intercross similar resistant plants. 

 On the other hand, resistance to nematodes seems sometimes not to 

 be one of the variations occurring in a plant. Such a plant can not 

 be selected, as there is no foundation on which to build. However, by 

 crossing it with some nearly related nonsusceptible sorts, some of the 

 progeny may possibly show desirable qualities of resistance while at 

 the same time preserving the best qualities of the parent sorts. 



In all such breeding it must be borne in mind as a very important 

 principle that this work should be done in badly infested fields. If 

 naturally infested fields are not available, provision should be made to 

 do this work where the disease is abundant. 



No attempt will be made here to describe the methods of selection 

 or hybridization. These are known to all seed growers and breeders. 

 They can be found described in detail in many publications. ^ 



Every farmer ought to be able at least to carry on this simple 

 selection: When any plants in an infested field show special vigor and 

 freedom from root-knot they should be marked and the seed collected 

 before the main crop is gathered. This should only be done, how- 

 ever, if these resistant plants are also up to standard in all other 

 features. 



SUMMARY. 



(1) The disease known as root-lmot, characterized by enlargements 

 of the roots and often leading to the death of the plant affected, is 

 caused by a nematode (Heterodera radicicola (Greef) Mull.). This 

 was probably originally native in the Tropics (of the Old World ?), 

 but has spread into nearly every part of both Temperate Zones. 



(2) The plants recorded as more or less subject to attack number 

 almost 480 species and varieties, including nearly all of the larger 

 families of flowering plants. Probably many more are actually 

 susceptible, but have not been reported yet as hosts. Most of the 

 important field and garden crops and ornamental plants are more 

 or less subject to root-knot. 



1 Hays, 1901; Bailey, 1906; Orton, 1909; Reed, 1909; Salmon, 1907; Spillman, 1909; Wilcox, 1903; Oliver, 

 1910. 



217 



