ROOT-KNOT— CAUSE AND CONTROL, INCLUD- 

 ING A LIST OF SUSCEPTIBLE HOST PLANTS. 



By Lerot Childs, Assistant Secretary State Commission of Horticulture. 



Very frequently roots of various plants, bearing knots and swellings, 

 are sent in to the Commission with inquiries relative to the cause of the 

 abnormality of the root system. More often than not this disarrange- 

 ment may be ascribed to the presence of a minute, semi-transparent 

 worm, Heterodera radicicola, which has established itself in the tender 

 tissue of the root system. The presence of this minute parasite stimu- 

 lates the plant tissues to such an extent as to caus6 an abnormal devel- 

 opment, characterized by the familiar knotty, disorganized roots of a 

 worm infested plant. (Figs. 390 and 397.) 



This small, semi-transparent worm has adapted itself readily to 

 many widely different varieties of plant life. At the present time 

 over 480 species of plants are known to be susceptible to the attack 

 of this parasite. A more thorough investigation would undoubtedly 

 swell the host list considerably. Again, Heterodera is unknown in 

 many localities, in which places the plant life of the region has never 

 been subject to a test, or its susceptibility or insusceptibility learned. 



Distribution. 



The eelworm, as it is more often called, seems to be of world-wide 

 distribution in the greater sense, being found in Europe, Asia, Africa, 

 Australia and both North and South America. Though found rather 

 universally all over the world, there are many localities in which the 

 pest has never been known. 



The original native habitat of this nematode has never been definitely 

 established; arguments of various authors, however, seem to favor the 

 tropics, from where, through the importation of various plants, both 

 useful and ornamental, Heterodera has been transplanted unknowingly 

 throughout the greater portion of the civilized world. 



In the United States the distribution is decidedly spotted and it is 

 at present regarded as a serious pest, and a menace to agriculture in 

 only a few rather definite localities, especially in irrigated regions, the 

 most prominent of which is an irrigated, potato-growing district in 

 Nevada, where the depredations of the pest threatened the industry. 

 The fact that the eelworm exists in so many places, doing little appreci- 

 able damage, is no criterion that all necessary care should not be 

 exerted in checking any further distribution of the pest. Heterodera 

 is a comparatively new-comer to our soils, and the maximum amount of 

 injury which it is capable of inflicting has undoubtedly not been 

 reached. Intensive farming and more thorough irrigation are two 

 factors which, in creating a better environmental condition, are decid- 

 edly advantageovi^ in increasing the number of worms which the soil 

 may harbor. 



The recent preliminary investigations in the citrus districts of the 



