UNCLASSIFIED PESTS 339 



It is especially troublesome in irrigated districts and in 

 greenhouses even in districts where it does not occur in the 

 open. 



Different species of plants vary greatly in their degree of 

 susceptibility to the root-knot disease. Among vegetable 

 crops those most susceptible to injury are potato, tomato, 

 eggplant, celery, beet, carrot, lettuce, pepper, endive, canta- 

 loupe, cucumber, watermelon, squash, pumpkin, lentil and 

 salsify ; the following also are sometimes seriously attacked : 

 asparagus, onion, cauliflower, cabbage, kale, collard, turnip, 

 pepper, bean, pea, radish, spinach and sweet potato. In some 

 localities potato tubers become badh' infested and acquire a 

 warty appearance. Infested potatoes are unsuitable for market 

 and should never be used for seed. 



The organism that causes the root-knot disease is not an 

 insect but a nematode worm. The young are produced inside 

 the old gall, from which they escape through cracks or by work- 

 ing their way through the tissue. In this stage the worm is 

 about T3- inch in length, slender and eel-like in form. The 

 young worms can live for several months in moist earth and 

 are able to move readily about among the soil particles. When 

 one of these worms finds a young and tender rootlet, it breaks 

 its way through the tissue by means of a spear-like organ located 

 in the mouth and then passes a short distance into the sub- 

 stance of the root. The worm then becomes inactive, swells 

 up and the female finally assumes a pear-shaped form. The 

 male becomes broadly spindle-shaped and then molts but 

 remains within the discarded skin. It then assumes a very 

 elongate form and becomes coiled in three or four folds within 

 its cyst. It finally escapes and, passing through the tissue of 

 the root, finds a female with which it mates before she has 

 become fully mature. The fully developed encysted females 

 are pear-shaped, about 2V inch in length and a little more than 

 half as broad and when the gall is broken open appear as little 



