412 heterodera radicicola. 



Susceptible Host Plants. 



The list of plants affected by root "knot" given bv ilessey 

 (1911) is a formidable one, containing over five hundred plants; 

 the list has been added to by other workers, and a large propor- 

 tion of the plants catalogued are of great economic importance. 

 Indeed, very few of our cultivated plants are altogether immune 

 from attack. 



Many plant roots on inspection are occasionally seen to be 

 somewhat disfigured by galls, which experience has shown can 

 not always be regarded as due to gall worm (Heterodera) until 

 the parasite has actually been isolated by microscopic investi- 

 gation. 



The presence of other nematodes, often species of the genus 

 Tylenchus, is sometimes manifested in cultures made of root 

 material infested by Heterodera. These worms often cannot easily 

 be differentiated from the larvae of Heterodera as (heir gross 

 morphology is identical with this genus. Their sizes and possibly 

 also the rapidity of their movement, would appear to be the 

 only means of distinguishing them. Thus, larvae of this nature 

 were found in the pith cavity of the stem and in the roots of a 

 snapdragon plant {Antirrhinum) which larvae were as long as 

 970/j,, and 21/x, broad. Others of similar structure, found in 

 roots and soil cultures were only 200/x in length and 20/x in 

 breadth, while the larvae of Heterodera vary in length between 

 345ja and 500/i, the average being about 360/a. 



The following is a list of plants that the writer has person- 

 ally examined and found parasitised more or less severely by 

 Heterodera in South Africa. 



It will be seen that most of the plants listed are cultivated 

 either for food or for ornament. 



Among tha few monocotyledonous hosts the presence of the 

 mealie, Zea mat's, is most important, for, as a rule, it has 

 been thought by most authorities to be one of the few immune 

 crops. In this case the mealie plants were found in an orchard 

 where conditions were optimum for the thriving of the parasite, 

 and, all hough the orchard was a new one, having been established 

 for only three years on ground that was covered with bush and 

 had never previously been under cultivation, most of the fruit 

 trees were severely infested. It would appear that the parasite, 

 under suitable conditions, is able to accommodate itself in most 

 plants, and that strains or varieties of supposedly immune plants 

 may occur which are not totally immune. 



List of Host Plants. 



A butilon indicum. 



A Hi u m porr u in — leek 



A maranthus paniculata. 



A ntirrhinum ma jus — snapdragon. 



Beta vulgaris — beetroot. 



Brassica campestris—tuxnxp. 



