216 ANNUAL REPORTS OF DEPARTMENT OF AGRICULTURE. 



It is becoming; more and more ck'ur (hat (he ^iall ncma is one of 

 the worst pests known to American a<:;riciilture and that any relief 

 from the losses due to its attacks on the roots of crops would be a 

 ver}^ great boon. Additional investioations have made it certain that 

 this organism, like most others, has its enemies: among them are 

 certain niononchs and other predatory nemas. Further study of 

 the subject makes it additionally probable that there are regions 

 suitable to the occurrence and growth of this nema in which it is 

 not regarded as a serious pest. It is strongl}'^ recommended that 

 examination be made of soils in such i)laces in order to determine 

 what are the factors that hold the pest in check. 



The explorers and other officers of the Office of Foreign Seed and 

 Plant Introduction have noted that in certain foreig-n countries the 

 chayote plant is often more thrifty than it aj)pears to be when in- 

 troduced into this country. When introduced here it is found to be 

 very susceptible to the attacks of the root-gall nema, Heterodera 

 radicicola. This has given rise to speculation as to whether preda- 

 tory nemas known to exist in foreign soils may be a factor in the 

 case. In consequence, experiments in the introduction of the foreign 

 predatory species are being undertaken. 



NEMAS IMPOKTED FKOJI FOIIEIGN COUNTRIES ON THE ROOTS OF PLANTS. 



Under this broad heading a number of investigations have been 

 undertaken during the ,year, the general results of which confirm the 

 growing belief that the indiscriminate importation of foreign plants 

 has been the source of enormous losses to American agriculture, 

 owing to the introduction of pests and diseases. 



It is impossible in this place to go into details, but it may be said, 

 in brief, that in no case where a small portion of soil removed from 

 the roots of imported plants has been examined for the presence of 

 nemas has the result been negative. In practically all cases nemas 

 have been discovered, as a rule in large numbers and in great variety. 

 As man}^ as 30 different species of nemas have been taken from a few 

 grams of soil removed from the roots of what would be called 

 " plants imported free of soil." The portion of the soil that most 

 closeh' adheres to the roots is the most densely populated with nemas. 

 Where the luimber of species is large, a great variety of biological 

 relationships exists. Along with the injurious species beneficial spe- 

 cies are found, and others whose economic relationships are not yet 

 understood. 



The following important facts may now, however, be considered 

 as comparatively well established: The importation of injurious 

 parasitic species of nemas is more likely than the importation of 

 beneficial species. This is because the parasitic species are found 

 within the tissues of the plants, while the beneficial species — that is, 

 the predatory nemas — are not found in these situations. From this 

 it follows that when the roots of plants undergo the so-called " clean- 

 ing process " preparatory^ to shipment, the predatory nemas are more 

 likely to be removed than the injurious ones. 



Of course, these facts with regard to the possible introduction of 

 diseases caused by nemas represent only a single phase of a much 

 broader problem, which includes the introduction in the same way of 



