416 HETERODERA RADICICOLA. 



have found maize to be susceptible to some degree, but in practice 

 both of these crops might be regarded as resistant and can often 

 be grown profitably. The growing of either of these cereals for 

 two or three years on infected ground might be a very practical 

 way of dealing with the problem. 



Desiccation is also very fatal to the parasite in all its stages. 

 In South Africa, where the sun's rays are very powerful and the 

 dry seasons are usually regular and sometimes prolonged, desicca- 

 tion often occurs as a matter of course. Where this can be 

 assisted by frequent ploughing and the use of unslaked lime 

 in fairly heavy quantities, the result should be beneficial. 



The presence of varieties or biological strains of Heterodera 

 is sometimes suggested by the fact that in heavily infested plots 

 the parasites often confine their activities to one particular kind 

 of host plant in preference to others that have been shown to be 

 equally susceptible to attack. This seems to indicate the possi- 

 bility of the creation of slight strains of the parasite which, once 

 having accustomed themselves to a special type of host, tend to 

 prefer it to other plant hosts. In the same way resistant varieties 

 of plants, such as the "iron" cowpea, may be simultaneously 

 evolved. Further and prolonged investigation will be required 

 before this hypothesis can be confirmed. 



Other curious phenomena occur connected with the habits of 

 Heterodera which make the study of the problem even more 

 interesting. On several occasions it has been observed that plots 

 which have shown themselves to be heavily infested with the 

 parasite have quite suddenly become free of the pest without their 

 having been specially treated in any way. Further, they have 

 not reappeared when very susceptible crops were subsequently 

 grown. It is difficult to explain these occurrences on any climatic 

 basis, but it is possible that the mystery may be cleared up by a 

 thorough investigation of the fauna and bacterial flora of the 

 soil. 



In conclusion, it may be a platitude to remark that, in view 

 of the difficulty entailed in the control of the pest the old adage 

 of prevention and cure should be the guiding principle of all 

 gardeners and horticulturists. Great precautions should be 

 observed in the introduction of new nursery stock that cannot be 

 guaranteed free from Heterodera, and seed potatoes, whereby in 

 agriculture the organisms are often spread, should be carefully 

 examined before planting. 



Acknowledgments. 



I wish to acknowledge the general assistance that many friends 

 have given during the course of these investigations. Primarily 

 am I indebted to Professor H. B. Fantham, of the Department 

 of Zoology, University of the Witwatersrand, Johannesburg, and 

 to Dr. Annie Porter, of the South African Institute for Medical 

 Research, Johannesburg, for their kindly interest and encourage- 

 ment. I desire also to acknowledge gratefully the numerous facili- 

 ties given me by many officers of the Divisions of Entomology and 



