Ke3.(Heter.)s6 



RLeld tests have given results comparable with pots. Figure 3 gives 

 the results of two years' plot work (Williams, vinpublished) . In the 

 first year the cultivated variety Majestic gave a large population 

 increase, while a resister gave a slight reduction compared with 

 fallow. In the second year, the whole area was sown with a resister. 

 This reduced the population after Majestic, after fallow, and after 

 the resister. Two years' cultivation of the resister resulted in a 

 very low potato eelworm population, indeed, and, with this particular 

 population, did not suggest any breakdown of resistance. 



Reference to the presence of occasional cysts on resistant plants has 

 been made. The different lines of S. andigena vary, C.P.C. 1673 

 especially producing fewer than others. The appearance of these cysts 

 was puzzling. They might be explained by variations in the suscep- 

 tibility of plant roots, variations in the conditions of tests, or by 

 the existence of resistance-breaking biotypes. Evidence is now 

 accumulating in favour of the last explanation. Van der Laan (1957) 

 obtained cysts from Peru and found that these broke resistance, and 

 this was subsequently confirmed by tests of tubers sent from Holland 

 to Peru (Quevedo, Simon & Toxopeus 1956). Meanwhile, evidence of a 

 similar kind had been found by Dunnet (1957) in Scotland. Here, one 

 garden population was found to be very aggressive. Confirmatory 

 tests done by Jones (1957 and unpublished work) have shown that 

 British populations vary considerably in their content of resistance- 

 breaking biotypes (Tables 5, 6, 7, and 8). 



In one series of tests, the proportions, crudely measured, ranged 

 from less than 1% to 75^. Subsequent tests with these populations 

 have indicated that there is only a very slow swing, when resisters 

 are grown, t'^wards increase in the percentage of resistance-breakers 

 at 1% level, and that increase is more rapid at higher percentages. 

 This is shown in Table 9 and also in Table 10. In the latter, a 

 population raised on susceptible and resistant plants was grown on 

 resistant plants in single cyst culture (i.e. 1 cyst/pot). On the 

 susceptible plant, the population remained static at h% of resistant- 

 breakers, while on the resistant plant, the proportion of resistance 

 breakers increased 3-U times. These results are not inconsistent 

 with the hypothesis that the resistance-breaking character in nematodes 

 is recessive, but the results, so far, are slender and require con- 

 firmation of various kinds. 



Dunnet (1957) could find no evidence that resistance in Solanum "vernei 

 was broken by his aggressive population. It would be valuable if 

 resistance here were of a different type, although breeding from 

 S. vernei is likely to be more difficult than from S, andigena. 



Acknowledgments 



I should like to thank T. D. Williams, H. W. Howard, and P. A. van 

 der Laan for allowing me to mention some of their work. 



