DISEASE RESISTANCE IN ANNUAL CROPS 55 



other (metabolic) functions give them survival value. Thus their exis- 

 tence is not due to a differential selection pressure by a specific race 

 of the pathogen. Polygenes have no differential effects; they convey 

 uniform resistance. Polygenic resistance is a quantitative character and 

 quantitative genetics should be applied in breeding for polygenic 

 resistance . 



The uniform resistance to P. infestans found in late maturing potato 

 cultivars is often ascribed to "gene balance", a lucky combination of 

 gametes with unpredictable inheritance. Toxopeus (1959) showed that 

 transgression of (supposedly) uniform resistance occurred, some ¥\ plants 

 being more resistant to late blight than either parent. Hooker (1967, 

 1968), studying the adult plant resistance of maize against maize rust 

 {Puccinia sorghi) , determined the infection severity of individual 

 plants in P, Fj, F2 and F3 populations. Inbred parents and F^ popula- 

 tions were uniform; F 2 and F3 populations showed much variation. The 

 partial resistance of mature maize plants is polygenic, slightly dominant 

 and highly heritable. Transgression of resistance was not unusual. So- 

 called minor genes for resistance against stripe rust in wheat seem to 

 be additive in effect (Lewellen and Sharp, 1968). 



ENVIRONMENT AND DISEASE EXPRESSION 



Monogenic qualitative inheritance and polygenic quantitative inheri- 

 tance have been represented as two contrasting systems . The data presented 

 on stripe rust of wheat in the Netherlands suggest that the contrast 

 should not be stressed too much. It is better to regard these two systems 

 of inheritance as extremes of a wide range of possibilities. 



Detailed investigations into cotton blight, caused by the bacterium 

 Xanthomonas malv ace arum , illustrate that the host-parasite relationship 

 is a dynamic system in which disease expression depends on the inter- 

 action (in the statistical sense) of environment with the genetic systems 

 of host and pathogen. A population which is uniform in disease expres- 

 sion in one environment can show a great variability under another set 

 of environmental conditions. In the latter case types could be selected 

 resistant in both environments (Arnold and Brown, 1968). The magnitude 

 of the gene effects, reflected in the terms "minor" and "major" gene, 

 can be a function of environment. What seems to be a monogenic system 

 in one environment may appear as a polygenic system in anotner environ- 

 ment. A concept like gene penetrance bridges the apparent discrepancy. 



RESISTANCE BREEDING 



In annual crops, the breeding for high resistance conditioned by one 

 or a few genes often follows a simple scheme. A resistant parent is 

 crossed with a susceptible parent of high agricultural value. The 

 resulting hybrid is resistant but contains unwanted germ plasm from the 

 resistant parent. This is eliminated by repeated backcrossing to the 

 agronomically valuable parent. The plants of the successive daughter 

 generations are exposed to infection and all susceptible plants rejected. 



Tactics in breeding for uniform resistance are being developed in 

 recent years. Among the selection criteria used by the breeder are the 

 infection ratio (Umaerus , 1968) and the latent period (Rudorf and 

 Schaper, 1954). The infectious period is difficult to measure and, 



