Selecting for late blight resistance
The strategy for breeding late blight resistant potatoes during the first half of the 20th century was utilisation of the major dominant R-genes which had been discovered in the Mexican wild species Solanum demissum. However, they failed to provide durable resistance, either singly or in combination, owing to the evolution of new races of Phytophthora infestans.
As a consequence, breeders at the Institute and its predecessors, started to select for quantitative field resistance by using races of P. infestans compatible with the R-genes present in their material.
Recently researchers in a number of laboratories have found, mapped and cloned new R-genes in species other than S. demissum. There is much interest and debate over whether or not these R-genes will be durable, or can be deployed in a durable way. It can be argued that new R-genes should be combined with high levels of field resistance, so that the near immunity conferred by R-genes is utilised until they are defeated, then the high levels of field resistance come into operation.
Our recent research has concentrated on how easily major-gene resistance can be distinguished from field resistance and how to select for high levels of field resistance, of which there is more than one type. Some field resistance is associated with late maturity (Bradshaw et al., 2004) and cannot be utilised in earlier maturing varieties.
Although the S. demissum derived R10 and R11 genes are alleles at the R3 locus, when the respective R-gene differentials of Black were crossed with a susceptible potato, the first cross displayed a continuous distribution of blight scores whereas the progeny in the R11 population could be categorised as resistant or susceptible (Bradshaw et al., 2006). Thus R10 behaved like a QTL whose resistance can be overcome by compatible isolates of P. infestans.
The QTL of large effect on field resistance on linkage group IV of Stirling (Bradshaw et al., 2004) and PDH247 (Bradshaw et al., 2006) can be overcome by genotype blue 13 of P. infestans which is now prevalent in the UK. Thus QTL x isolate interactions can occur, and this type of field resistance is no more durable than major R-gene resistance.
There is field resistance associated with defeated R-genes (Stewart et al., 2003) but it is not clear whether this increased resistance is conferred by the defeated R-gene or linked genes. Although small in effect, it is nevertheless useful resistance.
Finally, there is field resistance that does not yet display resistance x isolate interactions (for example, as seen in clone 8204a4 which is a parent of Stirling). Is this desirable variation, which is not associated with QTLs of large effect, horizontal resistance as proposed by Van der Plank back in 1968?
Commercially successful potato cultivars with adequate levels of durable field resistance to make protective spraying unnecessary are probably not achievable in practice.
Nevertheless, selection in potato breeding programmes for higher levels of field resistance would still seem to be desirable, albeit with a view to reduced fungicide use in Integrated Pest Management.
The best strategy to select for high levels of field resistance is probably selection for high levels of resistance with the currently prevalent complex races of P. infestans, but also taking into account the blight scores of a clone’s parents and sibs (Solomon-Blackburn et al., 2007).
References
Bradshaw, J.E., Pande, B., Bryan, G.J., Hackett, C.A., McLean, K., Stewart, H.E. and Waugh, R. 2004. Interval mapping of quantitative trait loci for resistance to late blight [Phytophthora infestans (Mont.) de Bary], height and maturity in a tetraploid population of potato (Solanum tuberosum subsp. tuberosum). Genetics 168, 983 - 995.
Bradshaw, J.E., Bryan, G.J., Lees, A.K., McLean, K. and Solomon-Blackburn, R.M. 2006. Mapping the R10 and R11 genes for resistance to late blight (Phytophthora infestans) present in the potato (Solanum tuberosum) R-gene differentials of Black. Theoretical and Applied Genetics 112, 744 - 751.
Solomon-Blackburn, R.M., Stewart, H.E. and Bradshaw, J.E. 2007. Distinguishing major-gene from field resistance to late blight (Phytophthora infestans) of potato (Solanum tuberosum) and selecting for high levels of field resistance. Theoretical and Applied Genetics 115, 141 - 149.
Stewart, H.E., Bradshaw, J.E. and Pande, B. 2003. The effect of the presence of R-genes for resistance to late blight (Phytophthora infestans) of potato (Solanum tuberosum) on the underlying level of field resistance. Plant Pathology 52, 193 - 198.
