Determining the importance of combining ability for late blight resistance in early generations of potato breeding when susceptible clones are discarded

At times it may be desirable to determine the importance, rather than the actual values, of general (GCA) and specific combining ability (SCA) in parental materials. In screening large populations for disease resistance over years it may be difficult or impossible to accurately estimate these effects. Once a clone has been found to be susceptible it is usually discarded or lost to the disease, but multiple evaluations are desired before declaring a clone resistant. The purpose of this study was to determine the importance of GCA and SCA for late blight resistance in tetraploid potatoes. Five late blight resistant females were crossed to four males in a design II mating scheme to generate 20 families. Progeny were evaluated for foliar late blight from 2002 to 2004. In the first 2 years progeny that had ≥90% late blight infected foliage were discarded; progeny with <90% infected foliage were retested. Sixty-seven clones had no more than 50% infected foliage each year. Individual χ ² tests demonstrated the significance of GCA in four of the five female parents, however, GCA was not significant in any of the male parents. The proportion of resistant progeny from the female and male parents was used to calculate the expected proportion of resistant progeny for each parental combination. The probbnml function in SAS was used to calculate the probability that the actual number of resistant clones observed was as great or greater than expected. SCA was important in four of the families. Although this method does not give estimates of GCA or SCA, it does determine if GCA or SCA are significant and which parents or parental combinations have significant GCA or SCA, respectively.