FINE SCREENING SOLANUM (POTATO) GERMPLASM ACCESSIONS FOR RESISTANCE TO COLORADO POTATO BEETLE

This study was conducted to investigate the utility of systematic scre ening at the genotype level within Solanum accessions highly resistant to the Colorado potato beetle. Evaluations of clonally replicated gen otypes showed that most accessions reported to be uniform when screene d as populations contained small but significant variation among genot ypes for resistance to oviposition, larvae, and defoliation, differenc es for numbers of larvae being most common. Adult counts and percentag e defoliation were not as useful in evaluating among-genotype variabil ity in beetle resistance. Genotypes of S. pinnatisectum WRF 343 and S. tarijense PI 473227 were the most uniformly and highly resistant to C olorado potato beetle. Genotypes of S. berthaultii PI 473331, S. chaco ense PI 473405, and S. tarijense PI 473336 were moderately to highly r esistant, and genotypes of S. bukasovii PI 473494 and S. canasense PI 230511 were uniformly susceptible to Colorado potato beetle. Nonparame tric correlation analyses indicated that number of egg masses, small l arvae, large larvae, and defoliation scores were positively correlated , negatively correlated, or not correlated, depending on the species. One generation of selection attempting to segregate resistance and sus ceptibility in nearly uniform and highly resistant S. pinnatisectum WR F 343 resulted in ''divergent'' populations that could not be distingu ished from each other or the base population. Thus, through genotype ( ''fine'') screening and selection, we showed that some existing popula tions are virtually pure for extreme resistance. Use of individuals fr om such families would make screening breeding populations more effici ent, and reduce the risk of losing resistance genes that have non-domi nant effects. Fine screening, recurrent selection, and maintenance of such elite populations is recommended as an extension of population-ba sed evaluation usually done by genebanks.