2005 Annual Report 4d.Progress report. This report serves to document research conducted under a reimbursable agreement between ARS and UC Berkeley. Additional details of research can be found in the report for the parent project CRIS 5354-21220-002-00D “Potato Variety Improvement through Gene Transfer and Virological Studies.” This grant is specifically to assist in the mapping and cloning of the RMC1(blb) gene from Solanum bulbocastanum for resistance to Columbia root-knot nematode. We first tested mapping population for resistant gene in Solanum bulbocastanum (RMC1(blb)). This population of 220 F1 progeny was inoculated with Meloidogyne chitwoodi, race 1, and resulted in a confirmed 95 out of 220 resistant or about 43% which does not deviate significantly from a 1:1 segregation ratio indicating a single gene. The parents and resistant and susceptible bulks from this intra-bulbocastanum cross were compared for AFLP polymorphisms. We now have ten closely linked markers, three of which are AFLP markers and one is a Polymerase chain reaction (PCR) based Cleavage Amplified Polymorphic site which is restricted by MSE I. We have found a new source of highly useful markers. It had been previously reported that genes similar to the virus resistance gene “N” in tobacco were found on the upper arm of chromosome 11 in potato. After probing bacterial artificial chromosome (BAC) library of Solanum demissum using sequence of “N”, BAC clones were identified that hybridized with these probes. The ends of these clones were sequenced and this data used to design primers for PCR amplifications. These amplifications were compared between Columbia root-knot resistant and susceptible bulks of the intra bulbocastanum cross we have been using. PCR amplicons were found that map right on top of the RMc1(blb) gene. These amplicons are amplified uniquely from resistant materials and do not produce any product from susceptible clones and thus are highly suitable for marker assisted selection. We have determined that Solanum bulbocastanum with or without nematode resistance is a good source of resistance to the potato aphid Macrosiphon euphorbiae, but not to Green Peach Aphid (Mysus Persicae) (Mp). Mp is definitely the most important aphid pest and major virus vector of potato worldwide, whereas M. euphorbiae is not important. As a consequence we have surveyed five new accessions of Solanum bulbocastanum and found a number of seedlings that appear to be highly resistant to Green Peach Aphid. The Solanum bulbocastanum clone that was resistant to late blight was found to be resistant to Mp, but somatic hybrids with cultivated potato had completely lost their resistance. We surmise that the trichomes of S. bulbocastanum are highly important in the expression of resistance, and that their expression is highly attenuated in the somatic hybrid. We selected Potato leafroll virus (PLRV) resistant progeny from the cross with Abnaki, and extracted haploids and made crosses to combine this extreme form of resistance with immunity to Potato virus Y (PVY) at the tetraploid and diploid levels. The intent is to provide materials that when used as parents in crosses will provide double resistance to PLRV and PVY in 80% of the progeny.