2006 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 5354-21000-002-00D, "Potato Variety Improvement Through Gene Transfer and Virological Studies (National Program 301)." 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 resistance. We first tested mapping population for resistant gene in Solanum bulbocastanum (RMC1(blb)). 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. We have further refined this and now have sequence tagged sites (STS) whose primers can be used in a simple PCR without resorting to restriction enzyme treatment. These markers are 99% coincident with the resistant root phenotype in BC5 breeding populations. We have developed introgression populations for the three wild species sources of root resistance to M. chitwoodi. Out of the marker set that co-segregates with resistance in Solanum bulbocastanum, one of those markers co-segregates perfectly with resistance phenotype in the S. hougasii derived population and the entire five marker set co-segregates perfectly with resistant phenotype in the S. fendleri derived population. This is confirmation of synteny between all three species. In field tests we noted that partial damage of tubers occasionally occurred. We found that presence of nightshade weeds, a good host for M. chitwoodi, could overcome the resistance in most of the resistant materials. However, we found that certain clones from advanced introgression generations resisted tuber damage even in the face of abundant juvenile M. chitwoodi provided by reproduction on nightshades. These clones have an additional layer of resistance in that the tubers are very resist invasion by free juveniles. We have found recombinant root-susceptible/tuber resistant types as well, suggesting that there is separate genetic control. Out of a population of thirty six root-resistant progeny 34 were also tuber resistance suggesting close linkage between factors controlling the two resistance. Combining root and tuber genetic resistance factors will provide the stablest resistance for the long run. Race 2 of M. chitwoodi, although defeating the Rmc1 gene, does not break through the tuber resistance. Furthermore, a newly discovered resistance- breaking biovar of race 1 that defeats Rmc1 does not overcome the new tuber resistance.