Using lateral stem cuttings to evaluate the resistance of Solanum phureja genotypes to Spongospora subterranea f. sp subterranea

In order to evaluate the resistance to Spongospora subterranea f. sp. subterranea on 105 genotypes of Solanum phureja (local name in Colombia papa criolla), bioassays were conducted through propagation by lateral stem cuttings. Inoculation was performed with a 1 x 10(6) sporeballs mL(-1) solution. The level of resistance or susceptibility of each genotype to the pathogen was determined at three different times: 1) At 15 days after inoculation (DAI), the presence of zoosporangia was evaluated in roots under optical microscopy; 2) At flowering, the presence of galls was observed in roots and 3) At harvest, the tubers were screened for the detection of pustules. Asymptomatic plants, (i.e. plants seemingly without pathogen's structures or disease symptoms) were evaluated by PCR using specific primers. At 15 DAI it was not possible to determine the degree of resistance in the genotypes and later there were no disease symptoms on tubers. Nevertheless, disease was detectable through gall formation on roots at the flowering stage. The most susceptible genotype was Criolla Colombia. Thirty genotypes, including the commercial variety Criolla Paisa, were highly resistant and presented neither pathogen's structures nor symptoms.