Characterization of Phytophthora infestans isolates infecting transgenic and somatic hybrid potatoes resistant to the pathogen in the TolucaValley, México.  H. Lozoya-Saldaña,*, C. Belmar-Díaz, J.M. Bradeen, and J.P. Helgeson. Autonomous University of Chapingo, Chapingo, México, 56230, PICTIPAPA, Conjunto SEDAGRO, Metepec, Méx., 52142, Univ. Minnesota, St. Paul, MN, USDA/ARS, U. Wisconsin, Madison, WI.  

In the summer of 2003 transgenic and somatic hybrid clones with resistant genes against  the late blight pathogen (Phytophthora infestans)from Solanum bulbocastanum were exposed to natural infection of the oomicete in the Toluca Valley of México. Most clones were resistant to the pathogen in the early stages of growth but gradual infection occurred  along the growing cycle. The aim of this work was to identify the P.i. genotypes that specifically infect the clones in time. Twenty two different P.i strains were identified on the basis of mating type and glucose-phosphate-isomerase (GPI) and peptidase (Pep) profiles. The first infections by four different isolates were observed in the non-transformed control (cv. Katahdin). The following week six P. i.. strains, mostly different from the previous ones, were obtained from the same host. Early infections by five different P. i.’s also occurred in a clone that did not take the transformant. Both clones died at midseason. The most frequent P. i. Genotypes were A1, 100/100, 100/100 (M.T. Gpi, Pep, 19%), and A2, 100/100, 100/100 (14%), infecting all clones, mostly at the end of the season. The A1:A2 ratio for the total population was 1:1 (86:80, 52%:48%). The transgenes conferring resistance worked well in young plants, and no host pathogen specificity was observed.