Characterization and Deployment of Recessive Resistance to Phytophthora capsici in Capsicum annuum

Resistance to two highly virulent isolates of Phytophthora capsici was confirmed in Capsicum annuum lines Fidel and CM334.  In order to assess inheritance of the resistance in Fidel, segregating populations were created by controlled pollinations in a greenhouse. F₁, reciprocal F₁, F₂ and backcross seed were generated with five susceptible lines or cultivars: Hidalgo Serrano, TM6, YB50, MJ201, and Mesilla. Seeds were planted in pasteurized commercial growing media consisting of ground peat moss and perlite.  Inoculum was prepared by culturing two isolates of P. capsici (New Mexico and Zacatecas) on V8 agar for 10 days to allow mycelial coverage of the plates.  The plates were flooded with sterile water and allowed to sit for 3–4 days, followed by shocking at 10 °C to induce zoospore release. Zoospore counts were conducted with a hemacytometer under a microscope at 10x, followed by dilution to 5,000 spores/ml.  Seedlings were inoculated after 35 days at the 4–6 true leaf stage with 2 ml of the spore suspension, providing approximately 10000 spores per plant.  Resistance expression in the F₁ was consistently close to zero, regardless of the susceptible parent.  Segregation ratios for resistance in the F₂ families varied based on the susceptible parent.  The ratios of susceptible:resistant progeny for families with TM6, Hidalgo and MJ201 parents suggested the possibility of duplicate, dominant gene action for susceptibility.  This would mean that two recessive genes would be required for resistance.  The families with the susceptible parents- YB50 and Mesilla, had much fewer resistant progeny. The Chi-square values for their susceptible:resistant ratios suggested that 3 recessive resistance genes would be a better fit.  One possibility could be that TM6, Hidalgo, and MJ201 share a common recessive resistance allele with Fidel.  Another explanation could be that the resistance threshold effect of inoculum was lower for YB50 and Mesilla, leading to more susceptible plants at 10,000 spores.  The threshold for these isolates was established at 20,000 spores/ml for Fidel, above which resistance expression began to break down.  At 50,000 spores, resistance was completely overcome for all resistant genotypes—Fidel, CM334, and PI 201234.  Nevertheless, this recessive resistance has been successfully backcrossed into New Mexico chile and Ancho breeding lines, which will soon be trialed in infested grower fields.