2018 ASHS Annual Conference

Phytophthora blight caused by Phytophthora capsici, a soil-borne pathogen, can be a devastating disease for cucumber. In cucumber, P. capsici specifically infects fruit, especially young fruit, leaving vines and leaves uninfected, necessitating a search for resistance in young fruit. The objective of this work is to identify quantitative trait loci (QTLs) associated young fruit resistance to P. capsici in cucumber. Cucumber accession, PI 109483, was previously identified as a source of resistance. The S₅ generation (line 109483-53) was used for doubled haploid (DH) production, and three resistant lines selected. The S₆ generation (109483-53 B5) was crossed with the susceptible pickling cucumber inbred line, Gy14, to develop F₂ and backcross populations. In the summer of 2017, 400 progeny from the F₂ population [(Gy14 x B5)x] along with parent lines and F₁ were screened in the field. In order to facilitate accurate phenotyping, plants were trellised to reduce contact of the fruits with the soil. This reduced wounding that can occur during the cleaning process to remove soil prior to inoculation, as well as lessening the probability of cross-contamination resulting from other pathogens in the soil. Harvested fruits were sanitized with 1% bleach, placed in sealed trays to maintain high humidity, and inoculated with 1×10⁴ zoospores/mL of P. capsici isolate Bartley’s 1. For the initial stage of screening, three replicate harvests were performed providing 5-25 fruits for each plant. The normal distribution of disease scores for the F₂ population indicated that young fruit resistance is a quantitative trait. Based on the initial result, the top and bottom 12% of plants were selected. Fruits were harvested from these individuals three additional times and inoculated with an elevated pathogen concentration (5×10⁴ zoospores/mL). The results from re-screening showed reproducibility of the disease scoring and accuracy of selection. To provide a second population for QTL-seq analysis, 400 F₂ progeny derived from DH line (A4-3) [(Gy14 x A4-3)x] is currently being phenotyped in the greenhouse. For QTL-seq analysis, 15 resistant and susceptible individuals will be selected from each F₂ population. DNA will be extracted and pooled for sequencing and analyzed for QTL to determine genomic regions associated with the resistance.