2017 ASHS Annual Conference

A key challenge in plant breeding is to differentiate among genotypes in an efficient and reliable manner. For disease resistance, greenhouse screenings are often used to select as early as possible. Greenhouse-based tests should correlate with field results. Our objective was to develop protocols for phenotyping the reaction of genotypes of tropical pumpkin (Cucurbita moschata) inoculated with Zucchini yellow mosaic virus (ZYMV) or Papaya ringspot virus (PRSV). We inoculated cotyledons, then used ELISA to measured titer in the first 4 leaves. Samples were taken as each leaf expanded. For ZYMV, differences among genotypes were clear early: in the 1^st or 2^nd leaf. Symptoms of ZYMV could also be observed early, by the time the 2^nd leaf expanded. The situation for PRSV was different. Genotypes could not be differentiated until the 4^th leaf. Mottling could sometimes be observed earlier in susceptible genotypes, however symptoms were clear in the 4^th leaf. In another experiment seedlings were inoculated with PRSV or ZYMV and ELISA readings were taken in the greenhouse and at two dates in the field. In general, Spearman correlations between ELISA in the greenhouse and field were low. For PRSV, the correlation between greenhouse readings and field readings (5 weeks later) was r = 0.48. The correlation was not significant between greenhouse and the 2^nd field readings 14 weeks post-seeding (r = 0.26). Results for ZYMV were similar: r = 0.54 (p<0.001) and r = -0.14 (NS) between readings in the greenhouse and the 1^st and 2^nd field ELISA tests, respectively. Finally, we conducted a study to determine if leaves from multiple apices from a single plant need to be sampled when evaluating resistance. We sampled two newly-expanded leaves from plants that had been inoculated and transplanted to the field. For both PRSV and ZYMV, rank correlations between the two samples from a plant were high (r = 0.81 for PRSV and r = 0.66 for ZYMV). Based on our studies, we recommend that the 4^th leaf of an inoculated tropical pumpkin seedling be used to compare genotypes for PRSV resistance using ELISA. Genotype differences for ZYMV can be tested in the 1^st or 2^nd leaf. In the field, a single leaf sample may be adequate to establish the level of resistance in a plant. More study is needed concerning the relationship between greenhouse and field results.