Screening the watermelon germplasm collection for resistance to Papaya Ringspot virus type-W

Papaya ringspot virus watermelon strain (PRSV-W), formerly watermelon mosaic virus-1, is a major disease of watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai]. The objectives of this study were (i) to screen the USDA watermelon germplasm collection for PRSV-W resistance, (ii) to verify the disease rating for the most resistant and most susceptible accessions, (iii) to determine the number of escapes on the basis of the retest of the germplasm screening test. The experiment was a randomized complete block with five replications and 1275 accessions. 'Charleston Gray' susceptible checks were used to verify that the PRSV-W inoculum was virulent. Enzyme-linked immunosorbent assay (ELISA) was performed after the last rating to determine whether the virus was in the plant tissue. The PI accessions with the highest resistance to PRSV-W that also had resistance to other watermelon viruses (ZYMV, zucchini yellow mosaic virus or WMV, watermelon mosaic virus, formerly watermelon mosaic virus-2) were PI 244018, PI 244019, PI 255137, and PI 482299. The first retest of the most resistant 21 PI accessions showed that there were some escapes that were not resistant to PRSV-W. Of the 21 PI accessions in the retest, seven PI accessions were identified for further testing. Of the 60 resistant PI accessions in the final retest, eight had resistance with a rating of 3.6 or less for the best, average, and maximum ratings: PI 244017 (best over all tests), PI 244019, PI 482342, PI 482318, PI 485583, PI 482379, PI 595203, and PI 244018.