Methods for screening watermelon for resistance to papaya ringspot virus type-W

Papaya ringspot virus-watermelon strain (PRSV-W) affects all agriculturally important species of the Cucurbitaceae, and is of economic interest because of its destructiveness. The objective of this study was to develop a consistent and reliable method to screen watermelon for resistance to PRSVW. PRSV-W isolates 1637, 1870, 2030, 2038, 2040, 2052, 2169, 2201, 2207, and W-1A were maintained in ‘Gray Zucchini’ squash, and were used in the inoculations. Three experiments were run, a preliminary experiment to determine the important factors involved in disease development, a main experiment to quantify the effects of those factors, and a retest of three cultigens to determine test variability. The experiment was a split-plot treatment arrangement in a randomized complete block design with four replications. Whole plots were growth stage (cotyledon, first true leaf), subplots were pot size (55 or 100 mm), and sub-subplots were the 10 isolates. Plants were rated on a scale of 0–9 for each of three traits: leaf necrosis, mosaic symptoms, and leaf deformation. We found the best method for a screening of the watermelon germplasm collection for resistance to PRSV-W is to grow the seedlings in square, 100 mm diameter pots (or 55 mm diameter pots if uniform germination is expected) and inoculate plants at the first true leaf stage using PRSV-W isolate 2052 and the rub method. Significant differences were obtained (with LSD values of 0.6–1.5) using four replications of five plants per plot, but fewer replicates and plants may be adequate for a large germplasm screening experiment. The method can be used by researchers interested in screening for PRSV-W resistance in watermelon, verifying that resistance, studying its inheritance, and transferring it to elite cultivars. # 2002 Elsevier Science B.V. All rights reserved.