Identification of sources of resistance to damping-off (Rhizoctonia solani) in two phenological phases of watermelon

Sources of resistance to damping-off caused by Rhizoctonia solani were researched in watermelon accessions, in which thirty accessions were inoculated in two phenological phases (at seeding and seedling stages). The completely randomized design with five replications per accession was used in both phenological stages. The inoculation in seeding stage was carried out at the time of sowing, while for seedling the inoculation was done 15 days after sowing. In both stages the evaluation occurred 15 days after inoculation using a rating scale. At seeding stage, six accessions were moderately resistant to CMM-2967 of R. solani. However, no accession resistant to CMM-1053 and CMM-1052 isolates was recognized. Three accessions were resistant to CMM-1053 at the seedling stage. Posteriorly, inoculation was carried out using only accessions classified as resistant in the previous evaluations using both phenological stages. For CMM-1053, factorial scheme of 2x4 were used (two stages and four accessions). For assessments against isolate CMM-2967 a factorial scheme of 2x12 (two stages and 12 accessions) was used. We confirmed the results of first two experiments for both isolates. At seeding stage, all accessions were classified as susceptible to CMM-1053. However, at the seedling stage, only Crimson Swett was susceptible. For the CMM-2967, inoculated at the seeding stage, the accessions were classified into two groups (resistant and susceptible) (Scott-Knott at 5%). At seedling stage there was no statistical difference among eleven accessions and they were all classified as resistant. Due to difficulty of finding sources of resistance to damping off, the method of inoculation at seedling stage showed more efficient for this pathosystem and could facilitate the work of breeders and plant pathologists.