Introduction: Spondias tuberosa is a tree endemic to the semiarid region of Brazil with fruticulture potential. Objective: To estimate the diversity and genetic structure of S. tuberosa accessions from four areas of the semiarid region of Brazil, in order to facilitate conservation genetic resources studies in this species. Methods: DNA was extracted, using the CTAB 2x method, from leaf samples of 24 accessions of S. tuberosa available in the germplasm bank at Embrapa Semiárido, Brazil. Ten microsatellite loci were used in this study. Results: The UPGMA dendrogram, generated with a Jaccard coefficient similarity matrix, contains four groups at a 0.44 cutoff point. The similarity coefficient ranged from 0.30 to 0.84, indicating great divergence among the accessions. A Bayesian analysis conducted with the software Structure suggests there are two subpopulations, one formed by accessions from the Januária region and another by accessions from the Juazeiro, Uauá and Petrolina regions. The ΦST value of 0.12 for the analysis of molecular variance indicates moderate genetic differentiation among the four populations, suggesting that the genetic variability is moderately structured in function of region. Conclusions: Together, the analyses indicate that the genetic diversity of S. tuberosa is not uniformly distributed in the studied regions. Thus, germplasm from a greater number of populations should be collected to increase the germplasm bank genetic diversity of the species.

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.

No abstract

This study aimed to find sources of resistance to damping-off and to determine the inheritance of resistance in watermelon.Firstly, 72 watermelon accessions were inoculated with CMM 1053 isolate of Rhizoctonia solani. Only two accessions (BGH 29 and BGH 76) were considered moderately resistant to the disease. The inoculated plants without symptoms from accession BGH-29 identified as resistant were self-fertilized and the inbred line S 1 was used for the crosses. The reaction to R. solani after inoculation of the segregating populations of the cross and backcrosses between susceptible cultivar Crimson Sweet and the inbred line BGH-29 was evaluated. In other experiment eight S1 lines of watermelon were inoculated with six different isolates of R. solani and the reaction was evaluated by partial diallel analysis using the Griffing's method IV to obtain estimates of general resistance ability (GRA), general aggressiveness ability (GAA), and specific interaction ability (SIA). According to ratings of the susceptible and resistant parents, generations F 1 , F 2 , BC 1 (F 1 × P 1 ) and BC 2 (F 1 × P 2 ), resistance to damping-off was conferred by at least nine genes with additive effect and low heritability which indicates polygenic inheritance. The results of the diallel analysis corroborate with the analysis of segregating population, indicating inheritance of resistance was horizontal, since 46.1% of the total sum of squares of the variation observed in the crosses was due to GRA, while the SIA corresponded to only 20.39% of variation. This is the first report about inheritance of resistance to damping-off caused by R. solani in watermelon.

No abstract