Resistência à murcha bacteriana em linhagens e híbridos de tomateiro

Abstract

Among the Solanaceae family, tomato plant is the second most produced in the world, behind only the potato. In Northeast of Brazil, its cultivation has been limited to sub regions of the Agreste and the Sertão, as a result of plant diseases problems, especially bacterial wilt. It is a disease caused by a complex of Ralstonia solanacearum species, and it has been reported in more than 450 species of plants. The losses caused by this disease are severe in plantations occurred during rainy summers and inside of greenhouses. This study aims to identify and obtain tomato germplasms resistant to bacterial wilt from 23 lines of tomato and 24 experimental hybrids. It was utilized the isolates CRM 74, CRM 76 and CRM 77 of Ralstonia pseudosolanacearum for the evaluation of lines, and the isolates CRM 74 and CRM 77 for evaluation of the hybrids. The 23 lines, with seven controls, were sowed, transplanted after 21 days and inoculated with 15 ml of the bacterial suspension with 5x108 CFU ml-1 per vase of 500 ml. The experiment had a completely randomized with three repetitions and every parcel had four vases with one plant each. Evaluations were carried out observing the incidence and severity of the disease and based on the result it was calculated: bacterial wilt index (IMB), incidence (INC), latency period (PL 50) and area below the curve of disease progression (AACPD). The data were submitted to analysis of variance and the means were grouped by the Scott-Knott test at 5% probability. Besides that estimating the phenotypic, genotypic and environmental correlation coefficients between the resistance components and calculating the dissimilarity of the Mahalanobis distance (D) to determine the genetic divergence among the lines. The Yoshimatsu and Hawaii 7996 were used as testers in crosses with the 10 lines of best results tomatoes and cultivars IPA-6 and Santa Clara. Subsequently, F1 hybrid was evaluated for bacterial wilt resistance during 15 days, analyzing the incidence and disease severity (SEV) using a descriptive scale notes from 0 to 4. The data were used to determine the general (CGC) and specific combining (CEC) capacities to four resistance components, estimating genetic parameters such as heritability, phenotypic, genotypic and environmental variance. The interaction genotypes x isolates presented significant difference at 1% probability only for the variables latency period and area below the disease progress curve. Considering bacterial wilt index, lines L04, L42, L49, L53, L82, L120, L125 and L128 were classified as resistant, but for incidence there were no lines that behaved similarly to the resistant controls Hawaii 7996, Yoshimatsu and Woodstock. As for genotype correlations, in 100% of the pairs of characters, values equal to or slightly higher than the phenotypic correlations were observed. Similar to these two, in 50% of the cases, were slightly higher than the environmental correlations, showing that the environment favored as the same form for IMB x PL 50, IMB x AACPD and PL 50 x AACPD. For the analysis of the dendrogram, a cut around 15.2% dissimilarity allowed the formation of four distinct groups and four subgroups. Group I was composed of resistant genotypes containing: Hawaii 7996, Woodstock, Yoshimatsu, Tropithai, L04, L42, L49, L53, L82, L125, L120 and L128, in addition to lines with moderate resistance. Considering the combining abilities of the experimental hybrids, the crosses that showed the greatest potentials for resistance were the YOS x L04 with the two isolates, and the HAW x L125 with the isolate CRM 74, since they were the only ones with a positive CEC for PL 50 and negative for the others characters. Also, presenting values from intermediate to superior that highlighted for the character PL 50, besides a high value for CEC, and at least one of the parents presented a high CGC value, which it is desirable. Observing the genetic parameters, the estimated values of the genetic variance were higher than the environmental variance for all the resistance components studied for the two bacterial isolates. However, it was observed in this study that the gene expression of the tomato resistance phenotype is resulted from the action of additive and non-additive gene effects, where the non-additive gene effects are involved in the four resistance components and that the additive effects are involved in the IMB, INC and AACPD.