Seleção de progênies de alface para resistência a doenças e caracteres morfoagronômicos

Abstract

Lettuce (Lactuca sativa L.) is the most important leafy vegetable economically for Brazil, being one of the most consumed in salads and with great popular acceptance. Due to its short cycle, areas planted with lettuce are usually submitted to successive crops, favoring the occurrence of soil pathogens such as Meloidogyne spp. and Pectobacterium carotovorum subsp. carotovorum, which promote large losses in the field. The lettuce breeding for multiple characteristics economically makes possible the crops of this vegetable in the edaphoclimatic conditions of the Brazilian Northeast, by providing cultivars with resistance to diseases and adapted to the crops in tropical regions. In this sense, the objective of this work was to obtain butterleaf lettuce progenies resistant to soft rot, with tolerance to early bolting and with commercially desirable morphoagronomic characteristics. The lettuce breeding program started with the hybridization between the Vitória de Santo Antão cultivar, resistant to soft rot, and the Beta lineage, resistant to root-knot disease, and the advancement of generations to obtain F2 progenies. Five experiments were then carried out with parental and cultivares Grand Rapids, Regina 71 and Tainá used as control: Experiment 1 - in greenhouse, 608 F2 plants were inoculated with Pcc-23 isolate of P. carotovorum subsp. carotovorum and with M. incognita race 1. Selected for resistance to soft rot by evaluating the symptoms of the disease with scale from 1 to 9 and preselected for resistance to root-knot disease by evaluating the galls in the lettuce roots visible still with substrate with scale from 1 to 5. The plants with scores up to three in the two evaluations were selected and obtained the F2:3 progenies; Experiment 2 - in greenhouse, ten F2:3 progenies were grown in trays of expanded polystyrene - EPS under blocks at random with three replicates and eight plants per plot. The plants were inoculated with Pcc-23 isolate of P. carotovorum subsp. carotovorum and evaluated for resistance to soft rot; Experiment 3 - in field at UFRPE, six F2:3 progenies were arranged under blocks at random with three replicates and eight plants per plot. The progenies were selected for tolerance to early bolting, by number of days from sowing until the first anthesis of the plant, and were obtained F2:4 progenies; Experiment 4 - In greenhouse, 50 F2:4 progenies were crop in EPS trays under blocks at random with four replicates and sixteen plants per plot, inoculated with Pcc-23 isolate of P. carotovorum subsp. carotovorum and evaluated for resistance to soft rot; Experiment 5 - in field at IPA-Vitória de Santo Antão, 50 F2:4 progenies were arranged under blocks at random with four replicates and sixteen plants per plot and the following morphoagronomic characteristics were evaluated: plant diameter, planta height, fresh mass of the plant, number of leaves, leaf length, leaf width, border type, leaf leaflet, leaf color and leaf brightness, and colorimetric analysis on the leaf. In the experiment 1, 333 progenies with scores up to 3 were selected for both diseases, of which 92.5% were died and 16 progenies produced F2:3 seeds. Among the F2:3 progenies evaluated in experiment 2, progenies 023; 075; 172; 202; 568 and 610 were moderately resistant to soft rot. In experiment 3, the progeny 202 showed transgressive performance and was tolerant to early bolting. Among the 50 F2:4 progenies evaluated in experiment 4, 27 progenies were moderately resistant to soft rot and the progeny 023-27 obtained the lowest mean (2.12). And in experiment 5, the progeny 202-11 presented morphoagronomic characteristics of interest for the lettuce breeding program, obtaining the highest averages for most of the characteristics. It was identified a negative genotypic correlation of leaf width with number of leaf (-0.90) at 1% probability and with fresh mass of the plant (-0.59) at 5% probability. These two characteristics, of great importance for being directly related to lettuce production, obtained good genetic gains with value of 12.09% for number of leaf and 13.44% for fresh mass of the plant. By evaluation the most important commercially morphoagronomic characteristics, progenies F2:4 172-12; 202-11; 172-08; 202-42; 023-02; 202-37 and 202-30 were indicated for selection.