Cultura da alface : produção de mudas utilizando Bacillus spp., escala diagramática para cercosporiose e levantamento da doença em Pernambuco

Abstract

Lettuce (Lactuca sativa L.) is one of the most consumed vegetables in the world. In spite of the importance of this crop in Pernambuco Brazil, there are no studies on the use of plant growth-promoting rhizobacteria for production of organic transplants, as well as a diagrammatic scale for evaluating the severity of Cercospora leaf spot (Cercospora longissima) in epidemiological surveys. Epiphytic and endophytic bacterial strains applied through simultaneous bacterization on seedlings and plants were tested in greenhouse using cv. Verônica. In field the strains C25 (Bacillus thuringiensis subvar. kenyae) and C116 (Bacillus pumilus) were tested separately and in mixture using cvs. Verdinha and Verônica. Seedlings treated with C116 showed significant increase in relation to controls for root fresh weight, shoot fresh weight and total fresh weight as well as those treated with C25 for root fresh weight and total fresh weight. The strain C25 also significantly increased the level of foliar N. Aiming tostandardize methods to quantify the severity of Cercospora leaf spot of lettuce, a diagrammatic scale was developed with 1, 3, 6, 11, 20, 34, 51 and 68% diseased leaf areas. The diagrammatic scale was validated by 10 raters using 50 leaves with different levels of severity, previously measured by the software AutoCAD® R14 for Windows. There were performed two evaluations with a 7-day interval in which different sequences of the same leaves were visually estimated by the same raters. The accuracy and precision of each rater was determined through simple linear regression between actual and estimated severity. The scale proportioned good levels of accuracy and precision, and despite severity tended to be underestimated, the absolute errors were around 10%. Generally the raters showed good repeatability of estimative. Surveys ofCercospora leaf spot severity were performed in commercial plantations, including conventional and organic systems, in Mata and Agreste of Pernambuco, in two planting seasons. Disease prevalence in conventional systems ranged from 92 to 100% and in organic systems from 88 to 100%. In conventional plantings the severity significantly varied from 0.84 to 16.86 in January-April and 0.00 to 2.44 in June-September. In this system significant differences were observed among the counties Chã Grande (4.99) and Vitória de Santo Antão (9.14), only in January-April. Still in the conventional farming, there were no significant differences of disease severity among groups, cultivars of lettuce and number of fungicide applications within each season. In the organic system disease severity varied significantly from 0.07 to 22.53 in January-April and from 0.00 to 15.18 in June-September. In this system there were no significant differences fordisease severity averages in Bom Jardim, Chã Grande, Glória de Goitá and Gravatá, within each season. In January-April, but not in June-September, the average of Cercospora leaf spot severity significantly differed among groups and cultivars of lettuce. The group Lisa cv. Verdinha (0,68), showed significantly less severity than the groups Crespa cv. Verônica (10.61) and Americana cv. Lucy Brown (7.71). The Pearson test did not find significant correlations for severity between the studied seasons in both conventional and organic systems. The Kruskall-Wallis test did not detected significant differences for severity among the conventional and organic systems in each period. However there was significant difference for severity between the two seasons in each system.