Qualificação da severidade da requeima em tomateiro por escalas diagramáticas e suas correlações com a fluorescência da clorofolifa A e acúmulo de biomassa e nutrientes

Abstract

Defining methods of quantification of tomato late blight, caused by Phytophthora infestans, is important to many researches about strategies of management of the disease. The aim of this present study was to assess and to validate the diagrammatic grading keys - Simplified, James-modified and Broad - and to determinate the possible correlations between the measures of severity and plants’ physiological and nutritional state and biomass accumulation. Two simultaneous experiments were carried out: one in greenhouse and the other in field conditions. Two cultivars were studied in greenhouses, Super Sweet and ‘Perinha Água Branca’, a resistant and a susceptible to late blight genotypes, respectively. The plants were inoculated or not with a sporangia suspension, and submitted to ten evaluations: 1, 2, 3, 4, 6, 7, 9, 10, 17 and 24 days after inoculation. The avaliations were performed in the fifth oldest leaf, and eighth and eleventh leaves, by visual estimation of severity, using three diagrammatic grading keys, Simplified, Broad and James-modified, but also measuring photosynthetic potential (Fv/Fm), by chlorophyll “a” fluorescence emission. In field conditions, four cultivars were evaluated – ‘Perinha Água Branca’, Super Sweet, Santa Clara and Carmen F1. Periodically, late blight severity was quantified using the same three grading keys in leaves of different plant parts, at 51, 62, 74, 94 and 102 days after transplant (DAT). At the day following each severity disease evaluation, one plant was harvested per plot to measure the accumulation of biomass and macronutrients of different plant organs. Fruit yield was determined by summing fruits harvested between 68 and 114 DAT. In greenhouse conditions, the estimative of late blight severity with the three diagrammatic grading keys proved to be adequate to the disease’s quantification, enabling the discrimination between inoculation effects and differences between the cultivars in four and in nine days after inoculation, respectively, which were confirmed by measures of the photosynthetic potential of the leaves. In field conditions, the diagrammatic grading keys Simplified and Broad were more adequate to tomato late blight quantification. Quantifications in the youngest leaves, in the medium and superior third part of the plant, were the most representative of the disease progress and also discriminated the cultivars regarding disease resistance. Negative correlations were observed between severity, estimated with the three grading keys considering the whole plant and its medium third part, and leaf dry mass and leaf concentrations of K and P. The cultivar ‘Perinha Água Branca’ showed the lowest stem and leaf dry mass at the beginning of plant evaluations, and also late fruit yield, but had the higher stem and leaf mass and at the end of the experiment. The cultivar Perinha Água Branca and the hybrid Carmen F1 had higher commercial fruit yield. Based in the results, it is recommended the use of Simplified diagrammatic grading key and evaluations in the medium or superior tomato third to quantify the late blight disease.