Epidemiologia comparativa das ferrugens marrom e alaranjada em genótipos de cana-de-açúcar

Abstract

Diseases are considered limiting factors to sugarcane productivity. Among the most relevant are brown rust (Puccinia melanocephala) and orange rust (Puccinia kuehnii), causing losses between 10 and 50% in crop productivity. The epidemiological study of these diseases can elucidate relevant factors in the plant-pathogen-environment interaction, at a regional level, and point out parameters and useful variables for the selection of resistant varieties within crop genetic improvement programs. In this context, the objective of the present study was to evaluate the reaction of sugarcane genotypes to brown and orange rust, through the quantification of parameters and epidemiological variables. In the field, the severity of the disease was quantified in 17 genotypes, under conditions of natural infection of the pathogens, with the aid of diagrammatic scales, every 30 days. From the severity values, the progress curves were constructed and the values of the Area Below the Disease Progress Curve (AACPD) were calculated. The reaction of the 17 genotypes to orange rust was also evaluated in a greenhouse. For this purpose, 60-day-old plants were inoculated with a suspension of 1x104 urediniospores.mL-1. Incubation and latency periods were determined from daily assessments. After the appearance of the first symptoms, the severity and the number of pustules per cm² were quantified on leaves +1, +2 and +3. Under field conditions, Ferrugens brown and orange progress curves showed gradual evolution, with different rates of progress between genotypes. Orange rust showed higher severity peaks than brown rust. Four genotypes showed complete resistance to brown rust and 12 showed partial resistance. For orange rust, there were three and 13 genotypes, respectively. In a greenhouse, the incubation period of orange rust varied between 7 and 11 days and the latency period between 15 and 22 days, in most genotypes. Leaf +1 was the one that best represented the evolution of the disease in the greenhouse, being the most suitable for evaluating epidemiological variables in this condition. There was agreement between the results observed for orange rust under field conditions and in a greenhouse. As observed in this study, the epidemiology of the diseases differs in some aspects, which were elucidated through the differential responses of the evaluated genotypes. Genotypes with high levels of resistance to both diseases have been identified and are promising for commercial use and/or as elite parents in sugarcane genetic improvement programs.