Avaliação da sensibilidade de isolados de Cercospora lactucae-sativae Sawada estrobilurinas e avaliação de componentes epidemiológicos entre isolados sensíveis e resistentes

Abstract

The spot cercospora caused by Cercospora lactucae-sativae Sawada is a common disease in lettuce production fields. The disease occurs in the leaves, especially the older ones, and the main symptom is necrotic spots may generate large losses for the producer. Control of the disease in the wild region of Alagoas is mostly done by the use of a fungicide of the strobilurin chemical group, which is even used without being registered for the control of disease in the crop. Studies were conducted to evaluate the effects of the fungicide on the populations of the pathogen by susceptibility testing and evaluation of epidemiological components. We selected 20 isolates and the variable tested was the mycelial growth plates. The sensitivity tests were conducted with and without the presence of salicylhydroxamic acid (SHAM). The resistance factor (RF) was determined to ascertain the level of resistance among isolates less sensitive compared to the most sensitive. Additionally, four strains (two most sensitive and least sensitive 2) were used for epidemiological review of the following components: incubation period (IP); latent period (LP); severity of the disease; and percentage of germination of spores produced in vivo and in vitro (in the pathogenesis). To effectively isolates whose concentration was able to inhibit 50% of the mycelium growth (EC50) can be estimated with and without SHAM, EC50 values were 0.07 to 1.98 μg ml-1 and from 0.007 to 0.339 μg ml-1 SHAM absence and presence, respectively. For the remaining isolated, with lower sensitivity, so called as resistant, there were two situations: i) the EC50 was estimated only in the presence of SHAM to two isolated, 77D and 76C, whose values were, respectively, 11.32 and 13, 52 μg ml-1; and ii) the EC50 could not be estimated with or without the presence of SHAM for four isolates (77B, 77C, 77E, and 76E). FR ranged from 1 to 26 for the isolated considered sensitive and 147-175 for the isolated considered insensitive. Due to the high RF found, it is suggested that the G143A mutation may be isolated occurring insensitive. With regard to the epidemiological components, PL and severity values were significantly (P = 0.05) lower and higher, respectively, for the resistant isolates. For the other variables, there was no difference between sensitive and resistant isolates.