Cancro-de-mirotécio do meloeiro : variabilidade do patógeno e avaliação da resistência em genótipos

Abstract

The Myrothecium stem canker caused by the fungus Myrothecium roridum is a disease associated to the “collapse" of melon (Cucumis melo) and it is increasing in importance in the fields of the Brazilian Northeast. In order to identify the variability in M. roridum isolates and to evaluate host genotype resistance to the pathogen, two studies were carried out. First, the variability of 53 isolates of Myrothecium roridum, obtained from melon fields in the Agropólo Mossoró/Assu (State of Rio Grande do Norte, Brazil), was estimated based on Myrothecium stem canker development and pathogen physiology related variables. Twenty-two-old melon plants (cvs. AF-682 and Orange Flesh) grown under greenhouse conditions, were wounded in the crown and inoculated with a pathogen suspension (2,8x106 conidia/ml), and the epidemiological components as area under the disease incidence progress curve (AUDPC) and disease severity six days after inoculation (SEV) were analyzed. Additionally, mycelium growth rate (MGR), spore production (SPO), and sensitivity to fluazinam (MGI) were measured in each isolate. High levels of variability were verified among the M. roridum isolates, with exception of the MGR. In the multivariate context, in which all variables wereused, was adjusted the model Yijk(AUDPC, SEV, MGR, SPO and MGI) = μ + αi + βj(i) + eijk (μ=general average, αi= municipality effect, βj(i) = effect of isolated nested inside municipality and eijk = experimental error. Was not effect of the municipality of origin of the isolate, however, there was high variability among the isolates of M. roridum inside of the municipality. In the second study, 150 melon genotypes were evaluated for resistance to one isolated of M. roridum (3x106 conidia/ml) and at this time, evaluations were performed, with the aid of a descriptive note scale varying from 0 to 4, until six days after the moist chamber been removed. The genotypes were distributed in five class of resistance reaction using the average data of the last evaluation. None of the genotype were immune or highly resistant to the pathogen, 26.7% were intermediate resistant (IR), 51.3% were susceptible (S) and 22.0% highly susceptible (HS). These results show the difficulty in obtaining genetic sources with high levels of resistance to M. roridum. The groups Charentais, Non-grouped, Galia and Cantaloupe showed the largest frequency of genotypes with IR reaction and the smallest frequency of HS genotypes. Most of the genotypes of the groups Yellow Valencian (66.7%), Cantaloupe(57.4%), Galia (60.0%) and Non-grouped (53.8%) were S. The genotypes ‘PL 420149’, ‘Caroline’, ‘A3’, ‘Chilton’ and ‘PS-1 Pele de Sapo’ showed the smallest values of disease severity and constituted promising resistance sources to the pathogen.