Monosporascus root rot and vine decline caused by Monosporascus cannonballus is one of the most important melon yield-limiting diseases in northeastern Brazil. This study investigated the fitness components of 57 isolates of M. cannonballus obtained from Brazilian melon fields by evaluating: i) their mycelial growth rate (MGR), and perithecia and ascospore production (PP and AP) on potato dextrose agar (PDA); ii) their sensitivity to the fungicide fluazinam; and iii) their virulence to melon seedlings. All M. cannonballus isolates showed variability in their MGR, PP and AP values. They were sensitive to the fungicide fluazinam, showing some degree of mycelial growth inhibition (MGI), and were pathogenic to melon seedlings, with a mean disease severity index (DSI) of 62.1%. By univariate analysis, the formation of groups of similarity amongst the isolates of M. cannonballus within each variable was not limited by the area of origin of each isolate, given that in most situations, different isolates of the same area were distributed into distinct groups of similarity. A multivariate cluster analysis allowed the separation of the 57 M. cannonballus isolates in 18 groups of similarity. The fitness variability among M. cannonballus isolates found in this study should be considered when possible sources of resistance are evaluated in melon breeding programs.

A variabilidade de 53 isolados de Myrothecium roridum, obtidos de diferentes cultivos de meloeiro (Cucumis melo) do Estado do Rio Grande do Norte, foi estimada com base em variáveis relacionadas à: severidade do cancro-de-mirotécio nas cultivares AF-682 e Orange Flesh, fisiologia do patógeno (taxa de crescimento micelial - TCM - e esporulação - ESP) e sensibilidade ao fungicida fluazinam (ICM). Todos os isolados foram patogênicos às cultivares AF-682 e Orange Flesh, porém, nessa última houve maior discriminação entre isolados, quanto às variáveis mensuradas. A variabilidade entre os isolados foi alta quando se analisaram as variáveis medidas, com exceção de TCM e sensibilidade a fungicidas (todos sensíveis). Não foi verificada correlação significativa da SEV com as demais variáveis. A variação entre isolados dentro de município (60%) foi maior que a variação entre municípios. Provavelmente, há alta variabilidade genética nas populações de M. roridum e não foi possível detectar associações entre as variáveis analisadas e localidades amostradas.

The variability of 53 isolates of Myrothecium roridum obtained from melon (Cucumis melo) fields in the State of Rio Grande do Norte (Brazil) was estimated based on: severity of Myrothecium stem canker in the cultivars AF-682 and Orange Flesh, the pathogen's physiology (mycelium growth rate - MGR - and sporulation - SP) and its sensitivity to the fungicide fluazinam. All isolates were pathogenic to AF-682 and Orange Flesh; however, better discrimination of pathogen variability was achieved in the latter. Variation among isolates was high when assessing all variables, except for MGR and sensitivity to fungicides (all sensitive). There was no correlation between SEV and any other variables. The variation among isolates within municipalities (60%) was higher than the variation among municipalities. There is evidence of high genetic variability in the populations of M. roridum, but no association between the assessed variables and locations was detected.