Pathogenicity of fungi associated with melon vine decline and selection strategies for breeding resistant cultivars

Melon Vine Decline is a severe rot root disease of increasing world-wide im portance. In Eastern Spain it is related to the presence of Acremonium cucu rbitacearum and Monosporascus cannonballus. The strong influence of environ mental conditions on the progress of this disease has made its study diffic ult. A field screening of Cucumis melo accessions has been conducted over f our years. Simultaneously, the pathogenicity of isolates of the two fungi r ecovered from the screening field was studied. These were more aggressive t han other Spanish and American isolates. Percentage of vine decay was score d, together with root damage, the latter being evaluated by using four scor ing systems based on root characteristics and disease severity. Root inspec tion allowed the selection of resistance sources, even when aboveground sym ptoms did not appear, due to the lack of environmental stresses at time of fruit maturity. The root damage scoring systems provided for each genotype a measure of the potential risk of suffering vine decline if environmental stresses occur during fruit maturity. The accession C. melo var. agrestis Pat 81 consistently exhibited high fiel d resistance level, expressed as a higher percentage of symptomless plants, together with a significant delay in symptoms appearance. The F-1 hybrids derived from the cross Pat 81 x C. melo susceptible varieties showed an int ermediate level of resistance between the parents, suggesting a partial dom inance gene action. The high resistance level found in Pat 81, and also in its derived hybrids, against the aggressive isolates found in this area, ma kes it promising for breeding melon varieties resistant to melon vine decli ne.