New sources of melon accessions with resistance to bacterial fruit blotch at different phenological stages of melon growth and to multiple strains of Acidovorax citrulli

Abstract

Bacterial fruit blotch, which is caused by Acidovorax citrulli, is one of the most common diseases of melon crops. To date, there is no disease-resistant commercial cultivar, and management strategies have not been efficient. Therefore, the objectives of this study were to identify melon accessions with genetic resistance to bacterial fruit blotch at different phenological stages of plant growth and to multiple strains of the pathogen. The disease severity was estimated using descriptive scales and evaluated by inoculating A. citrulli into seeds (55 accessions), seedlings, and plants before flowering (38 accessions). After assessing the stability of seven accessions to three strains of the bacterium, two accessions were selected to evaluate their resistance at the flowering and fruiting stages and to perform a transmission assay and optical and scanning electron microscopic analyses. Most accessions showed varied responses (resistance vs. susceptibility) at the different phenological stages of melon growth. However, accessions I-136 and A-43 showed high levels of resistance in most assays and in the stability assay. The fruits of these accessions showed no symptoms of bacterial fruit blotch, and seed transmission of A. citrulli showed values of only 0.6% (I-136) and 2.5% (A-43). Microscopy showed few structural and temporal changes in the colonization of A. citrulli in the leaf mesophyll tissues of these accessions, which can be associated with resistance. The results suggest that accessions A-43 and I-136 can act as donors of bacterial fruit blotch resistance genes, which can be introduced into commercial cultivars through genetic improvement programs.