Pathogenicity phenotypes of Fusarium spp. associated to citrus dry root rot: primary results

Many Fusarium spp. are supposed to be casual agents of citrus dry root rot, whose identification can be very problematic. The aim of this work was to characterize Fusarium spp. into non-pathogenic, mild, intermediate and severely pathogenic strains. Six Fusarium oxysporum and seven F. solani strains, representative of 79 isolates, collected from citrus rhizosphere that grouped according the sequence of ?-tubulin and ?-elongation factor loci into 9 phylogenetic clusters, were used in this study. Three pathogenicity characters were assessed: phytotoxicity of fungal excretions, capacity to colonize the surface of citrus host plant, and ability to colonize the internal tissues. All trials were carried out on Carizzo citrange. Symptoms of wilting were assessed over two months with a phytotoxicity of culture filtrate trial. Citrus plantlets grown in vitro under controlled conditions, were inoculated with Fusarium strains, and the ability of the pathogen to surface colonize and evoke alterations to the host were visually assessed. The ability of the pathogen to proliferate and colonize host tissues was estimated by CFU/g of plant fresh weight. The results highlighted the significant correlation between phytotoxic excretions and invasion ability. No correlation of this two traits with ability to colonize citrus plantlet surface was detected. This study also revealed the considerable variation of severity among Fusarium strains belonging to different phylogeny clusters. In conclusion both genotype and excretion phytotoxicity assay seem good predictor of aggressiveness.