Phytotoxins from Dactylonectria torresensis involved in replant disease of fruit trees

Abstract Dactylonectria torresensis, together with Fusarium spp. is one of the most frequently soil-borne fungal pathogens associated to “replant disease”, a phenomenon of multicausal origin that manifests as growth reduction, dead on newly planted trees and crop decline in adult orchards. Due to difficulties to reproduce with pathogenicity tests the impact on root development of the major soil borne fungi associated to “replant disease” and recent evidences on mediation of Fusarium phytotoxins on root development at rhizosphere level, a bioassay was carried out to evaluate the impact of extracellular compounds of D. torresensis on fruit tree growth. The response of in vitro shoots of grapevine, apple and peach rootstocks (K5BB, M.26 and, GF677 respectively) to extracellular exudates of D. torresensis was evaluated. Toxicity of crude sterile culture filtrates of two D. torresensis isolates from apple roots were tested at 3 concentrations in a common medium (MS medium) for in vitro plant propagation. Low molecular weight (MW) secondary metabolites (50-1000 g mol-1 MW) were also identified using liquid chromatography time-of-flight mass spectrometry. In vitro plants of all tested plant genotypes showed significant root inhibition and shoot reduction from lowest dose in filtrate amended MS medium as compared unamended control, suggesting phytotoxicity of extracellular exudates of D. torresensis. Chlorosis and abnormal leaf growth observed in above ground part of in vitro plants grown in filtrate-amended-media suggested interferences on plant functionalities rather than a simple growth inhibition due to severe root reduction. Those findings were consistent with the mode of action of three phytotoxins found in fungal filtrates from both fungal isolates, they were: tentoxin, HC toxin and zearalenone. These phytotoxins are non host-specific or low host-specific and act at cell and plant functional level without causing specific symptoms such as tissue necrosis. Biological properties of these phytotoxins along with the other compounds detected in culture filtrates, among which antibiotic and antifungal compounds such as nidulin, rabelomycin and biochanin A, should help to elucidate the complex patho-system D. torresensis - fruit tree.