Studies on the potential role of root exudates in the interaction between musk melon roots and Fusarium oxysporum f. sp melonis

The interactions of resistant (Bingo, Giusto, Sweetness) and susceptible (Cantalupo di Charantais, Harper, Retato degli Ortolani) muskmelon cultivars with Fusarium oxysporum f. sp. melonis races 0, 1, 2 (FOM0, FOM1, FOM2) were studied through the effects of root exudates (REs), released by 14 and 21 days old hydroponic plantlets, on the percentage of germinated conidia. REs caused a moderate promotion or inhibition of conidial germination or did not cause any effect. The observed patterns mostly depended on pathogen race, plant cultivar and plant age: among the races, only FOM2 conidial germination was reduced by almost all REs but never totally; among the cultivars, none affected FOM0 conidial germination at any plant age. FOM1 conidial germination was either not affected or enhanced, depending on plant age. REs from Bingo and Retato degli Ortolani were also tested for their effects on FOM2 hyphal growth, number and length of germ hyphae and for root callose deposition after FOM2 challenge. Inhibitory effects of REs on the formation of germ hyphae by conidia of FOM2 were not noted. In case of the macroconidia the number increased. The length of germ hyphae was increased in both macro and microconidia and more pronounced with REs from cultivar Bingo. REs from Bingo and Retato degli Ortolani enhanced hyphal growth of FOM2. Fluorescence microscopy showed that FOM2 infection of roots caused callose deposition in root cortical cells in the resistant cultivar Bingo, but not in the susceptible Retato degli Ortolani. In conclusion, under our experimental conditions, REs appeared to be only partially involved in plant-pathogen interaction, whereas callose deposition in the roots of the resistant cultivar Bingo was elicited after inoculation with FOM2.