Vegetative compatibility and heterokaryon stability in Fusarium oxysporum f.sp radicis-lycopersici from Italy

Fusarium crown and root rot, caused by Fusarium oxysporum f.sp. radicis-lyc opersici (Forl), is one of the most destructive soilborne diseases of tomat o in Italy. Chlorate-resistant, nitrate-nonutilizing (nit) mutants were use d to determine vegetative compatibility among 191 isolates of Forl collecte d in five geographic regions (Calabria, Emilia-Romagna, Liguria, Sardinia, Sicily) in Italy. The isolates were assigned to five vegetative compatibili ty groups (VCGs): 65 isolates to VCG 0090; 99 to VCG 0091; 23 to VCG 0092; two to VCG 0093; and two to VCG 0096. The population structure of Forl in I taly is similar to that reported for Israel, and differs from that found in North Atlantic European countries, where VCG 0094 is predominant. The stab ility of prototrophic heterokaryons originating from hyphal anastomosis bet ween compatible complementary nit mutants was assessed through conidial ana lysis and mycelial mass transfer. Most monoconidial cultures (84%) recovere d from 117 prototrophic heterokaryons were nit mutants, indicating that het erokaryons generally do not proliferate well through conidiation; most of t he 177 prototrophic heterokaryons examined were unstable, and only 9% susta ined prototrophic growth through the tenth mycelial transfer upon subcultur ing. The prototrophic growth is proposed to be maintained through restorati on of the heterokaryotic state by continual anastomosis between adjacent ho mokaryotic hyphae. Since heterokaryosis is a prerequisite for parasexual re combination, we speculate that this mechanism is unlikely to play a major r ole in generating the VCG diversity found among Forl or other strains of F. oxysporum.