INDEPENDENCE OF SEXUAL AND VEGETATIVE INCOMPATIBILITY MECHANISMS OF THANATEPHORUS-CUCUMERIS (RHIZOCTONIA-SOLANI) ANASTOMOSIS GROUP-1

The soilborne plant pathogen Thanatephorus cucumeris (anamorph Rhizoct onia solani) is a basidiomycete that occurs worldwide and causes damag e to a large variety of agricultural crops. The lack of knowledge of t he genetic basis of incompatibility in T. cucumeris hampers the develo pment of environmentally friendly control measures for this plant path ogen. To clarify incompatibility mechanisms in T. cucumeris, sexual an d vegetative compatibility were investigated simultaneously in anastom osis group (AG)-1. Sporulation was induced in vitro for a field isolat e belonging to AG-1, and single spores were isolated, giving rise to h omokaryotic colonies. The homokaryons were paired, and the contact are a between isolates was studied macro- and microscopically. Mating proc esses (formation of heterokaryotic tufts between paired homokaryons) o ccurred independently from vegetative incompatibility processes (lysis of anastomosed cells), showing that in T. cucumeris AG-1 sexual and v egetative incompatibility are two mechanisms that operate independentl y. Vegetative incompatibility was variable and irreproducible, indicat ing vegetative compatibility in T. cucumeris AG-1 is a complex mechani sm. Furthermore, heterokaryotization of homokaryotic mycelium (Buller phenomenon) was observed. A novel phenomenon is described, consisting of the spontaneous lysis of the cells of some of the homokaryotic prog eny of the field isolate.