Independence of Sexual and Vegetative Incompatibility Mechanisms ofThanatephorus cucumeris(Rhizoctonia solani) Anastomosis Group 1

The soilborne plant pathogen Thanatephorus cucumeris (anamorph Rhizoctonia solani) is a basidiomycete that occurs worldwide and causes damage to a large variety of agricultural crops. The lack of knowledge of the genetic basis of incompatibility in T. cucumeris hampers the development of environmentally friendly control measures for this plant pathogen. To clarify incompatibility mechanisms in T. cucumeris, sexual and vegetative compatibility were investigated simultaneously in anastomosis group (AG)-I. Sporulation was induced in vitro for a field isolate belonging to AG-I, and single spores were isolated, giving rise to homokaryotic colonies. The homokaryons were paired, and the contact area between isolates was studied macro- and microscopically. Mating processes (formation of heterokaryotic tufts between paired homokaryons) occurred independently from vegetative incompatibility processes (lysis of anastomosed cells), showing that in T. cucumeris AG-1 sexual and vegetative incompatibility are two mechanisms that operate independently. Vegetative incompatibility was variable and irreproducible, indicating vegetative compatibility in T. cucumeris AG-1 is a complex mechanism. 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 progeny of the field isolate.