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Independence of Sexual and Vegetative Incompatibility Mechanisms of Thanatephorus cucumeris (Rhizoctonia solani) Anastomosis Group 1. María C. Julián, Department of Virulence and Resistance, DLO-Research Institute for Plant Protection (IPO-DLO), Wageningen, Netherlands; Fons Debets(2), and Jaap Keijer(3). (2)Department of Genetics, Agricultural University of Wageningen, Wageningen, Netherlands; (3)Department of Virulence and Resistance, DLO-Research Institute for Plant Protection (IPO-DLO), Wageningen, Netherlands, and Department of Microbiology and Biotechnology, Rikilt-DLO, Postbus 230, 6700 AE, Wageningen, Netherlands. Phytopathology 86:566-574. Accepted for publication 22 February 1996. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/Phyto-86-566.

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)-1. Sporulation was induced in vitro for a field isolate belonging to AG-1, 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.