Agriculture and Agri-Food Canada, Cereal Research Centre, 195 Dafoe Road, Winnipeg, Manitoba, Canada, R3T 2M9
Vegetative compatibility has been used to assess the population biology of many fungal plant pathogens. However, for many species, including Fusarium graminearum, this has meant making auxotrophic mutants to force heterokaryon formation. A method was developed to observe barrage zones of thick, raised mycelium at the junctions of vegetatively incompatible F. graminearum isolates. The appearance of the barrage zones was influenced by the growth medium and the light. Barrage zones on V8 agar were thicker and better defined than those on potato dextrose agar, Spezieller Nahrstoffarmer agar, and water agar. The addition of ground wheat kernels to V8 agar enhanced barrage zone formation. Incubating the cultures under constant light at 2,150 lx produced more distinct barrage zones than constant light at 3,400 lx, constant darkness, or ambient room light. Forty-three F. graminearum isolates from 34 vegetative compatibility groups, determined previously using nit auxotrophic mutants, were paired in all combinations using these optimized conditions. Isolates in different vegetative compatibility groups typically formed distinct, thick barrage zones at their junctions. Pairs of isolates in the same vegetative compatibility group had a very slight or no visible reaction, or rarely, a distinct “line gap” of sparse mycelium. Subcultures from the same isolate typically had no visible reaction at their colony junctions; however, subcultures from some isolates had thin, slight barrage zones. This method was used to identify the proportion of each of four F. graminearum isolates from infected barley spikes in the field, inoculated previously with a mixture of conidia from these four isolates. Barrage zone formation represents a rapid method to screen vegetative compatibility groups in F. graminearum and may be useful for other Fusarium species.