EXPERIMENTAL MYCOLOGY 14,255-267 (I!%)) Mycelial Interactions in Sclerotinia sclerotiorum

sclerotiorum. Experimental Mycology 14,255-267. Mycelial interactions were examined among 35 isolates of Sclerotinia sclerotiorum and two Asian species, Sclerotinia asari and an unnamed, Japanese species. Pairings were scored as compatible when strains merged to form one colony and incompatible when strains grew to form two distinct colonies. Incompatible mycelial pairings resulted in an interaction zone in which a distinct reaction line and abundant aerial mycelium or thin mycelium were observed with some variation among replicates. All pairings of a strain with itself were compatible. Ofthe 31 strains of S. sclerotiorum tested, 21 were mycelially incompatible with all others. Among the remaining 10 strains of S. sclerotiorum, there were four mycelial compatibility groups consisting of two or three strains each. Pairings of S. asari with all other strains resulted in a unique incompatible reaction, a mycelium-free interaction zone. Two of three strains of the Japanese species were intercompatible, but pairings of each of the three strains with all other strains were incompatible. Microscopically, mycelial interactions in pairings of strains were complex. Anastomosis between paired strains was not always observed. This may be due in part to the conversion of many hyphal tips, in both compatible and incompatible interactions, to sites of microconidiogenesis no longer capable of hyphal fusion. Incompatible pairings were fol