USE OF NITRATE NON-UTILIZING MUTANTS TO STUDY VEGETATIVE INCOMPATIBILITY IN FUSARIUM-SOLANI (NECTRIA-HAEMATOCOCCA), ESPECIALLY MEMBERS OF MATING POPULATION-I, POPULATION-V AND POPULATION-VI

Nitrate non-utilizing (nit) mutants were obtained from 89 (out of a sa mple of 94) strains of Fusarium solari isolated from several plant hos ts in Japan, New Zealand and North America. Nit1 (unable to utilize ni trate) and nit3 (unable to utilize nitrate or nitrite), mutants were r eadily recovered from all 89 strains when these were grown on a minima l medium containing KCLO(3) at 30 g l(-1) (MMC), but NitM (unable to u tilize nitrate, hypoxanthine or uric acid) mutants were only obtained from 53 strains on this medium. Modified MMC containing L-threonine in stead of L-asparagine enabled recovery of NitM mutants from an additio nal 11 strains. Sulphate non-utilizing mutants (sul) were generated fo r a few strains that did not produce NitM mutants. Based on complement ation between nit1 (or nit3) and NitM (or sul) mutants, 57 strains wer e heterokaryon self-compatible, of which 31 were assigned to single-st rain, vegetative compatibility groups (VCGs). Nine multi-strain VCGs w ere identified among New Zealand strains: here members of each VCG cam e from the same host and usually from the same location even though th ey were often isolated at different times. Two pea strains from North America were in the same VCG, and a third North American pea strain wa s compatible wih a pea strain from New Zealand. Although the VCG resul ts were consistent and repeatable the vigour of individual heterokaryo ns varied widely. Frequently the intra-strain complementations produce d heterokaryons of greater vigour from those that formed between compa tible strains. There was a high frequency of self-incompatibility due either to simple or complex genetic mechanisms among Nectria haematoco cca MPI strains.