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A total of 540 isolates of Pyricularia grisea from rice in the United States were examined for vegetative compatibility, MGR586 DNA fingerprint diversity, and mating type based on hybridization with the mat1-1 and mat1-2 sexual mating type alleles. The collections contained both archived and contemporary field isolates representative of the known MGR586 lineages and races that occur throughout the United States. Complementary nitrate nonutilizing (nit) or sulfate nonutilizing (sul) mutants were used to assess vegetative compatibility in P. grisea. There was a complete correspondence between vegetative compatibility groups (VCGs), MGR586 lineage, and mating type among 527 contemporary isolates (collected between 1991 and 1997) from Arkansas, Louisiana, Missouri, Mississippi, and Texas; all isolates in MGR586 lineages A, B, C, and D belonged to VCGs US-01, US-02, US-03, and US-04, respectively. In addition, all isolates tested in VCGs US-01 and US-04 had the mat1-1 mating type allele whereas those in VCGs US-02 and US-03 had the mat1-2 allele. The strict association of independent markers during this sample period was consistent with a strictly asexual mode of reproduction. However, examination of archived isolates collected in the 1970s and 1980s and contemporary isolates revealed an incongruent relationship between the independent markers. MGR586 C and E isolates were vegetatively compatible which indicated that multiple robust MGR586 delineated lineages could be nested within certain VCGs. Although isolates in lineages C and E were vegetatively compatible, they were of opposite mating type. Several hypotheses, including recombination, could account for the incongruence between the various markers. Among the eight MGR586 lineages (A through H) that occur in the United States, all isolates in lineages A, D, E, G, and H had the mat1-1 allele, whereas isolates in lineages B, C, and F had the mat1-2 allele. Nit mutants can be recovered relatively easy from P. grisea and should allow large numbers of individuals within a population to be assessed for vegetative compatibility. VCGs may prove to be an effective multilocus marker in P. grisea. Thus, VCGs should be a useful means for characterizing genetic structure in populations of the rice blast fungus worldwide, provide a useful genetic framework to assist in interpreting molecular population data, and may provide insight into potential sexual or asexual recombination events.

Forty-four isolates consisting of Fusarium solani (Mart.) Sacc. f.sp. glycines Roy, Fusarium solani f.sp. phaseoli (Burkholder) W.C. Snyder & H.N. Hans., and F. solani, collected from a variety of hosts and locations, were compared based on pathogenicity on soybean and mitochondrial DNA (mtDNA) restriction fragment length polymorphisms (RFLPs). The 24 isolates of F. solani f.sp. glycines caused more severe sudden death syndrome (SDS) foliar symptoms and root rot on soybean compared with all other isolates. All isolates of F. solani f.sp. glycines belonged to a single mtDNA RFLP haplotype. The other isolates of F. solani belonged to nine mtDNA RFLP haplotypes distinct from that of the SDS pathogen and did not cause significant levels of SDS. Three isolates of F. solani f.sp. phaseoli in a single mtDNA RFLP haplotype were very similar to isolates of F. solani f.sp. glycines in culture. One of these isolates consistently produced SDS-like symptoms in pathogenicity tests, although at a low frequency. Fusarium solani f.sp. glycines represent a genetically distinct subgroup within F. solani but may be related to F. solani f.sp. phaseoli.Key words: Fusarium solani f.sp. phaseoli, Glycine max, sudden death syndrome (SDS), mtDNA.

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