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Genetic Diversity in California and Florida Populations of the Pitch Canker Fungus Fusarium subglutinans f. sp. pini. J. C. Correll, Department of Plant Pathology, University of Arkansas, Fayetteville 72701; T. R. Gordon(2), and A. H. McCain(3). (2)(3)Department of Plant Pathology, University of California, Berkeley 94720. Phytopathology 82:415-420. Accepted for publication 22 November 1991. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/Phyto-82-415.

Isolates of Fusarium subglutinans that are pathogenic to pines, recently given the designation F. subglutinans f. sp. pini, were collected from slash and loblolly pines in Florida (116 isolates) and from various pine hosts (Monterey, bishop, aleppo, and Canary Island pine) in California (209 isolates). These isolates were characterized by vegetative compatibility, using nitrate nonutilizing mutants, and by mitochondrial DNA (mtDNA) restriction fragment length polymorphisms (RFLPs). Attempts also were made to characterize F. s. pini for sexual compatibility. Using four different restriction enzymes, no RFLPs were detected among isolates of F. s. pini. However, restriction patterns of mtDNAs of all nonpine isolates of F. subglutinans examined were different from those of F. s. pini. The characteristic restriction fragment patterns of mtDNAs of isolates of F. s. pini indicate that they are genetically related and may represent a distinct mating population within the morphological species F. subglutinans. However, we were unable to demonstrate the sexual fertility of any strains of F. s. pini. Vegetative compatibility group (VCG) diversity in the Florida population, where the disease had been established for at least 15 years, was high with 45 distinct VCGs identified among 117 isolates examined; one Florida isolate collected in 1977 was vegetatively self-incompatible and avirulent on pine. The predominate VCG at any one collection site in Florida made up 1133% of the sampled population. In contrast, in the California population, only five VCGs were identified among 209 isolates examined. Furthermore, 70% of the entire California population sampled was composed of a single VCG (C1), indicating that the population is rather homogeneous with respect to VCG. This VCG structure would be indicative of an asexually reproducing pathogen and/or one that has recently been introduced.