1 Department of Microbiology and Plant Pathology, FABI, University of Pretoria, Pretoria, South Africa; 2PROMEC, Medical Research Council, PO Box 19070, Tygerberg, South Africa; 3 Department of Plant Pathology, Kansas State University, Manhattan, KS 66506-5502, USA; 4Department of Plant Pathology, University of California, Davis, CA 95616, USA

Background and objectives
Fusarium subglutinans is a cosmopolitan pathogen that occurs on a wide variety of host plants including mango, pineapple, sugarcane and pine. Some F. subglutinans strains have formally been proposed as pathotypes. One of these pathotypes is F. subglutinans f. ;sp. pini (F.s. pini), the causal agent of the pitch canker in the United States, Haiti, Japan, Mexico and South Africa. F. subglutinans, F. moniliforme, F. proliferatum, F. thapsinum and F. nygamai encompass seven mating populations (indicated by the letters A to G) in the Gibberella fujikuroi complex. Mating populations in G. fujikuroi can be distinguished from one another based on genetic isolation. Therefore, members of the same mating population produce sexually fertile crosses among themselves, but not with members of other mating populations. F. subglutinans isolates from sugarcane usually belong to the B and isolates from maize to the E mating population. Isolates of F. subglutinans from other plants including mango, pine and pineapple have not been formally described as a mating population of G. fujikuroi. Considerable confusion exists regarding the appropriate placement of F. s. pini in G. fujikuroi. In this study we have, therefore, investigated whether F.s. pini should reside in a new and unique mating population in the G. fujikuroi complex.

Materials and methods
The fungal isolates used in this study include the B and E tester strains of G. fujikuroi, as well as a substantial number of F.s. pini isolates from California, Florida and South Africa. Crosses were made on carrot agar, as described in the procedure of Klittich and Leslie [1]. Reciprocal crosses, where the male parent was used as the female parent, were done when fertile crosses were obtained. All the field isolates of F.s. pini from South Africa, California and Florida were crossed as males with the B and E mating tester strains. Isolates of F.s. pini from each geographical area were also crossed with each other in all possible combinations. Vegetative compatibility groups (VCGs) were identified using nitrate non-utilizing (nit) mutants according to a standard protocol [2]. The thallism of isolates pertaining to the sexual crosses was determined by identifying the VCGs of the parent isolates and progeny.

Results and conclusions
Fertile crosses were produced between F.s. pini isolates from South Africa, California and Florida. In crosses between F.s. pini isolates and the testers for the B mating population, and in control crosses between the B and E mating population tester strains, fertile perithecia were occasionally seen. Ascospores were recovered from these perithecia, and tested for VCG, and all the progeny were in the same VCG as the B parent. We interpret these results to mean that the B mating type testers can occasionally produce perithecia and ascospores homothallically. Such homothallic perithecia probably explain earlier reports that F.s. pini isolates belong to the B mating population. None of the other F.s. pini isolates produced fertile crosses with either the B or E mating tester strains. We conclude that fertile F.s. pini isolates belong to a unique mating population of G. fujikuroi: mating population H.

1. Klittich CJR, Leslie JL. 1988. Genetics 118, 417-423.
2. Correll JC, Klittich CJR, Leslie JL. 1987. Phytopathology 77, 1640-1646.