POPULATION-STRUCTURE OF THE PITCH CANKER PATHOGEN, FUSARIUM-SUBGLUTINANS F-SP PINI, IN CALIFORNIA

Isolates (170) of Fusarium subglutinans f. sp, pini, the cause of pitc h canker, were collected from infected Pinus spp. throughout the known range of the disease in California, between 1993 and 1995. Of these, 152 isolates were vegetatively compatible with tester strains represen ting a previously characterized vegetative compatibility group (VCG) o f F. s. pini: VCG CI, 83 isolates; C2, 12 isolates; C3, 51 isolates, a nd C4, 6 isolates. The remaining 18 isolates were associated with thre e new VCGs: C6, 9 isolates; C7, 4 isolates, and C8, 5 isolates. The po pulations of F, s. pini in the original disease centres in Santa Cruz/ Monterey Counties and east of San Francisco Bay (Alameda and Contra Co sta Counties), were dominated by VCG CI, as they were when originally sampled (1987-9). In contrast, nearly all isolates from a large infest ation in San Luis Obispo Co. were associated with VCG C3, which was pr eviously identified as the sole VCG in an isolated infestation near Sa nta Barbara and at a tree nursery in Los Angeles Co. This distribution implies that movement of infected trees might have contributed to the spread of pitch canker in California. The limited diversity in the F, s. pini population statewide is consistent with a recent introduction of the pathogen into California and the absence of sexual reproductio n in this population.