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 pitch 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 representing a previously characterized vegetative compatibility group (VCG) of F. s. pini: VCG CI, 83 isolates; C2, 12 isolates; C3, 51 isolates, and C4, 6 isolates. The remaining 18 isolates were associated with three new VCGs: C6, 9 isolates; C7, 4 isolates, and C8, 5 isolates. The populations of F, s. pini in the original disease centres in Santa Cruz/Monterey Counties and east of San Francisco Bay (Alameda and Contra Costa Counties), were dominated by VCG CI, as they were when originally sampled (1987-9). In contrast, nearly all isolates from a large infestation in San Luis Obispo Co. were associated with VCG C3, which was previously identified as the sole VCG in an isolated infestation near Santa 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 reproduction in this population.