Fields W. Cobb

Heterokaryosis is not required for virulence of Heterobasidion annosum

Intensive sampling of the pathogenic fun- gus Heterobasidion annosum in white fir stands yielded a large percentage of homokaryotic isolates. To assess the stability and virulence of homokaryotic isolates, four homokaryons and four heterokaryons were in- oculated on 200 roots from 50 mature trees. Roots were sampled after 4 and 12 months. Eighty-three percent of the inoculated roots were infected, and 95% of the isolates retrieved were identified as the isolates originally inoculated. Homokaryons were not heterokaryotized and no significant colonization dif- ference was observed between homokaryons and het- erokaryons. One year after the removal of inoculated roots and the subsequent creation of large basal wounds, fungal isolates that had reached the root col- lar and colonized the main stem of the tree could still be isolated. Eight putative heterokaryons and six putative homokaryons were obtained from stems of a subsample of the inoculated trees and were grown in the laboratory. Six of eight heterokaryons re- mained unchanged, whereas all six homokaryons had been heterokaryotized.

VARIETIES OF VERTICICLADIELLA WAGENERI

Ninety-eight strains of Verticicladiella wageneri Kendr., cause of black stain root disease on conifers, were isolated from species of Pinus, Pseudotsuga, and Tsuga across the known range of the pathogen in western North America. Examination of these strains (45 of which were examined in detail) resulted in recognition of three apparently host-specialized morphological variants. Mycelial pigmentation, presence or absence of a hyaline margin on advancing mycelia, growth rate at 25 C, capacity for conidiophore production, and presence or absence of a swelling at the apex of conidiophore stipes were used to delineate the variants. The variant isolated primarily from hard pines (Pinus ponderosa, P. jeffreyi and P. contorta) is described as V. wageneri var. ponderosa var. nov. The variants from pinyons (Pinus monophylla and P. edulis) and Douglas-fir (Pseudotsuga menziesii) are not consistently distinguishable from each other and are retained in V. wageneri var. wageneri.

Nuclear behaviour and evolution of two populations of the western gall rust fungus

Peridermium harknessii , cause of western gall rust of pines, comprises two populations of multilocus electrophoretic types (zymodemes) in the western United States. When stained with a DNA-specific fluorochrome, mature, ungerminated aeciospores from zymodeme I were found to be predominantly binucleate (70%), as were those of the related macrocyclic species, Cronartium quercuum (74%), whereas aeciospores from zymodeme II were predominantly uninucleate (93%). Within each zymodeme, aeciospores with two nuclei had significantly ( P = 0.01) more DNA than spores with one nucleus, and numbers of nuclei in germlings increased arithmetically over time. These data suggest that aeciospore nuclei in both zymodemes I and II divide mitotically, not meiotically, as is consistent with an asexual life cycle. Photometric measurements also indicated that the amount of DNA in one nucleus of a uninucleate zymodeme II aeciospore was similar to the total amount of DNA in a binucleate zymodeme I aeciospore. These data, coupled with recent isozyme studies, suggest either that zymodeme II evolved after karyogamy of zymodeme I and an unidentified zymodeme, or that zymodeme I evolved after haploidization of zymodeme II.

Identification of Armillaria species in California

AbstractMating experiments were conducted with 30 California isolates of Armillaria species to determine their intersterility grouping. Basidiomes were collected from a variety of coniferous and ha...