Canadian Forest Service Publications
Measuring shifts in forest pathogen populations. 1999. White, E.E.; Morrison, D.J.; Kinloch, B.B., Jr.; Westfall, R.D.; Gitzendanner, M.A.; Foord, B.M.; Dupper, G.E. Pages 153-162 in F. Lieutier, W.J. Mattson, and M.R. Wagner, Editors. Physiology and genetics of tree-phytophage interactions. Les Colloques No. 90, August 31-September 5, 1997, Gujan, France. Institu national de la recherche agronomique (INRA), Paris, France.
Issued by: Pacific Forestry Centre
Catalog ID: 5466
The genetics of trees and phytophagous organisms converge at their interactions, producing resistance or susceptibility. In insect and disease interactions, resistance is the product of two sets of genes, those of the host and the pest. Knowledge of the genetics of both is necessary to understand the interaction. Genetic analysis of pathogens has been hindered when morphological differences are limited, as with some obligate parasites, or when morphological structures are rarely produced, as with some fungal fruiting structures. DNA analysis provides a profitable addition to the analysis of pathogen genetics, mating system, and population and species genetic diversity. This report reviews examples of our use of DNA analysis in studies of the genetics of two diseases, Armillaria root rot and white pine blister rust.
The study of Armillaria used variation in the ribosomal DNA region of the fungus to compare species genetic diversity. Genetic studies of Cronartium ribicola, causal agent of white pine blister rust, used this and other molecular markers and examined mating system and population structure. In both cases, the results provided information which was difficult or impossible to obtain on the basis of morphology alone.
In the Armillaria study, the ribosomal DNA marker provided a rapid, convenient species diagnostic. Epidemiological analysis requires species differentiation since only one of the four species commonly occurring in the study area is pathogenic. A large number of cultures generated by studies of Armillaria species distribution in British Columbia were identified with the diagnostic much more quickly and accurately than with conventional mating tests. For Cronartium ribicola, the results showed the pathogen is heterothallic, highly outcrossing and in genetic equilibrium. The demonstration of virtually uninhibited outcrossing has profound implications for the stability of tree resistance, because of the recombinational potential in the pathogen to generate races with multiple forms of virulence.
Major differences in the patterns of variability of DNA markers in the two fungi reflect major differences in their life cycles and modes of infection. Low variation in the DNA of Armillaria is consistent with reproductive isolation and vegetative spread. In Armillaria, reproductive isolation appears to have fixed the neutral markers used to differentiate the pathogenic species. In contrast, relatively high within-population variation in DNA markers occurs in C. ribicola, which is highly out-breeding and infects by spores.