The coarse-scale population structure of pathogenic Armillaria (Fr.) Staude species was determined on approximately 16 100 ha of relatively dry, mixed-conifer forest in the Blue Mountains of northeast Oregon. Sampling of recently dead or live, symptomatic conifers produced 112 isolates of Armillaria from six tree species. Armillaria species identifications done by using a polymerase chain reaction based diagnostic and diploid–diploid pairings produced identical results: 108 of the isolates were Armillaria ostoyae (Romagn.) Herink and four were North American Biological Species X (NABS X). Five genets of A. ostoyae and one of NABS X were identified through the use of somatic incompatibility pairings among the putatively diploid isolates. Armillaria ostoyae genet sizes were approximately 20, 95, 195, 260, and 965 ha; cumulative colonization of the study area was at least 9.5%. The maximum distance between isolates from the 965-ha A. ostoyae genet was approximately 3810 m, and use of three estimates of A. ostoyae spread rate in conifer forests resulted in age estimates for the genet ranging from 1900 to 8650 years. Results are discussed in relation to possible mechanisms that influenced the establishment, expansion, and expression of these genets; the genetic structure and stability of Armillaria; and the implications for disease management in this and similar forests.

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The following study investigates the timing and mechanism of impact of Swiss needle cast on Douglas-fir (Pseudotsuga menziesii) needle physiology (i.e. gas exchange). Swiss needle cast is a foliar disease caused by the fungus Phaeocryptopus gaeumannii, which occurs throughout the range of Douglas fir and until recently has been considered unimportant. However, recent surveys show the Swiss needle cast currently affects 52 611 ha of forested lands in western Oregon, USA, causing a reduction in growth of c. 23% or an implied growth loss of c. 3.2 m$ ha −" yr −" for 1996 alone. Gas exchange of artificially inoculated 2-yr-old Douglas-fir seedlings was monitored on a monthly basis using A\C i curve analysis. No effect of fungal presence on gas exchange was noted until the emergence of fungal fruiting structures (pseudothecia) from needle stomata. However, once present, maximum stomatal conductance and CO # assimilation rates were inversely proportional to the presence of pseudothecia. A\C i curve analysis showed that declines in CO # assimilation appeared to be due to both stomatal and nonstomatal limitations. Stomatal limitations to CO # assimilation were the direct result of reduced CO # diffusion through blocked stomata. Nonstomatal limitations arose, in part, from an indirect effect of pseudothecia development on Rubisco activation. For example, in both Swiss needle cast-infected foliage and foliage with artificially blocked stomata (by external application of petroleum jelly), the amount of Rubisco activation showed a strong, positive relationship with daily maximum stomatal conductance. A mechanism is proposed that outlines the impact of pseudothecia development on stomatal conductance and CO # assimilation rates.

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Widespread mortality of Cordilleran cypress (Austrocedrus chilensis) occurs in developed and pristine forests in south-western Argentina and possibly south-eastern Chile. Affected trees may die rapidly but mortality is commonly preceded by several decades of severely restricted radial stem growth. Roots are often affected by one or more types of decay. Cypress mortality, locally termed 'mal del cipres', is similar to a major forest disease in North America: Port-Orford-cedar (Chamaecyparis lawsoniana) root disease in Oregon and California and a major tree decline: Alaska yellow-cedar (Chamaecyparis nootkatensis) decline in south-east Alaska. This paper discusses several hypotheses concerning mal del cipres and compares current forest decline research in Patagonia with North America.