Evidence for environmental determination of uredinia and telia production in Chrysomyxa pirolata (inland spruce cone rust)

Abstract

Chrysomyxa pirolata Wint., the cause of inland spruce cone rust, is a serious pathogen in natural spruce forests and seed orchards. Cone infection is caused by basidiospores produced by telia on alternate hosts in the genera Pyrola, Moneses, and Orthilia. The disease cycle of this rust and the influence of moisture on the differentiation of telia on Pyrola asarifolia Michx. were studied over two growing seasons at a wet site adjacent to a spruce seed orchard and a drier site adjacent to another orchard at Smoky Lake, Alberta, and at Edmonton and Hinton, Alberta. The proportion of uredinia to telia varied with microsite conditions, with more frequent production of telia in moist sites. The effect of moisture on formation of telia was tested by subjecting infected plants with immature sori to 90-100% relative humidity under various temperature and light conditions. Plants under high humidity, regardless of other conditions, formed mainly telia from immature sori, even if uredinia had already formed on parts of the leaves before the experiments. Telia formation, however, was much slower at 4-6°C than at 22°C. Cross-sections of sori showed that young uredinia could sometimes convert to telia. Results of a field experiment also suggested that increasing moisture increases the percentage of telia produced on plants. The production of undifferentiated sori that can become either uredinia or telia in response to environmental conditions may allow the fungus to maximize vegetative proliferation when conditions are unfavorable for sexual reproduction. It might also explain the large variation in cone rust levels from year to year in a given location.Key words: Picea, Pyrola, telia induction, seed orchard, Uredinales.