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

Chrysomyxa pirolata Wint., the cause of inland spruce cone rust, is a serio us 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 influ ence 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, Al berta, and at Edmonton and Hinton, Alberta. The proportion of uredinia to t elia varied with microsite conditions, with more frequent production of tel ia in moist sites. The effect of moisture on formation of telia was tested by subjecting infected plants with immature sori to 90-100%, relative humid ity under various temperature and light conditions. Plants under high humid ity, regardless of other conditions, formed mainly telia from immature sori , even if uredinia had already formed on parts of the leaves before the exp eriments. Telia formation, however, was much slower at 4-6 degrees C than a t 22 degrees C. Cross-sections of sori showed that young uredinia could som etimes convert to telia. Results of a field experiment also suggested that increasing moisture increases the percentage of telia produced on plants. T he production of undifferentiated sori that can become either uredinia or t elia in response to environmental conditions may allow the fungus to maximi ze vegetative proliferation when conditions are unfavorable for sexual repr oduction. It might also explain the large variation in cone rust levels fro m year to year in a given location.