Epidemiology of comandra rust on jack pine and comandra in Minnesota

In the greenhouse, comandra plants (Comandraumbellata (L.) Nutt. ssp. umbellata Piehl) inoculated with aeciospores of Cronartiumcomandrae Pk. became infected after only 6 h in a mist chamber. Plants became infected if misting was not delayed more than 8 days after inoculation with aeciospores.Rust developed on comandra more rapidly in open areas than in shaded areas. In open areas in the field, uredia occurred as early as May 17, and telia, as early as June 30. Some uredia were present until about July 28 and telia were present up to mid-August, by which time rust-infected comandra leaves had abscissed.Aeciospores were produced as early as May 13 on all twig, limb, and main-stem infections, but peak aeciospore production coincided with jack pine (Pinusdivaricata (Ait.) Dumont = P. banksiana Lamb.) pollination. Rodent damage to rust infections was associated with production of spores, especially of pycniospores. The period of maximum infection on 4-week-old pine seedlings occurred from mid-July through mid-August. The 4-week-old pine seedlings exposed during this period had up to 4.4 times more infection than older seedlings.