ABSTRACTUredia ofCronartium quercuumf. sp.fusiformeoccurred commonly on leaves of northern red oak seedlings after inoculation and incubation at 9–18 C. On water oak, uredia were most frequent at 15 C. Almost six times as many urediospores were produced on northern red oak leaves after incubation at 16 C than at 18 C. Leaves that were 50–80% mature when inoculated produced urediospores over a longer time period than did 95% mature leaves. Urediospores were better inoculum than aeciospores for the production of urediospores. The ratio of urediospore production to spore inoculum ranged from 3.6 to 19.7, depending on inoculum type and density with the larger ratios at the lower densities.