Resistance to white pine blister rust in sugar pine is simply inherited and can be identified by distinct needle spot morphs. After artificial inoculation [with Cronartium ribicola] at 2 yr of age, seedlings from selfed and full-sib families developed either fleck spots, characterized by a pale, yellow margin with a necrotic fleck in the center, or typical yellow or red spots (or both). Seedlings segregated for needle spot reaction in monohybrid ratios with fleck dominant. Mycelium in secondary needle tissues of fleck spots, in contrast to yellow and red spots, was relatively sparse and confined by dense tannin deposits. Bark infection and mortality was heavy on seedlings with yellow and red spots. On seedlings with fleck spots, no bark symptoms developed from secondary needle infection but small, abortive cankers did develop on some of these seedlings as a result of primary needle infection. The atypical cankers did not sporulate or spread extensively and healed by the 2nd yr after inoculation. The gene responsible for the fleck reaction elicits a hypersensitive response in secondary needles and, apparently, in bark tissues also.