Michigan State University Extension
Ornamental Plants plus Version 2.0 - 00001678
01/01/98
White pine blister rust, caused by Cronartium ribicola, is a serious problem in many areas of the north central states. The severity of the disease is related to the weather conditions. Cool, wet weather with ambient temperatures averaging below 67 degrees F from July to September favor infection of pine by the rust fungus. The fungus infects needles and young stems through their stomates (air exchange pores). The fungus grows into the wood, causing cankers (areas of dead bark). The bark becomes discolored and turns yellowish at the canker margins.
Cankers will kill a tree if they grow into the trunk. If branches contain cankers about 4 inches from the trunk, the tree will invariably die. Pruning out limbs with cankers more than 4 inches from the trunk will protect the tree. Cankers are most easily seen in the spring when orange-yellow pustules protrude from the bark of the cankers. In late spring and early summer, these areas will form blisters containing a sticky, yellow-orange fluid that later turns dark and hard. The pustules and blisters on the pines are part of the spore formation cycle of the rust fungus. Spores produced on the pine, however, can infect only plants in the genus Ribes, which includes gooseberries and currants. Ribes infected with white pine blister rust form orange and brown spores on the undersides of their leaves. The brown spores form in hairlike projections under the leaves in late summer and early fall. From these projections arise the spores that infect the pines.
Control: Homeowners can avoid loss by planting species such as red pine that are not susceptible; certainly, eastern white pine should not be planted in areas of high disease incidence. In areas of low disease incidence, look for evidence of rust on all currants and white pines within at least 400 feet of the planting site. If no evidence of disease is seen, eradicate Ribes within 400 feet of the planned planting site and proceed with planting. If rust is evident, plant a species not susceptible to the blister rust.