We've had a busy few weeks a the Plant Clinic, with new boxwood blight confirmations and another exotic, rare disease! We received a sample of a pine branch earlier in May with some blistering on the bark and large amounts of sap exudate. After collecting spores and examining them under the microscope, we identified the pathogen as Cronartium ribicola, causal agent of white pine blister rust. Several of us in the Plant Clinic had never seen this pathogen before, so it was quite the exciting day!
White pine blister rust is an exotic rust disease and was the impetus for the 1912 Plant Quarantine Act, the first law restricting the introduction of plants to the United States. The pathogen was probably first introduced to the United States in the early 1900s on imported susceptible white pine seedlings grown in Europe, though the rust appears to be native to Asia. Today, it is found across most of the United States where environmental conditions are conducive to its development. Eastern white pines are particularly susceptible, making it a concern in higher elevation forested areas.
Cool, wet conditions favor development of the disease. Because of this, we rarely find the disease in Illinois though it is not uncommon in Minnesota and other north-eastern states. This sample was from northern Illinois.
White pine blister rust is a typical, complicated rust. It is heteroecious, meaning it requires two hosts to complete its life cycle. In this case, the alternate host is most frequently Ribes spp. (currants and gooseberries). The submitter noted that wild gooseberry plants nearby had rust spots on the leaves. White pine blister rust is also macrocyclic, meaning that it produces all 5 types of spores associated with rusts. Two types of spores are produced on the white pine host: spermatia (aka pycniospores) and aeciospores. Spermatia are male sex cells, allowing for sexual recombination in the rust's life cycle. Aeciospores are produced on white pines, but cannot infect white pines; instead, they infect Ribes spp. Other types of spores are produced on the Ribes hosts, one of which can infect the white pine host.
Aeciospores of Cronartium ribicola
Management consists of pruning out cankered branches several inches below the canker. Heavily infected trees will die. Removing the alternate host is sometimes recommended; it may or may not be helpful, depending on the population of other Ribes spp. nearby.