White Pine Blister Rust

Two destructive organisms often take their toll on white pine in Vermont. One, the white pine weevil, wrecks the sawlog potential of white pines by killing the leading shoot: when the side branches compete for dominance, multi-stemmed trees that are aptly called "cabbage pines" result. The white pine weevil does not kill the tree, but it makes such a mess of it that some people wish it did. The cabbage-like specimens seem to grow ever more vigorously.

The other organism, white pine blister rust, is a killer. Almost all the trees that it infects are dead within six years. The disease cycle begins when a single spore enters a pine needle through a stomate, or pore, in the needle. The first visible symptom is likely to be a dead branch still holding its brown needles. As the girdling fungus progresses down the branch and into the main trunk, cankers form. The trunk is shriveled or sunken and often oozes resin.

This fungus was introduced from Europe on planting stock at the beginning of the century. All five-needled pines are susceptible, but eastern white pine is the only five-needled pine in these parts.

There are several five-needled western pine species and one, whitebark pine, plays an important ecological role at high elevations. It has been classed as the most susceptible of all the pines. As many as 90% of the trees in some stands have been killed by blister rust and, partly because its seeds are an important food for grizzly bears and many other wildlife species, its survival is a matter of real concern.

Several different types of spores are produced during the complex life cycle of this fungus and one spore type can only survive and reproduce on currants and gooseberries, which make up the genus Ribes. Beginning soon after blister rust was introduced, Ribes plants across the country have been methodically torn up or, more recently, destroyed with herbicides, in an attempt to eliminate this link in the chain of infection.

Except in Maine, and in the vicinity of tree nurseries, the effort was abandoned as being futile. There are over 15 native Ribes species and their seeds may lie dormant for years before sprouting. To compound the problem, plants can grow back from small pieces of their roots.

It appeared that eliminating Ribes in areas where blister rust was a real hazard - mainly cold, damp environments - was not effective. If even a few transplants survived, the infection rate was still high. Eliminating Ribes from non-hazard areas, on the other hand, appeared to be unnecessary because there the disease occurs infrequently, regardless of the number of Ribes bushes.

These eradication programs have longbeen used as an example of expensive and misguided forest protection. In a recent study in Maine, however, forest pathologist Bill Ostrofsky and others found reason to continue hacking away at Ribes. When they compared the incidence of blister rust on saplings, poles and sawtimber in areas where Ribes control has been practiced with other areas that have been left alone, they found less than half the infection rate in the eradication areas. Their economic analysis showed that Ribes removal was cost effective. The benefit of removing Ribes is even greater than their study showed since they did not consider the seedling-sized trees that were saved.


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