Save trees from foreign invaders

By Gary Raham
Nature Columnist

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Two foreign invaders threaten area trees this year. One, the white pine blister rust, has probably entered North America many times since its 1910 entry into the Vancouver area of Canada, according to Anna Schoettle, a researcher at the Rocky Mountain Research Station on Prospect Road. However, it had not been found in this immediate area until it was identified in Red Feather Lakes in 1998.

Blister rust

White pine blister rust is a fungus that attacks limber pine, whitebark pine, southwestern white pine and bristlecone pine. All these trees have needles that occur in bundles of five, unlike the lodgepole and ponderosa pine that have two and three needles per bundle. The fungus penetrates the needles, grows into the wood and forms cankers (areas of dead bark) that turn yellowish at the canker margins. During late spring and early summer, orange-yellow pustules protrude from the bark of the cankers and form blisters containing yellow-orange spores. The wound produces sap that later turns dark and hard. The infection will kill the tree if it reaches the main trunk. If branch cankers have formed within 4 inches of the trunk, the tree has probably been fatally compromised. If branches are pruned at least 6 inches beyond any visible symptoms on the trunk side of the cankered branch, infection can be stopped because the fungus quickly dies.

This rust produces spores in the bark pustules that will germinate on gooseberries and currants--plants in the genus Ribes. The orange or brown spores produced on these intermediate hosts are the ones that can infect the pines. Removing Ribes plants within 200 feet of the pines by uprooting or herbicide application provides another means of control.

Schoettle urges people not to transplant susceptible pines from the Red Feather area and to make sure nursery plants are healthy. Nonsusceptible pines, like red pines, might provide a better planting choice. Schoettle would love to hear from anyone who spots what might be a rust infection in areas south of the Poudre River. Although the rust is most likely a permanent feature on this continent now, Schoettle is working to limit its impact by increasing the general health of the trees and the ecosystems of which they are a part so that the infection is not devastating.

Part of this ecosystem strengthening involves understanding the pines relationship with other creatures. The pine depends on Clarke's Nutcracker, for example, to disperse its large, wingless seeds. If the pine's population becomes too small and isolated, the birds stop coming and the white pine's future turns bleak.

Elm bark beetle

The second invader is a Chinese immigrant called the banded elm bark beetle, otherwise known as Scolytus schevyrewi. Jose Negron studies "schevy" at the Rocky Mountain Research Station. "They're found everywhere in the west," said Negron. In May 2003 the Agricultural Research Service Systematic Entomology Lab confirmed that specimens were found in packing materials at the Denver Zoo and other sites. The beetle attacks Russian olive, willow trees, pea shrubs, fruit trees and Siberian elms. Siberian elms have been most affected in the Cheyenne area. They usually succeed in their invasion when trees are already weakened and stressed.

Negron's research is looking at beetle attractant chemicals and details of its hosts and lifecycle to get a better handle on how to control it. The beetle can complete its development in about a month and has two peaks per year, starting as early as late March and the beginning of April. Most likely the beetle can transmit Dutch elm disease, although it hasn't been formerly confirmed yet. The Dutch elm fungus has been isolated from the insect. The larval form of the insect causes the damage by feeding on the cambium and phloem layers beneath the bark. Sap often flows from beetle entry holes as the trees attempt to flush the critters out. By the time holes are visible, the insect has come and gone and the damage done.

Gardeners should take the same action with trees infected with either the banded elm bark beetle or the Dutch elm fungus: remove infested wood, burn it, or chip it and bury it in a landfill.

Those with questions or information for the researchers can find them at the Rocky Mountain Research Station, 240 W. Prospect Road, in Fort Collins. Schoettle can be reached at 498-1333. Negron's number is 498-1252.