Research Topics: Invasive Species
White Pine Blister Rust in Western North America
A disease native to Asia, white pine blister rust was introduced separately into both eastern and western North America early in the 20th century. In both cases, the vector was seedlings of native eastern white pine imported from European nurseries, where they had become infected. Blister rust had first appeared in Europe in the mid-nineteenth century, and within three decades had spread across the continent wherever the popular eastern white pine had been planted.
Mechanisms and Inheritance of Resistance
A multi-year survey of western white pine species has identified major resistance genes to the white pine blister rust fungus (Cronartium ribicola) in sugar pine, western white pine, southwestern white pine, and limber pine. Virulence to at least two of these genes (in sugar pine and western white pine) was found to be highly specific, in a classic gene-for-gene relationship.
Studies of the population genetic structure of the blister rust pathogen (Cronartium ribicola) revealed that it was (a) highly outcrossing, (b) depauperate in genetic variation, and (c) had a metapopulation structure. Virulence in the pathogen is inherited cytoplasmically. Distribution and frequency of two virulence genes were mapped in the western U.S.
In both alternate hosts, infection takes place through open stomata on leaves. On pines, a basidiospore sends a germ tube through a stomate, and develops a bulbous substomatal vessicle. From this, a primary infection hypha extends into mesophyll tissue, where a mass of mycelium grows into a "pseudosclerotium" like structure. At this point, a macroscopic symptom of the disease is expressed as a yellow spot. Soon, mycelium penetrates into the vascular cylindar, grows down the phloem to the base of the needle, and establishes in living bark tissues of the shoot. The visible incipient symptom is a pink/orange discoloration of the shoot. This is followed about a year later by pycnia, spore structures that bear pycniospores (spermatia), that function as gametes. A year later, aecia and aeciospores are formed in the same locus.
Virulence in C. ribicola
Sugar pines with MGR that resisted early waves of infection for 14 years in a test plantation suddenly developed multiple infections from a new and virulent race of blister rust. Happy Camp, Siskiyou Mountains California.
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