Montana Field Guide

White Pine Blister Rust is a fungal pathogen of five-needle pines native to China. It was introduced into North America around 1900. Since its introduction it has devastated Western White Pine (Pinus monticola), Sugar Pine (P. lambertiana), Limber Pine (P. flexilis) and Whitebark Pine (P. albicaulis) across the continent. In the Rocky Mountains, it has caused the decline of Whitebark pine which has contributed to the species continued candidate status for listing under the Endangered Species Act (US Department of the Interior 2016).

The life cycle of the fungus requires not only a susceptible pine host but also an intermediate host plant of the genus Ribes (currents and gooseberries). Within this alternate host it causes discoloration of the leaves but does not impact these plants beyond this. In pine hosts it initially causes discoloration in the needles and as the infection progresses, cankers on the branches and trunk. These cankers coincide with destruction of the cambium and if present around the circumference of the trunk, will cut off nutrients and cause the death of the tree (Maloy 2001).

Symptoms of White Pine Blister Rust vary by host species. In its intermediate Ribes host infection causes discoloration of leaves. Specifically, the lower surface of the leaves becomes pale and the fungus produces small orange fruiting bodies called uredinia which produce and release spores which reinfect the leaves. At the end of the season, short yellow-brown filaments grow called telium, which produce spores that disperse to pines (Geils et al. 2010).

In pines, initial infection manifests as discoloration of the needles late in the summer or autumn. Over several years the infection spreads into the cambium causing the characteristic cankers or blisters (aecia). These blisters appear yellow in the spring, eventually rupture and dry by the fall. As they have a relatively high sugar content, blisters are often chewed by rodents and expel sap (Maloy 2001).

This species can be found in any habitat where suitable host species and climate characteristics are present. In Montana, typically mid- high elevation pine forests.

Pine host species include all five-needle pines. Species native to Montana are: White Bark Pine (P. albicaulis), Western White Pine (P. monticola), and Limber Pine (P. flexilis). Alternate hosts are plants in the genera: Ribes, Castilleja, and Pediculari (Geils et al. 2010).

Whitepine Blister Rust requires two hosts to complete its lifecycle, a five-needle pine and a Ribes shrub or other alternate host (Castillija spp., Pedicularis spp.). Pine trees are infected when spores enter through needle stomata. These spores germinate and the fungus grows through intercellular spaces of the bark toward the main stem. After several years, the fungus matures and produces the characteristic blisters that release aeciospores into the environment, typically in the spring. These spores are carried by the wind, and will infect alternate host species if deposited on the leaves, entering through stomates. As the summer progresses the fungus grows within the host leaves, and the rust produces reproductive structures called basidiospores in fall which if carried to a suitable pine host complete the cycle (Geils et al. 2010).

As the fungus is reliant on abiotic processes in the environment to distribute its spores, environmental factors can impact host suitability and distribution. Spores may be damaged by low-humidity so survival of spores is higher in damp areas. Cooler temperatures also favor survival of spores. As such local environments and microclimates may play a role in where rust is present and the severity of local outbreaks. In some areas that are generally too warm and dry for the fungus, the pathogen may be present in areas where local topography creates suitable climate conditions. Younger trees in the understory may be more susceptible due to the microclimate present within the forest (Van Arsdel et al. 2006).

Many strategies have been employed to limit the damage of this fungus including mechanical and chemical treatments, with little effect on slowing its spread or restricting its distribution. There is no treatment that can be economically applied at the stand or forest level. Individual trees may be treated by pruning infected branches. Currently, efforts are underway to breed five-needle pines resistant to this pathogen for use in reforesting efforts in impacted areas (as reviewed in Geils et al. 2010).

For information and resources on plant pests and diseases see the USDA's Animal and Plant Health Inspection Service's Plant Pests and Diseases Profiles