White Pine Blister Rust

The early signs and symptoms of white pine blister rust are typically harder to find but become quite apparent as the disease progresses.

Needles: The disease initially appears as yellow/orange spots on white pine needles. The needles eventually turn brown as the infection spreads and will remain attached to the stem when they die.

Stem/Trunk: Orange/brown cankers form when the infection reaches the stem. Over time, these swelling cankers turn brown and secrete heavy resin that attracts animals such as rodents. White blisters that eventually turn yellow will appear at the canker site during spore production and release. The canker eventually girdles the branch, causing it to break off in the wind or under heavy snow.

Canopy: White pine blister rust can also be seen from afar by looking at the canopy. Common symptoms of WPBR include flagging branches, broken branches, and dying treetops (topkill). In all 3 cases, the needles will appear brown or orange in colour instead of green.

Ribes leaves: The disease appears as orange or yellow spores on the underside of Ribes leaves. These spores grow into tiny hair-like projections in the late summer and early fall.

White pine blister rust has a complex life cycle that alternates between 5-needled (white) pines and Ribes species. There have also been reports of the disease using Pedicularis species (Louseworts) and Castilleja species (Paintbrushes) as alternative hosts instead of Ribes.

The disease cycle begins in late summer and early fall when an airborne spore infects the needles of a white pine tree. Yellow or orange spots begin to appear on the needles shortly after infection as the spores spread towards twigs and branches. After a year of infection, spindle-shaped orange swellings called cankers will appear on the branches. Over time, these bright orange canker sites turn dark, rough, and swollen. They interrupt water and nutrient flow towards the top of the tree, resulting in the death of higher branches. White blisters will develop on the cankers 2-5 years after initial infection but turn yellow in colour when they’re ready to rupture and release aeciospores. These aeciospores are carried in the wind to infect the leaves of nearby Ribes, which eventually produce basidiospores that are released in the fall to infect white pine.

White pine blister rust spores thrive most in cooler, moist conditions during the late summer and early fall. These ideal conditions are usually present closer to the ground where there’s a higher moisture content. For this reason, lower branches and shorter, younger trees that stay wet longer are at a higher risk of infection.

*White pine blister rust can only survive in living tissue. It cannot be found in dead trees or dead plant material.

White Pine blister rust is native to Asia and was introduced into Europe before coming to North American. It is now found throughout the range of white pine in Ontario and throughout Canada: Eastern white pine can be found from Newfoundland to Manitoba; Western white pine can be found in British Columbia; limber pine and white-barked pine are found in British Columbia and Alberta. 

To report a sighting or to check up-to-date distribution maps of recent reports, visit EDDMapS.

Ecological Impacts

White pine blister rust kills white pine trees of all ages and sizes. Younger trees typically die soon after infection as their smaller size makes it easier for the fungus to reach the trunk from the needles. This fungal pathogen can therefore limit regeneration, making it difficult for white pine trees to grow and establish themselves. White pines are of great ecological importance and the loss of these trees can negatively impact the wildlife that rely on them for food and shelter. For example, white pines are common nest sites for a variety of birds, such as bald eagles and great horned owls. Additionally, it can take many years for white pine blister rust to kill a larger tree. During this time, top-kill (canopy death) in larger pines can interfere with cone production and dispersal, limiting reproduction. This, in turn, can reduce the available resources for wildlife. For example, several songbirds and small mammals that consume eastern white pine seeds will need to search for alternative food sources when diseased tree produce less cones. 

 
Economic & Social Impacts

White pines are a valuable source of softwood lumber as its wood is durable yet easy to work. They are used to make a variety of important products, including doors, cabinets, paneling, furniture, and moldings.

Dying and dead white pine trees that interrupt a green landscape are usually thought to be displeasing. These types of trees are generally unappealing to the public, resulting in less tourism and contentment.

White pines are also valued culturally, with reports of them being used for medicine by several Indigenous peoples in North America.