Pine tree infections: White Pine Blister Rust

Pine Tree Infections

The Eastern White Pine, our symbolic tree, is integral to our province’s ecosystem but is now threatened by invasive pine tree infections. 

One of the most dangerous pine tree infections threatening Eastern white pines in Ontario is White Pine Blister Rust (Cronartium ribicola). There is no cure for White Pine Blister Rust once it has taken hold within an infected tree. As such White Pine Blister Rust poses a substantial risk to the native white pine populations in Ontario. 

What is White Pine Blister Rust?

Pine Tree Infections White pine blister rust

White Pine Blister Rust is a non native, invasive pine tree infection caused by the fungus Cronartium ribicola. It infiltrates the trees and forms cankers, causing their bark to burst open. Cankers appear first on branches and move to the trunk once the disease has taken hold. Once cankers appear on the trunk, the eventual death of the tree is inevitable.  

Which trees are affected by White Pine Blister Rust?

White Pine Blister Rust targets white pine species, with the Eastern White Pine (Pinus strobus) being the most susceptible in Ontario. The Eastern White Pine is the largest species of tree in our area, growing to 30 m in height with a crown spreading to 12 meters. However, all five needle pine species can be affected. Due to the dominance of white pines within our forests this is one of the pine tree pests and diseases most threatening to our local ecosystem.

Damage Caused by White Pine Blister Rust

White Pine Blister Rust causes significant damage to infected trees, impacting their growth, reproduction, and overall health. The fungus disrupts the normal physiological functions of the tree, leading to reduced photosynthesis, nutrient uptake, and water transport. The development of cankers on branches interferes with the flow of essential nutrients, resulting in weakened and stunted growth. Eventually, the cankers reach the trunk of the tree, where they completely encircle it, leading to the death of the tree. Ultimately, if not addressed, White Pine Blister Rust can lead to the decline and mortality of white pine populations.

Pine Tree Infections White pine blister rust

White pine blister rust symptoms

Infection takes place in the late Summer or early Fall, but the disease takes several years to complete its life cycle, so you will notice progressive symptoms over a number of years.

White pine blister rust life cycle

White Pine Blister Rust needs two unrelated hosts to complete its life cycle. The first stage of this pine tree infection begins in the spring when white blisters, called aecia, appear on trees infected the previous year. Within the blister, fungal spores are produced, which are released into the air when the blister eventually bursts. 

These spores cannot infect other pine trees – they require a different host – species of the genus Ribes, which includes currants and gooseberries. The fungus forms infections on their leaves and stems but does not tend to do much damage to this host and may not even be visible. The infection develops on the plant, finally producing basidiospores that can only infect pine in the Fall. 

Pine Tree Infections White pine blister rust on Blackberry Leaf

The spores are carried on the wind, and if they find a suitable species of pine within a few kilometers will infect them by entering through the needles. The fungus then will spread from the needles into the twigs and branches over the next 12 to 18 months. White blisters will begin to appear on infected branches the following spring.

Meanwhile, the fungus on the originally infected tree has spread into living bark on the branch. Each year more aecia are produced in spring, release their spores, the bark on which they were living dies back and the fungus moves on to living bark. It keeps spreading until the branch or stem is girdled and killed.

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Early Signs of infection

The disease first affects the needles. The needles of infected trees may show yellow or orange spots before eventually turning yellow and then rusty brown. Infected branches will develop cankers.

Later Signs of infection

If unchecked, cankers begin to develop on the trunk. These are often oval shaped and surround an infected branch. The cankers secrete a sticky white resin which runs down the trunk and  will attract rodents so increased signs of rodent activity may take place. 

As the infection progresses, the cankers enlarge and white or yellow blisters appear at the margin. This will produce powdery orange spores which spread the infection. 

From a distance iInfected trees will have a brown, sparse and thinning canopy, whole branches where the needles are yellow, brown or red  and multiple broken branches. 

Pine Tree Infections White pine blister rust

Prevention of White Pine Blister Rust

There is no cure for white pine blister rust so prevention of its spread is critical to its management and the preservation of white pines. Maintaining tree health and sound arboricultural practices are the best prevention. Thin and selectively harvest white pines to improve tree vigor and reduce the risk of infection.

White Pine Blister Rust does not spread from tree to tree. The spores of the fungus that cause the disease are spread on the wind. When planting new white pine trees, select varieties that demonstrate resistance to the disease.  Plant in dry rather than moist locations, on mid or upper slopes that face south or south west.

White pine blister rust requires alternate hosts to complete its life cycle. Reduce the proximity of hosts, such as currants and gooseberries, to white pine stands.

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White Pine Blister Rust Treatment

While no cure exists for White Pine Blister Rust, management strategies can help reduce the impact of the disease. Detecting White Pine Blister Rust in its early stages is crucial for effective management. 

The infection will begin in the late Summer or early Fall, but you may not see signs of infection. The infection develops over several years, so regularly check your trees for any signs. 

Prune infected branches. Promptly remove and destroy branches or trees exhibiting White Pine Blister Rust symptoms. Remove at least 4 inches of healthy wood before the first sign of infection. The pathogen cannot survive in dead wood, so no special disposal measures are needed for the infected branches. 

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