What is Killing Our Trees?

White Pine Blister Rust A Complex Disease

This is the fifth in a six-part series that explores some of the diseases that affect Michigan trees and potentially impact wildlife.  

The Eastern white pine (Pinus strobus) is Michigan’s official state tree—and with good reason.  The species dominated Michigan’s virgin timber in the mid-1800s, and lands north of a line from Muskegon to Bay City, in particular, were re-shaped by the lumbering that depended on it.  Deer feed on white pine needles in harsh winters, and many species of birds—from warblers to eagles—nest in white pines.  

In his book, Michigan Memories, Larry B. Massie offers the recollections of an old lumberjack boasting in the Seney Bar in the Upper Peninsula:  

“I was one of the red sash brigade that finished axing our way across the state of Maine in the 1840s.  Then we heard about Michigan.  What we heard didn’t nearly measure up to what we saw when we got here.  Why, there were trees so big that three men together couldn’t get their arms around them.  They reached 150 feet into the air, straight as an arrow.  That virgin timber land lay as far as you could see in all directions.  Those pines blanketed the Saginaw River Valley, the Thumb, they stretched from the Kalamazoo all the way to the Strait of Mackinac and across the whole U.P.”  

“Oh, sure there was maple, walnut, cherry and oak aplenty too, but we didn’t much care for hardwood.  We wanted pine – white pine in particular – green gold we called it – cork pine so light it floated like a cork in the water.”  

“….Oh, those were glorious days when pine was king – there was so much pine – we’d never cut it all – but we did!  By the turn of the century we were logging the last in the U.P.  Where’d it all go?  Well we built Chicago – twice!  As well as many another town out on the treeless prairie.”  

“I sure do miss the sound of the wind sighing through those big trees – the smell of the pine pitch – the yell timberrr!  What I wouldn’t give to be able to sink this ax into a big old cork pine and send a chip flying the size of your head – but they’re all gone now boys, they’re all gone now.”  

While Michigan’s old white pines are now limited to a few preserves like Hartwick Pines east of Grayling, the species is still prevalent throughout the state.  White pines are still cut for lumber, and tall, stately white pines grace yards in both urban and rural settings.  

The future of the species is a bit cloudy though because of white pine blister rust, caused by the fungus Cronartium ribicola.  White pines, especially young trees, are very susceptible to the often-fatal disease.  Nationwide, it has caused more damage and cost more to control than any other disease of conifers.  

Curiously, the fungus also infects plants in the genus Ribes, which includes currants and gooseberries, as well as certain other pines that have five needles per bundle.  Foresters and wildlife biologists fear the disease enough that they sometimes forgo otherwise desirable white pine plantings in the Great Lake states.  It is one reason red pine (not susceptible) plantings have often been substituted for white pine.  

White pine blister rust is most severe in cool, moist habitats and bad years often follow unusually wet summers and falls.  The disease can show up anywhere white pine grows, but is much more prevalent in the northern part of the state.  Foresters have produced (and updated periodically) blister rust hazard maps for some areas of the Great Lakes Region.  

The disease originated in Asia and arrived in the U.S. in a shipment of infected seedlings from Europe between 1898 and 1908.  It has since spread throughout the white pine’s range, with a boost from a separate introduction to British Columbia in 1910.  

The life cycle of the fungus is incredibly complex.  The fungus causes swollen white to yellow “blisters,” bearing a sweet liquid that attracts insects which help spread the infection.  Eventually the blisters pop, releasing spores into the air.  The tree’s bark dies and the fungus spreads into living tissue at the margins of the blister.  So, it keeps spreading until the branch or stem is girdled, stopping the flow of water and nutrients.  The disease is especially devastating to seedlings and can spread rapidly via various types of spores.  A special type of spore, called an aeciospore, can disperse hundreds of miles.  The aeciospores can’t infect pines, but do infect currants and gooseberries (Ribes), shrubs commonly found in white pine range.  Orange-colored rediniospores form on the undersides of Ribes leaves and build up until they eventually produce a type of spore that infect pines.  These are called basidiospores and typically disperse less than 1,000 feet.  

The key features of the disease cycle are no overwintering on Ribes, only on pine, and no pine-to-pine spread.  So, the role of Ribes as an intermediate host is crucial.  Pines looked at from a distance will usually show stunted or dead branches (sometimes called flagging), and dead tops.  But Ribes in the area will usually appear normal, unless you look for the yellowish-orange leaf spots on the undersides of the leaves.  

Through the 1950s, there were huge and complex Ribes eradication programs in parts of the U.S.  They worked somewhat in the Eastern U.S., but were abandoned in the West where Ribes was too hard to kill.  Today, most management of the disease centers on pruning away branch infections before the pine stem is infected.  Preventative branch pruning usually involves initial cuts up to 50% of tree height from the ground, repeated until there are no branches within 10 feet of the ground.  Planting white pines as understory trees rather than in the open also seems to reduce infection rates and severity.  Research has so far failed to produce strains of white pines with proven long-term resistance to white pine blister rust.   

Perhaps the only good news about white pine blister rust in Michigan is that the disease hasn’t been as devastating as in other regions.  About 5 million cubic feet of white pine are lost annually to blister rust in the Pacific Northwest.  But in the Northeast, the incidence of the disease has declined substantially since World War II.  Weather will be a big factor in the future impact of this disease in the Great Lakes Region.  

Dr. Patrick J. Rusz
Director of Wildlife Programs

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