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Tuesday
Oct012019

White Pines Hit by Needle Disease Year After Year

By JOE RANKIN

Forests for Maine’s Future Writer 

Something’s not right.

That’s what my MacBook tells me when I’ve got a wonky internet connection. It’s also the phrase that pops unbidden into my head in early to mid-June when I look out the window and see yellowing needles on the white pines that fringe my meadow. Something’s not right.

And it isn’t. The pines are victims of early needle release disease. Also known as needlecast disease or white pine needle damage. But that doesn’t make me feel any better since there’s nothing I can do about it.

The eastern white pine, Pinus strobus, Maine’s iconic tree and a bulwark of its forest products industry, normally keeps its needles for three, sometimes four years. Older needles, the third or fourth year needles, are normally shed in the fall. Worn out, tired, they drift to the floor of the forest to add their organic matter to the soil, to be replaced the next spring by a new set.

But pine needles don’t fall in early summer. Normally. Unless the tree has early needle release disease.

And my pines, and probably yours as well, have suffered with this malady every year for the last decade. A disease that was once seen only occasionally has apparently become a recurring thing. And the results could have serious ramifications.

Defoliation of eastern white pines in the northeastern U.S. “is an escalating concern threatening the ecological health of northern forests and economic vitality of the region’s lumber industry,” a six-member team of researchers wrote in a 2017 article published in the scientific journal Global Change Biology.

Widespread defoliation was first documented in the spring of 2010 in Maine, where it affected pines across some 60,000 acres, the scientists note. Since then, white pine needle damage “has continued to spread and is now well established in all New England states,” they said.

Almost all conifers have some needlecast diseases. Reports from 50 to 100 years ago mention early needle drop as occasionally affecting white pines, said Kevin Smith, the supervisory plant physiologist at the U.S. Forest Service’s Northern Research Lab in New Hampshire. “But from 2010 forward we’ve been finding the same fungi that we used to see, but we’re seeing needlecast defoliation occur more frequently and over greater areas of the white pine forest.” 

The fungus, or, sometimes fungi, implicated in this disease are members of the sac fungi, the Ascomycota. You may be familiar with some of their relatives: yeasts are sac fungi, so is the fungus that produces penicillin. All four of the common species of needlecast fungi are native. Because there are multiple fungi implicated, that technically makes this what scientists call a “disease complex” rather than just a disease.

A needlecast fungus invades a pine’s newly emerging needles in the spring of one year. The following spring, in June or early July, the now second-year needles yellow and drop, leaving the tree with one set when it should have multiple sets of needles at the start of a new growing season.

Spread of the disease is intimately tied to the weather, said Aaron Bergdahl, the forest pathologist for the Maine Forest Service. Like most fungi, needle release fungi thrive in prolonged periods of moisture and Maine’s springs have gotten notably wetter, he said. The fungus is dispersed by raindrops and wind.

Then, if humidity remains high for some time, the spores invade emerging needles through the stomata, the pores the pine uses to breathe, as it were; and set up shop. The fungus uses the tree as a source of nutrition, and that nutrition allows it to . . . produce more spores.

But why the delay in needle drop?

Smith said the fungus is just taking advantage of the fact that the white pine holds onto its needles. Once in the needles it can take its time getting established, winding up to sporulate, or produce spores, in the second year.

Why wouldn’t the tree, knowing it’s infected, drop the diseased needles sooner? No one really knows. “I don’t think the needle drop is caused so much by a toxin produced by the fungus,” said Smith. “It may be a tree response to the question of ‘how do I get rid of the infection?’ If this was one needle out of a thousand it would be a good strategy. My gut level feeling is that the tree is shedding a needle that has become more a liability than an asset.”

You can see the results of premature needle shedding just by looking at the pines after a spring needle release:  A pine stand can look thin, wispy, like a person with thinning hair. Then a second set of needles grows in and the view goes back to normal. But . . . affected pines have lost some ground because they’ve been working with one set of needles rather than two. You could think of it like a solar farm where a large percentage of the panels aren’t working. Efficiency is lost.

The good news is that it’s third or second-year needles that are lost. Older needles aren’t as efficient as younger ones, notes Bergdahl. “New foliage is the most valuable to the tree and the hardest working tissue,” he said. So, losing an entire year’s crop of needles prematurely isn’t going to cost the tree half its photosynthesizing power.

And if it was only one year out of 20, or 10, or even 5 . . . ? But it hasn’t been, and there are big questions of how long white pines can take it when premature needle loss occurs year after year. You might think of it like a human who has some chronic disease.

If trees are defoliated repeatedly, said Smith, “eventually the tree will run out of energy even to do its business of making new foliage, setting buds in the fall.”

While needlecast disease weakens the tree by depriving it of energy, it’s also making the tree more vulnerable to other pests and pathogens, said Smith. “How long can you defoliate 50 percent of a mature white pine before armillaria root rot or other diseases come in? That could depend on land use history.”

Bergdahl said larger pines in the overstory could be less affected, their crowns up high in the canopy where needles dry more quickly. Younger trees in the understory, where humidity is higher and there’s less light, could be more vulnerable. Also, pines on good sites could hang in there while those on more marginal sites succumb.

The potential effect on pine regeneration, he said, “is one of the more alarming aspects of this whole story.”

There are a lot of questions about needlecast disease in pines. The disease is a hot topic now in research circles and scientists are looking hard for the answers. The Maine Forest Service is working with the U.S. Forest Service and its counterparts in other northeastern states on a region-wide study of the incidence of the disease and its causal agents.

As part of that study Maine researchers sampled needles from 35 sites across the southern and central parts of the state. “At every site that was sampled at least one needle disease was found,” said Bergdahl.

But even if we learn more about early needle release disease, we may not be able to do anything about it. Fungal diseases are notoriously hard to combat, especially in a forest setting: think Dutch elm disease or chestnut blight, both, incidentally, caused by sac fungi.

And, we can’t control the weather.

Climatologists are predicting increases in annual rainfall for Maine as the earth’s climate warms, with more rain in the spring and fall and less in the summer. That could be very good for rainsplash-spread fungi.

Stephen A. Wyka at the University of New Hampshire, Durham and the other researchers who authored the Global Change Biology study analyzed weather data and confirmed a trend of “increasing temperature and precipitation.” And that, the previous year’s May, June and July rainfall was “the best predictor of defoliation events in the following year.”

“Our results clearly demonstrate the role changing climate patterns have on the health of eastern white pine in the northeastern United States,” they said.

 

Joe Rankin has written feature articles for Forests for Maine’s Future since 2010. He lives on 70 acres of forest in New Sharon.