Forest pathologist’s grim outlook for tree diseases

NORFOLK — Robert Marra of the Connecticut Agricultural Experiment Station (CAES) told an audience that genetic engineering might prove a better strategy than creating hybrids in restoring American chestnuts. Marra, a forest pathologist at CAES, spoke to a group of about 50 people at Great Mountain Forest in Norfolk on Saturday, July 16.

Marra said the genetic chestnut idea is “highly contested” but could prove to be more effective than creating hybrid strains of American chestnut and blight-resistant Asian chestnut. (An example of the hybrid approach exists in a grove in Falls Village.)

Prior to widespread American chestnut blight, Marra said estimates put the percentage of American chestnut trees in hardwood forests in the eastern United States at somewhere between 30% and 50%.

The blight was caused by a parasitic fungus that was accidentally introduced into the U.S. in 1904, from imported Asian chestnut trees, which are resistant to the fungus.

By about 1950, the disease “was as widespread as the tree itself.”

Beech bark disease threatens American beeches and, to some extent, European beeches.

The villain in this case is am exotic scale insect, likely imported on European beech trees in the late 19th century.

The insect eats the bark, creating a white, waxy wooly substance.

The damage to the bark allows a native fungus to get into the tree. Marra said without the bug paving the way, the disease would not exist.

The scale insect was introduced in Nova Scotia around 1890, and Marra said the disease is widespread in the northeast and heading south at least as far as the Great Smoky Mountains National Park in North Carolina and Tennessee.

Beech leaf disease is a more recent development. Marra said it was first identified in 2012 in Ohio, and affects American, European and Oriental beech trees.

The leaves become banded, with darker areas the primary clue that the disease has struck.

Marra said foresters had no immediate answers for the disease, and that by 2019 it had spread into upstate New York, Pennsylvania, the New York City metro area, and Connecticut.

Marra showed photos of affected trees in West Rock Ridge State Park in New Haven and Hamden.

The result, he said, is trees that provide little or no canopy because of their degraded leaves.

“There’s not a lot of photosynthesis going on.”

Marra said the culprit has been identified as a nematode (aka eelworm), a plant parasite. The specific nematode here is litylenchus crenatae mccannii.

“This is confirmed and proven as causal,” Marra said firmly.

Any “wet event” (rain, drizzle, dew, or humidity) will trigger the movement and deleterious work of the nematode.

Marra wound up the grim litany of tree disease with oak wilt, first spotted in Wisconsin in 1944, and now in 21 states.

Marra said the fungus Bretziella fagacaerum grows on the oak trees, with disastrous results. The fungus is spread by oak bark beetles.

“Once it sets in it goes crazy and blocks water transport” within the tree. Marra said the result is often mistaken for effects of drought.

“All oaks are susceptible,” Marra said, red oaks most of all.

Symptoms include rapid leaf discoloration and wilting, starting at the top of the tree and moving down.

A diseased tree can die in as little as three weeks.

Is oak wilt in Connecticut?

“We don’t know,” said Marra. The CAES is collecting samples, and there was one tree in Guilford that received scrutiny.

Marra said New York state, where there is definitely oak wilt, the state has set up quarantine protocols.

But Connecticut does not have any measures in place.

Isolation and prevention measures are expensive, Marra said, showing a slide of a million-dollar piece of equipment, a vibratory plow, in use in Minnesota.

And the machine “probably wouldn’t work here,” Marra said. “Guess why?”

“Rocks!” said the audience.

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