Saturday, October 24, 2020

Elm Trees Still in Dutch


Beautiful but foreboding, the galleries of the European elm bark beetle spell trouble for this American elm. They also cut to the heart of a cautionary tale.

Some months ago, a female beetle found or was lured to this weakened tree to build a nursery. She burrowed through the bark to the outermost layer of wood, where she chewed a tunnel parallel with the grain. She laid eggs along the sides of her gallery, and when they hatched the larvae then tunneled away from her path. After metamorphosis, a phalanx of new adults exited the tree and flew to other elms to feed.

By themselves, the European beetles, Scolytus multistriatus, aren’t a disaster for elms. Neither are native elm bark beetles, Hylurgopinus rufipes, which have similar habits. The much greater harm comes from the fungi they can carry from diseased elms, as this one was, to healthy elms. The fungi, Ophiostoma ulmi and its more aggressive cousin, Ophiostoma novo-ulmi, are better known as the agents of Dutch elm disease.

This is a disease of deprivation. The spores carried into the tree by the beetles germinate inside the xylem, the water-conducting vessels of the wood. As the fungus grows and reproduces, it causes living cells in the wood to push balloon-like extensions called tyloses into the vessels. Potentially, the tyloses could inhibit the spread of the fungus, but they usually form too late. Instead of defending the tree, they combine with gums produced by the tree’s degraded cell walls and the fungal masses themselves to plug the vessels and block the flow of water to the leaves. The leaves then wilt, turn yellow and then brown, and eventually fall. This symptom, called flagging, is one of the first visible signs of trouble.

Just as elms respond too slowly to defend themselves, so communities responded too slowly to stop the spread of this disease. Introduced into the U.S. in the 1920s on imported elm wood, the fungus spread quickly across the country. In Minnesota, the first diseased trees were discovered in the early 1960s in St. Paul and Monticello. Aided by urban monocultures and initial doubt that the European beetles would survive here, Dutch elm disease soon spread to every county in the state. All three native elms - American elms (Ulmus americana), rock elms (U. thomasii) and red elms (U. rubra) – are susceptible, and thousands have succumbed.

The good news is that elms persist. Young elms that survived the first onslaught of the disease have matured, and it’s worth the effort to protect them. In addition, resistant – but not immune – elms have been selected from surviving American elms or hybrid American and Asiatic elms, and they are being planted in greater numbers. They, too, would benefit from sanitation practices that take the fungus out of circulation.

Among its many lessons, Dutch elm disease teaches the importance of tackling invasive species (ones that are introduced and harmful) before they cause widespread damage. Awareness is an important first step to doing that. The page about invasive species includes links to many resources that can help identify and manage such species while their numbers are low. It can be a wily game, but as anyone who’s lost an elm might say, it’s one worth playing well.

 

 References and further reading

History of Dutch Elm Disease in Minnesota, by David W. French. University of Minnesota Extension Service.

Dutch Elm Disease. Minnesota Department of Agriculture. Website accessed October 13, 2020.

Dutch elm disease. University of Minnesota Extension.

D’Arcy, C.J. 2000. Dutch elm disease. The Plant Health Instructor. DOI: 10.1094/PHI-I-2000-0721-02
Updated 2005
. Available on the website of the American Phytopathological Society,
https://www.apsnet.org/edcenter/disandpath/fungalasco/pdlessons/Pages/DutchElm.aspx.

Thursday, October 8, 2020

Plant Profile: New England Aster

New England Aster blooming at Baker Park Reserve in Maple Plain, Minnesota, in early October 2020.

New England Aster (Symphyotrichum novae-angliae) is a widespread native plant of open, low grasslands, meadows, roadsides and shore lands. Its stout, hairy stems grow 3-6 feet tall and branch at the top. Alternate, hairy leaves are up to 4 inches long and 1 inch wide. They clasp the stem at their bases. 

Pink to purple flowers bloom from August into October at the ends of the stems. At 3/4 to 1-1/2 inches wide, the flowers are among the largest and showiest of the asters, and they can trick the eye. What looks like one flower is actually a group of florets in a head, called a head inflorescence. The center of the head bears small, tubular, yellow to orange disk florets. Around the disk are 50 or more pink or purple ray florets, so called because each bears a single, petal-like ray. 

Below the head are green or greenish-purple bracts called phyllaries. The phyllaries of New England aster are narrow or lance-shaped with long points. They curve away from the head and are covered with short hairs. Some of the hairs are tipped with bulbous glands, appearing as tiny glistening dots under a magnifying lens. The stalk of each inflorescence, called a peduncle, is also hairy. 



If you find New England Aster in bloom -- and if you have a steady hand and lots of patience -- pinch off a few disk flowers and use a needle to slit them open lengthwise. A 20x or higher magnifying lens will help. Inside the flower you may see that the filaments supporting the anthers are fused into a column that surrounds the style, the neck of the pistil. When the anthers open, they drop their pollen into the column. As the style elongates, its hairy surface picks up the pollen and makes it available to pollinators. The fused filaments of the stamens could explain the genus name Symphyotrichum, which means "joined hairs."

Many other asters in this region bloom at about the same time as New England Aster, but this species blooms later than most. Also, no other native aster has the same combination of large flower heads, clasping leaves, and hairy stems, leaves and phyllaries.

New England Aster is a late-season source of pollen and nectar for bees, moths and butterflies. To grow it in a garden or landscape, choose a location with moist to mesic, rich soil and full sun to partial shade. The plant will spread by rhizomes to form clumps. To maintain the plant's vigor in a garden, divide it in spring every three years. 

References:

USDA Plants Database, https://plants.usda.gov/core/profile?symbol=SYNO2

Wetland Plants of Minnesota, by Steve W. Chadde. A Bogman Guide, 2012. 

Pollinators of Native Plants, by Heather Holm. Pollination Press, LLC, 2014.









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