What is Tar Spot?

 

The black scabs on these Silver Maple leaves are signs of tar spot, a common fungal disease that also affects Norway, Red and other maples as well as willows, holly and sycamores.

The spots, called stromata, are the overwintering form of the fungus. When infected leaves fall, they often land with their upper surfaces, and so the stromata, facing up. That puts them in a good position to release wind-borne or rain-splashed spores next spring. The freed spores then infect new leaves, eventually producing light green to yellow spots on the blades that enlarge as the season progresses. The spots turn dark and tarry-looking in late summer and early fall. 

Although it looks bad, tar spot is rarely a serious disease. It can cause early leaf drop, but otherwise it's just unsightly. Raking and destroying infected leaves can prevent reinfection in ­­spring. In most cases, fungicides are not recommended.

Inside a Stroma

If you slice through a stroma in spring and examine it with a microscope, you'll find its lower surface covered with tiny, bowl-shaped structures called apothecia. Inside each apothecium are hundreds of  sacs called asci ("as-eye" or "ask-eye," singular ascus). Each ascus contains eight needle-shaped spores that are released through cracks in the overlying stroma. 

Left: An apothecium of Rhytisma acerinum with the dark, overlying stroma ruptured. A clear layer of asci covers the bottom of the apothecium. Right: A closeup of the asci with emerging needle-like spores. Both photos by Bruce Watt, University of Maine, Bugwood.org. 

Because tar spot fungi need living tissue to survive, spores are not released in fall. Instead, they are released in spring, when leaves are emerging from their buds. Blown by wind or launched by splashes of water, many of the spores won't land on a susceptible host. With a good measure of luck, some will, and the life cycle begins again.  

The "Womb" Fungi

Tar spot fungi are in the genus Rhytisma. Three species typically infect maples: native R. americanum and R. punctatum and introduced R. acerinum.

All three species belong to a large group of fungi called Ascomycetes, or sac fungi. The apothecia of sac fungi typically are open at the top, but those of Rhytisma are covered by a layer of fungal tissue in the stromata. 

Because the protected apothecia resemble wombs, they're also called hysterothecia, from the Greek root words "hystero," meaning uterus or womb, and "theca," meaning case or cup.

References

Leaf Diseases of Hardwoods: Tar Spots. Dr. Robert Blanchette, College of Food, Agriculture and Natural Resources, University of Minnesota.

Tar Spot of Trees and Shrubs. Brian Hudelson, University of Wisconsin-Madison Plant Pathology. Last Revised 12/18/2018. 

Rhytisma acerinum and Rhytisma punctatum, two causes of Tar Spot of maple. Heather Hallen Adams and Tom Volk, University of Wisconsin-LaCrosse. Fungus of the Month, October 2007. 

 

Hopeful News About Emerald Ash Borer

 “Attack fungi” could help manage this destructive insect.

University of Minnesota researchers recently discovered insect-attacking fungi in the larval galleries of emerald ash borer. Their finding offers hope that the fungi could help manage this destructive pest.

Emerald ash borer (EAB) is an introduced beetle that kills all species of ash trees. It has been present in the U.S. since at least 2002 and in Minnesota since at least 2009. Even with a federal quarantine (now removed), the insect spread rapidly. According to the USDA, its range now covers most of the eastern U.S. and the Midwest, with isolated infestations as far west as Colorado.

The larvae of EAB cause the bulk of the damage. After eggs hatch, the larvae bore into the inner bark, the area that includes the water- and sap-conducting cells of the xylem and phloem. Their galleries of serpentine tunnels interfere with the flow of needed resources and kill trees in 2-4 years. Hundreds of millions of trees have succumbed to the insect.

Hope for managing EAB rests in part on biocontrols, organisms that prey on eggs, larvae or adults and so reduce their numbers. Three parasitoid wasps have been released in Minnesota as potential biocontrols. More information about that program is available here.

New biocontrols could emerge from recent research by the Minnesota Invasive Terrestrial Plants and Pests Center (MITPPC). In their study published in Fungal Biology, U of M researchers sampled affected trees from Rochester to Duluth and isolated the fungi associated with EAB larval galleries. They identified many types of fungi, including some that are entomopathogenic – fungi that attack insects.

One fungus they identified, Beauveria bassiana, has already been studied for EAB control. The other entomopathogenic fungi they found also need research to see if they, too, could be used to manage the insect.

This good news comes as EAB continues to spread in Minnesota. Since the November 2020 post about EAB, the insect has been confirmed in two more counties, Cottonwood and Blue Earth in southwest Minnesota. The state’s Department of Agriculture maintains a quarantine boundary that now includes 27 affected counties. 

The Two Lives of Cedar-Apple Rust

 

The dimpled, red galls on this eastern red cedar (Juniperus virginiana) are signs of cedar-apple rust, a fungus that divides its time between two completely different hosts. One part of its life cycle is completed on junipers, where these golf ball-like masses can be spotted in winter. The other part is completed on plants in the rose family, such as apple trees. On each host, the appearance of the fungus is so different that it can be hard to connect the two as belonging to the same organism.

In spring, cedar-apple galls on junipers sprout gelatinous, orange “horns.” These gummy tentacles produce and release spores that can infect the leaves of apples, crabapples and sometimes hawthorns. As the fungus grows on apple trees, the leaves develop yellow or orange spots on their upper and lower surfaces. In summer, spores released from the spots on the lower surfaces of the leaves are spread by wind back to junipers, where they form overwintering galls. And so the cycle is continues.  

Left: Cedar apple gall with orange, spore-producing "horns" in spring. Right: Spots on an apple leaf caused by the cedar-apple rust fungus. Spores produced on apple leaves then infect junipers.

Photos by James Chatfield, Ohio State University, Bugwood.org, through forestryimages.org.

Cedar-apple rust usually doesn’t have severe effects, although infection can cause susceptible apples and crabapples to lose their leaves early. Fruits may also develop unattractive spots. Many varieties of apples and crabapples are resistant to cedar-apple rust. For a list, see the University of Minnesota Extension Service link below.

Spots on apple leaves can also be caused by other fungi. Apple scab is one example.

References

Cedar-apple rust and related rust diseases. R. Koetter and M. Grabowski, University of Minnesota Extension Service.  Accessed online 2/27/21.

Plant of the week: Cedar apple rust (Gymnosporangium juniperi-virginianae Schwein.). D. Taylor, U.S. Forest Service. USDA. Accessed online 2/27/21.

Agrios, G. N. 1988. Cedar-Apple Rust. Pages 462-466 in Plant Pathology, third edition. Academic Press, Inc. New York.

Crusty Clues to Plant ID

In this patch of winter woods, black knot has a grip on the understory. Dark, lumpy galls crust over many stems and branches, flagging them against the snow. From a distance it looks like scat, but this isn’t animal stuff. It isn’t plant stuff either, not entirely. It’s a fungus, and it’s dropping hints about the plants growing here.

Many fungi that grow on plants, including black knot, have specific hosts. Some grow so consistently on one plant or another that when they’re found, they can help identify a plant to its genus, if not its species. This can be especially helpful in winter, when plant ID is challenging without leaves.

Many fungi serve as reliable guides to plant identification, but here are three that are especially common or commonly sought and easy to recognize.

Prunus and Black Knot

Black knot, Apiosporina morbosa, infects trees and shrubs in the genus Prunus, a group that includes native and introduced cherries and plums. In Minnesota, the disease is especially common on chokecherry, Prunus virginiana, a small tree found in open woods and woodland edges throughout the state. It also affects American plum (P. americana), Canada plum (P. nigra), pin cherry (P. pensylvanica), black cherry (P. serotina) and sand cherry (P. pumila). All these are native shrubs and trees, but several introduced species and cultivars of Prunus are also susceptible to the disease.

Black knot is spread in spring, when fungal spores produced in these knots are carried by wind or rain to new hosts. A year later, after the fungus has stimulated its host to grow a mass of large cells, the infection appears as a swelling with a light brown or olive-green surface. In the second summer after infection, the gall turns black. Some galls may have white or pink patches on the surface from other fungi parasitizing the knot.

Tiny pores on the surface of a gall mark the exit holes for spores. They’re easiest to see with magnification. Pores are one way to distinguish black knot from chaga, a similar dark, crusty fungus that grows primarily on birch trees. Unlike black knot, chaga is sterile -- it doesn't produce spores. More on that next. 

Birch Trees and Chaga

Chaga, Inonotus obliquus, is a parasitic fungus found in northern forests around the world, including the United States and Canada. It has been used in folk medicine for centuries and is still harvested and consumed for its purported health benefits. Formal studies of its medicinal use continue.

In this region, chaga grows mostly on birch trees, especially paper birch, Betula papyrifera. It also grows on yellow birch, Betula alleghaniensis, and much less frequently on alders, beech, oaks, maples and aspen.

On living birch trees, chaga looks like a hard, irregular, black mass erupting from the trunk. Likened to burned charcoal, the surface is a melanin-rich mass of dead fungus over an orange or brown interior. The masses, popularly called conks, appear on trunks or large branches, but not on small branches. An infected tree may bear 1-3 conks.

Chaga conks are the visible part of the fungus. The hidden part lives mostly in the heartwood, where it causes white rot. The tree may live with the infection for up to 80 years, producing slow-growing, sterile conks that take many years to mature.

Inonotus, the fungus that produces chaga, reproduces only after its host dies. The fruiting body (actually a spore-producing body called a basidiocarp) is a mat of slender, vertical tubes formed under the bark, typically above a sterile conk. As the fruiting body develops, it exerts so much outward pressure that it will suddenly rupture the bark. It’s a fascinating find, but a rare one. Fruiting bodies are produced only once in the life cycle of the fungus, and they live for just a few days.

Chaga look-alikes include black knot and several shelf mushrooms in the genus Phellinus. Unlike chaga, these fungi release spores through tiny pores. Black knot pores cover the gall, wheareas shelf mushrooms have a pore layer on their lower surfaces. 

A note about harvesting chaga:

Because of its purported medicinal value, chaga is in high demand, so harvest from some public lands is regulated. According to Ed Quinn, Natural Resource Program Supervisor for the Division of Parks and Trails at the Minnesota DNR, harvesting chaga is illegal in state parks, state recreation areas, state monuments and state waysides. Although state park rules permit collection of­­­­ mushrooms for personal use, chaga is technically not a mushroom because it isn’t a fruiting body -- it doesn’t produce spores. In addition, use of spikes, ladders, knives and hatchets to reach and collect the fungus can damage trees and create wounds that may leave them vulnerable to pathogens and insect pests. Finally, because the fungus slowly rots the heartwood of infected trees, climbing them can be unsafe.  

The rules in state forests differ from those in state parks. According to Dave Schuller, State Land Programs Supervisor for the Division of Forestry at the Minnesota DNR, collecting chaga for personal use is allowed without a permit, but host trees must not be damaged. Commercial harvest is allowed with a special products permit from a local DNR forestry office. The permit requires harvesters to take only the visible part of the fungus, without cutting into the stem or damaging live trees. More information about chaga harvest is available from the University of Minnesota Extension Service publication linked in the references below. Again, safety is paramount when harvesting chaga. Because trees bearing the fungus may have significant interior decay, climbing them is risky and can result in serious injury. 

Harvesting chaga on other public lands, such as regional or local parklands, may also be regulated. Check with the appropriate authority before heading out.

There are also concerns about the potential effects of collection on chaga biology and ecosystem health (Thomas et al., 2020). One issue is that overharvesting chaga has unknown effects on the ability of the fungus to reproduce. Taking too much, too often could delay or deny Inonotus its once-in-a-lifetime opportunity to make fruiting bodies, with consequent effects on its spread.  

Overharvesting also has unknown effects on the function of ecosystems where chaga is found. Other living things – insects, for example – may depend on chaga in ways not yet understood. Without that understanding, aggressive collection of chaga could have ripple effects on ecosystem health. More study is recommended.

 

White Oaks and Smooth Patch

Smooth patch looks like it sounds: It’s a smooth patch on otherwise rough bark. Usually the area is low on the trunk and sunken and lighter than surrounding bark. Smooth patch disease is caused by any of several fungi, especially Aleurodiscus oakesii. This fungus prefers trees in the white oak group, In this region the most common hosts are white oak, Quercus alba, and bur oak, Quercus macrocarpa. Less often, smooth patch is also found on birch, ash, willow and basswood.

The fungi that cause smooth patch feed only on dead outer bark and don’t directly harm the tree. Small, cup-like fruiting bodies are often seen within the smooth patch. They are light brown or gray with curled edges. In winter they may be shriveled and look like lichens. The genus Aleurodiscus translates to “flour disc,” named for the whitish, dusty appearance of the disks in spring and summer.

 

References

Black knot

American Phytopathological Society. Black knot. Website accessed February 11, 2021.

University of Illinois Extension, Department of Crop Sciences, University of Illinois at Urbana-Champaign. Black knot of plums and cherries. RPD No. 809 September 2000.

University of Minnesota Extension Service. Black knot. Website accessed February 11, 2021.

Chaga

Millman, L. 2012. Chaga’s Significant Other. Fungi 5:3, 11-12.

Min-Woong Lee, Hyeon Hur, Kwang-Choon Chang, Tae-Soo Lee, Kang-Hyeon Ka, L. Jankovsky. Introduction to Distribution and Ecology of Sterile Conks of Inonotus obliquus. Mycobiology. 2008 Dec; 36(4): 199–202. Published online 2008 Dec 31. doi: 10.4489/MYCO.2008.36.4.199

Natural Resources Canada. Sterile conk trunk rot of birch. Date modified:2015-08-04. Website accessed February 11, 2021.

Spinosa, R. and Bunyard, B. No, That’s NOT Chaga! Fungi 5:3, 45-47

Thomas P.W., Elkhateeb W.A. & Daba G.M. 2020. Chaga (Inonotus obliquus): a medical marvel becomes a conservation dilemma? Sydowia 72: 123–130.

University of Minnesota Extension Service. 2013. Chaga (Clinker Polypore). Pages 99-101 in the Minnesota Harvester Handbook. Available at https://conservancy.umn.edu/handle/11299/173824.

Smooth patch of oak

Smith, W. R. 2008. Quercus macrocarpa Michx., Bur oak. Pages 382-383 in Trees and Shrubs of Minnesota. Minnesota Department of Natural Resources. University of Minnesota Press, Minneapolis.

University of Minnesota Extension Service. Non harmful tree conditions. Website accessed February 11, 2021.

Vann, S. R. Undated. Smooth patch of oak trees. FSA7578, University of Arkansas, Division of Agriculture.

Volk. T. April 2006. Aleurodiscus oakesii, the oak parchment, cause of "smooth patch disease."https://botit.botany.wisc.edu/toms_fungi/apr2006.html, accessed February 16, 2021.

Status of Emerald Ash Borer in Minnesota

 

Range of confirmed emerald ash borer in Minnesota. Source: Minnesota Department of Agriculture.

Emerald ash borer, or EAB, has been confirmed in 25 counties in Minnesota. The latest discoveries were in late summer 2020 in Carver and Sibley Counties, southwest of the Twin Cities. All affected counties are shown in the map above, which is a snapshot of an interactive map maintained by the Minnesota Department of Agriculture. It shows quarantined counties in pink, the quarantine boundary in red, and the boundaries of generally infested areas in green. Click on the map or the link in this paragraph to view the interactive version.

Movement of some types of wood out of the quarantine area is regulated. Ash logs and lumber, ash tree waste, ash chips and mulch, and any hardwood (deciduous) firewood should not be moved outside the quarantine area without a Compliance Agreement with the MDA. The agreement requires that the wood be treated in any of several ways to minimize the risk of EAB spread. To get an agreement, call Arrest the Pest at 888-545-6684 or email arrest.the.pest@state.mn.us.

How to Detect EAB

EAB is a serious pest of all native ash trees. The insect, Agrilus planipennis, overwinters as larveae or pupae in the bark or wood. Metallic, green adults, just shorter than the width of a penny, emerge from the trees in spring or early summer and later lay their eggs on ash trees. The larvae burrow into the bark and feed on the inner bark and outer sapwood, tissues that conduct water, nutrients and sugars throughout the tree. Infested trees die 1-3 years later.

During the growing season, one symptom of EAB infestation is die-back of the canopy, starting at the crown and moving down. Some trees also develop epicormic sprouts, branches that grow from the lower part of the trunk. In fall and winter, look for woodpecker holes (the birds eat EAB larvae), cracks in the bark and, under the bark, S-shaped galleries of EAB larvae.

From left: 1) Canopy dieback; Source: Steven Katovich, Bugwood.org. 2) Epicormic sprouts; Source: Pennsylvania Department of Conservation and Natural Resources - Forestry, Bugwood.org. 3) Woodpecker damage; Source: Kenneth R. Law, USDA APHIS PPQ, Bugwood.org. 4) EAB galleries: Source: Kelly Oten, North Carolina Forest Service, Bugwood.org. Full photo credits are below.

To determine if an ash tree might be infested with EAB, use the MDA’s “Does My Tree Have Emerald Ash Borer (EAB)?”  If you then suspect an EAB infestation, call Arrest the Pest at 1-888-545-6684 or email arrest.the.pest@state.mn.us.

Help Find EAB

Emerald ashborer. Source:Leah Bauer, 

USDA Forest Service Northern Research 

Station, Bugwood.org.

Emerald ash borer beetles are inconspicuous, but a University of Minnesota citizen science project used a unique tool to find them: Wasps. Volunteers for the project, called Wasp Watchers, monitored nests of the smoky winged beetle bandit wasp, Cerceris fumipennis, for their beetle prey. Finding EAB alerted scientists to locations of affected ash trees, sometimes before the trees began showing symptoms.

The Wasp Watchers program ended in July 2020, but volunteers can still monitor wasp colonies for beetle prey and report their findings through iNaturalist. Wasps or nesting sites with EAB should also be reported to Arrest the Pest. These web pages offer more information:

•          Sunsetting Wasp Watchers
•          Wasp Watchers Training Material

What About Other States?

For information about EAB in neighboring states, visit these websites:

•       North Dakota
•         South Dakota
•          Iowa
•          Wisconsin

References and More Information

Haack, R. A., et al. Emerald Ash Borer Biology and Invasion History. Chapter 1 in Biology and Control of Emerald Ash Borer. USDA, March 2015. https://www.fs.fed.us/foresthealth/technology/pdfs/FHTET-2014-09_Biology_Control_EAB.pdf

Emerald Ash Borer Program. Minnesota Department of Agriculture. Website accessed October 29, 2020.

Questions & Answers About the EAB Quarantine & Compliance Agreements. Minnesota Department of Agriculture. Website accessed October 29, 2020.

Photo Credits

Canopy dieback by Steven Katovich, Bugwood.org, licensed under a Creative Commons Attribution 3.0 license.

Epicormic sprouts by Pennsylvania Department of Conservation and Natural Resources – Forestry, Bugwood.org, licensed under a Creative Commons Attribution 3.0 License.

Woodpecker damage by Kenneth R. Law, USDA APHIS PPQ, Bugwood.org, licensed under a Creative Commons Attribution 3.0 License.

EAB galleries by Kelly Oten, North Carolina Forest Service, Bugwood.org, licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

Emerald ash borer by Leah Bauer, USDA Forest Service Northern Research Station, Bugwood.org, licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

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.