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. 

 

Are Psyllids the Solution to Invasive Knotweeds?

Help is on the horizon to manage these aggressive plants.

Fallopia japonica  (Houtt.) Ronse Decr., Japanese Knotweed
Fallopia sachalinensis (F. Schmidt) Ronse Decr., Giant Knotweed
Fallopia x bohemica (Chrtek & Chrtková) J.P. Bailey, Bohemian Knotweed

A stand of Japanese Knotweed in October. Although these plants are green, others are starting to yellow. The spiky growths are inflorescences that have shed their flowers and fruits.

In Minnesota and around the world, invasive knotweeds are some of the most difficult plants to manage.

Aided by vigorous rhizomes that can extend several meters from a parent plant (1, 2), Japanese Knotweed, Giant Knotweed and their hybrid, Bohemian Knotweed, can displace native plants, change soil chemistry, alter microbe and invertebrate assemblages, and enhance soil erosion, especially on riverbanks (3, 4, 5). In addition, they are potentially allelopathic, meaning they can release compounds from living or decaying tissues that inhibit the growth of some plants (6, 7).

Knotweeds are spreading in North America and elsewhere, and as they do, so is the realization that controlling them is no easy task. (See EDDMapS for their distributions in North America; use the genus Reynoutria, a synonym.) Although a variety of chemical and mechanical control methods can be effective, all require years of repeated efforts and monitoring for renewed growth (8). Mechanical methods – digging and cutting – also demand meticulous attention, because fragments of rhizomes or stems left behind can regenerate the plants (9).

This large stand of Knotweed covers about 2,000 square feet along a property edge. The property is adjacent to a lake, offering the riparian habitat where knotweeds thrive and spread. 

Biocontrol – the use of a plant’s natural enemies – now offers some hope for an efficient alternative. Small, sap-sucking insects called Knotweed Psyllids, Aphalara itadori, have been collected from Japan, part of knotweeds’ native range. After years of study to determine that the insects would effectively target only knotweeds, scientists released the psyllids into several field locations in North America.  

According to the North American Invasive Species Management Association (NAISMA, 10), one race of the psyllids, called the Kyushu line, was released on Japanese and Bohemian Knotweed populations in several states in 2020 and 2021. (Minnesota was not one of them.) Another race, called the Hokkaido line, was released on Giant Knotweed populations in 2021. The nymphs and adults feed on knotweed stems and leaves, removing sap from the plants and causing the leaves to curl. The insects leave behind lerp, a flaky or stringy crystallized form of honeydew, a sugary solution they excrete.

When winter arrives, the adult psyllids take shelter and enter diapause, a time of suspended activity. Because the insects are native to a similar climate in Japan, it is hoped the adults will survive winters here and resume activity in spring, as they do in their original range. However, according to NAISMA, no sustained populations have been confirmed. Additional races or populations are being studied for future release.  

Although psyllids are not yet a sure solution to knotweed invasions, the research is promising. It also highlights the importance of accurate plant identification, because different races of psyllids are effective on different knotweeds. The Kyushu race is more effective on Japanese and Bohemian Knotweed, whereas the Hokkaido race is better for Giant Knotweed. A third race, called the Murakami line, is being studied for release on Bohemian knotweed in Canada.

In Minnesota, Japanese, Giant and Bohemian Knotweed are on the state Department of Agriculture’s noxious weed list. They are classified as “prohibited-control,” meaning all propagating parts, including seeds and vegetative parts, must be prevented from spreading. In practice, that means treating at least the above-ground growth before the stems flower in late summer, and if the stems are cut, collecting and disposing of the cuttings so they don’t escape and establish knotweed stands elsewhere.

In other words, it’s a lot of work, especially in large patches. With some additional research and field tests, psyllids may someday make that work easier.

Resources for Knotweed Identification and Management

Grevstad, F.S., J.E. Andreas, R.S. Bourchier, and R. Shaw. 2022. Knotweeds (Fallopia spp.): History and Ecology in North America. In: R.L. Winston, Ed. Biological Control of Weeds in North America. North American Invasive Species Management Association, Milwaukee, WI. NAISMA-BCW-2022-19-KNOTWEEDS-P. [Accessed online at 23198.pdf (bugwoodcloud.org)]

Dusz M-A, Martin F-M, Dommanget F, Petit A, Dechaume-Moncharmont C, Evette A. Review of Existing Knowledge and Practices of Tarping for the Control of Invasive Knotweeds. Plants. 2021; 10(10):2152. https://doi.org/10.3390/plants10102152.

Minnesota Department of Natural Resources. Non-native knotweeds: Japanese, Bohemian, and Giant knotweed. Website accessed October 10, 2022.

Mary H. Meyer, University of Minnesota Extension Service. Japanese knotweed, a major noxious weed. October 6, 2020.

Angela Gupta, Amy Rager and Megan M. Weber, University of Minnesota Extension Service. Japanese knotweed. Reviewed in 2019.

Minnesota Department of Agriculture. Knotweeds. (Brochure.) No date.

Minnesota Department of Agriculture. Japanese Knotweed. Website accessed October 10, 2022.

Minnesota Department of Transportation. Minnesota Noxious Weeds. February 5, 2020.

Linda M. Wilson. British Columbia Ministry of Forests and Range. Key to Identification of Invasive Knotweeds in British Columbia

References

1) Grevstad, F.S., J.E. Andreas, R.S. Bourchier, and R. Shaw. 2022. Knotweeds (Fallopia spp.): History and Ecology in North America. In: R.L. Winston, Ed. Biological Control of Weeds in North America. North American Invasive Species Management Association, Milwaukee, WI. NAISMA-BCW-2022-19-KNOTWEEDS-P. [Accessed online at 23198.pdf (bugwoodcloud.org)]

2) Fennell M, Wade M, Bacon KL. 2018. Japanese knotweed (Fallopia japonica): an analysis of capacity to cause structural damage (compared to other plants) and typical rhizome extension. PeerJ 6:e5246 https://doi.org/10.7717/peerj.5246

3) Lavoie, C. 2017. The impact of invasive knotweed species (Reynoutria spp.) on the environment: review and research perspectives. Biological Invasions 19: 2319-2337.

4) Matte, R., Boivin, M., and Lavoie, C. 2021. Japanese knotweed increases soil erosion on riverbanks. River Research and Applications 38 (3): 561-572. DOI: 10.1002.rra.3918

5) Colleran, B., Lacy, S.N., and Retamal, M.R. 2020. Invasive Japanese knotweed (Reynoutria japonica Houtt.) and related knotweeds as catalysis for streambank erosion. River Research and Applications 36 (9): 1962-1969. DOI: 10.1002/rra.3725.

6) Kato-Noguchi, H. Allelopathy of Knotweeds as Invasive Plants. Plants 2022, 11, 3. DOI: 10.3390/plants11010003.

7) Drazan, D., Smith, A.G., Anderson, N.O., Becker, R., and Clark, M. 2021. History of knotweed (Fallopia spp.) invasiveness. Weed Science 69(6): 617-623. DOI: 10.1017/wsc.2021.62.

8) Grevstad, F.S., J.E. Andreas, R.S. Bourchier, R. Shaw, R.L. Winston, and C.B. Randall. 2018. Biology and Biological Control of Knotweeds. USDA Forest Service, Forest Health Assessment and Applied Sciences Team, Morgantown, West Virginia. FHTET-2017-03. [Accessed online at https://bugwoodcloud.org/resource/pdf/Biology_&_Biological_Control_Series/FHTET-2017-03_Biological_Control_of_Knotweeds_508.pdf.]

9) Lawson JW, Fennell M, Smith MW, Bacon KL. 2021. Regeneration and growth in crowns and rhizome fragments of Japanese knotweed (Reynoutria japonica) and desiccation as a potential control strategy. PeerJ 9:e11783 https://doi.org/10.7717/peerj.11783

10) USDA. Field Release of the Knotweed Psyllid Aphalara itadori (Hemiptera: Psyllidae) for Classical Biological Control of Japanese, Giant, and Bohemian Knotweeds, Fallopia japonica, F. sachalinensis, and F. x bohemica (Polygonaceae), in the Contiguous United States. Environmental Assessment, January 2020.

11) Grevstad, F.S., J.E. Andreas, R.S. Bourchier, and R. Shaw. 2022. Knotweeds (Fallopia spp.): History and Ecology in North America. In: R.L. Winston, Ed. Biological Control of Weeds in North America. North American Invasive Species Management Association, Milwaukee, WI. NAISMA-BCW-2022-19-KNOTWEEDS-P. [Accessed online at 23198.pdf (bugwoodcloud.org)]