Tar Spot Research

November-December 2007

SPENDING SOME TIME ON THE ROAD in early to mid-October, you probably noticed the lovely display of fall colours that we had. I wasn’t expecting much of a show after such a long, hot, dry growing season but Mother Nature surprised us all in the end. The cool nights and sunny days were just perfect for expression and synthesis of the pigments that are responsible for those golden yellows, bright oranges and gorgeous reds that make us proud to be Canadian. By now there are just a few dull-coloured leaves left on the trees and the golden hues of our larches are beginning to fall away. November is a difficult month psychologically. I find the short days and dreary weather makes me want to crawl into bed right after dinner. I get “hibernation envy” in November. Wouldn’t it be nice to take the whole month off to just sleep and eat? That reminds me, I need to check my lottery ticket…

Recently, fall (and early winter) has become an extension of the already long construction season. The boom in building, road and landscape construction has benefited from recent trends towards mild autumns that seem to extend well into January. The El Ninos and El Ninas have certainly provided us with ample opportunities for late season landscape installations in the last few years. The dry, warm soils in autumn are a lot easier to work with than the cold, mucky mess we’re often dealt in spring. But with this opportunity also comes cost. Mild autumns and winters mean changes in the kinds of plants and pests that can overwinter successfully in our part of the world. They can have an effect on flowering and yields of our crops. But what I’ve been noticing is the effect on people. Although extended mild seasons can be very stimulating (especially to those who work indoors), I see and hear from a lot of very tired and “burned out” horticulturalists and arborists. It’s been a long, hot, busy season. Take some time for yourself, your employees and your families. Tell them how much you appreciate their hard work, their patience and their support.

Tar Spot Research at U of Guelph
Despite the dry spring, we managed to contract significant levels of tar spot on our maple trees again this year. Each year, we receive tons of calls about this fungal disease from horticultural professionals and homeowners. Tar spot is a leaf spot disease of maple (Acer spp.) that is caused by a fungus (Rhytisma spp., pronounced “Rye-teesma”). Thankfully, it has little effect on the health of the tree but it is pretty unsightly to behold, especially at the front of one’s home or business. Most notable is the tar spot (Rhytisma acerinum) we see on introduced Acer platanoides, A. campestre and A. pseudoplatanus (Norway, hedge and sycamore maples). But if you look around, you’ll see tar spot on some of our native red and silver maples too: is it the same species of Rhytisma? If it is, then why is it so uncommon on our native maples? And what can we do to manage this disease a little better? If you look in the literature, there isn’t very much detail on tar spot biology or etiology. The fruiting structures of the fungus overwinter in those tar-like spots on the leaves and will produce spores some time the following spring. We know that applications of fungicides are best used during the sporulation period… but when exactly is that?

Dr. Tom Hsiang (pronounced “Shung”), a professor in the Department of Environmental Biology at the University of Guelph, has been working diligently on tar spot over the last couple of years. He and his research team have been studying the biology of the fungal organism to 1) identify the sporulation period of Rhytisma acerinum on Acer platanoides in Ontario and 2) understand and correctly identify (using DNA sequencing) the various species of the Rhytisma fungi and their specific host ranges on Acer spp.. They are also looking at fungicide efficacy during the sporulation period but the results are not presented here. The following is from a recent research summary written by Dr. Hsiang.

To better predict the sporulation period, the research team collected and examined fallen, infected leaves over two growing seasons. From May-August 2006 and from March-October 2007, samples of infected, fallen maple leaves were gathered weekly from multiple locations in southern Ontario. These leaves were inspected microscopically for the presence of spores of the tar spot fungus. These spores are produced in microscopic sacs that remain closed until they mature in spring, when they open and eject spores after precipitation events. The spore sacs were examined weekly to see how many were full and how many were empty. For 2006, up until June 8, about 90% of the sacs were full. Over the next two-week period, by June 22, the spore sacs became almost 100% empty. For 2007, up until May 22, the sacs were 100% full at five sites across southern Ontario, but were empty by July 3. The practical implication of these findings is that fungicide protection against tar spot needs to be applied only during a very short period, and this period starts with the end of full leaf expansion (in Norway maple) and continues for the next three to five weeks. Keep in mind that spore release and infection is dependent on rainfall and that extended dry periods can delay spore release (which seems to have occurred in 2007).

Another objective of this study was to obtain DNA of this organism and confirm its identity. Researchers at Cornell University claimed that a European species of Rhytisma (R. acerinum) causes tar spot on European maples in North America and proposed that the species of Rhytisma causing tar spot on North American maples was a different species (R. ameri-canum). Dr. Hsiang’s research team was able to obtain DNA from tar spots on Norway maple and silver maple locally, as well as from big-leaf maple in British Columbia and from sycamore maple in Germany and England in 2006. In 2007, they expanded the study and, with the help of homeowners and horticulturalists, they obtained tar spot material on striped and red maples in addition to the previous maple hosts. They found that the tar spot found on Norway maple is the same species which occurs in Europe (Rhytisma acerinum) on various European maples, whereas our native tar spot which occurs on our native maples is indeed a separate, native fungal species (R. americanum). No wonder tar spot is so uncommon on our native maples. Dr. Hsiang also found that the third tar spot species, often found on striped maple, is the same native species that is found on big-leaf maple (R. punctatum). This is the first proof of species distinction based on full DNA sequence evidence.

It may interest you to know that tar spot on European maple species is just as severe in European countries as it is here in North America. Tar spot seems to be a lot more noticeable in the last 10-15 years on our European maples here in Canada… why is that? European researchers have found that the fungus that causes tar spot in Europe is extremely sensitive to air pollution. Some scientists believe that the increased presence of the tar spot fungus may be a response to better air quality and reduced levels of acid rain in the last little while. I’ve also heard that theory as an explanation for why lichens are becoming more prolific. It’s a nice theory but unfortunately, most homeowners that I know hate tar spot and aren’t too crazy about lichens on their trees either!

Contact Information

This column is written by Jen Llewellyn, Ontario Ministry of Agriculture and Food and Rural Affairs (OMAFRA) Nursery Crops Specialist

Email any questions you have directly to Jen and we'll publish her response.

P: 519-824-4120 ext. 52671 • F: 519-767-0755

OMAF website: http://www.omafra.gov.on.ca/scripts/english/crops/agriphone/index.asp

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