Growing up with a maple tree in the front yard meant that around this time, I had to rake. Honestly though, now that I think about it, my parents weren’t too strict on this chore. Shoveling snow was much more important in the months to come than a cosmetic rake. Also, my mother as a high school science teacher realized that this is a natural process, that gave our lawn nutrients. In reality, our lawn was barren and our neighbors hated us. On the positive side, year after year I got to see the gradual leaf color change. From bright green to shades of red and orange, to the final stages of yellow and brown. This change was predictable, but I soon recognized a leaf formation that didn’t really fit. Some of the green leaves at the end of the growing season had black spots on them. I assumed that this was just a process in leaf senescence, a signal that fall was on our doorstep. Upon further investigation, I learned that what I was looking at was a fungus. The tar spot fungus, Rhytisma acerinum.
Wherever you can find maple trees, you can find this tar spot fungus, however, the genus Rhytisma is not only limited to parasitizing maples. These fungi can also penetrate the leaf cuticle of several deciduous trees and shrubs like sycamore, elm, grape, and eucalyptus. There are around 18 species of Rhytisma found all over the world, but here, I will focus on the species I grew up with, R. acerinum. Before I could identify this specific tar spot fungus, I had to properly key out the maple tree on my parent’s lawn.
This past spring, the maple tree was in full blossom, and I decided to key the tree down to species. After all of these years, by looking at its flowers and bark, I finally learned that this was in fact a Norway maple. I was kind of bummed out too. I thought, “what’s wrong with native maple trees?” As it turns out, introducing non-native trees and shrubs was and is a cool thing to do. Having exotic species made people notice your front lawn more, and for the most part, many of these species are more disease resistant. Unfortunately for the Norway maple, humans brought its leaf parasite along for the ride as well. The tar spot R. acerinum was once thought to specialize in parasitizing Norway maple (Acer platanoides), silver maple (Acer saccharinum) and sycamore maple (Acer pseudoplatanus) but new research suggests otherwise.
In 2007, Tom Hsiang and Xiuling Lynn Tian published a paper that closely looked into the genomes of the fungi associated with the different tree species. Together they extracted and sequenced DNA from Rhytisma species located on native and introduced maples. They found that the species growing on Norway maples was Rhytisma acerinum, a tar spot that evolved in Europe. On the contrary, the fungus growing on native maple trees were identified as R. americanum and R. punctatum, parasites that evolved in North America. So, I could finally say with confidence that these tar spots that I’ve taken several pictures of are indeed Rhytisma acerinum.
Although I mentioned earlier that this fungus is parasitic, it is only weakly parasitic. If this fungus reduced photosynthetic output during the peak of plant growing season, then it would have a greater effect on trees, and be considered a stronger parasite. However, R. acerinum only begins to negatively affect leaves at the end of the growing season, when photosynthesis is beginning to slow. It’s at this time in which trees stop producing costly substances that safeguard against parasites and pathogens, that the fungus takes full advantage. Because of the warm autumn we are having, trees like my parent’s maple are holding on to their leaves longer, and are still producing compounds that reduce parasitism. For this reason, leaves are still green with chlorophyll, and there are very few patches of tar spot fungus, even this late in the season.
The leaves that are parasitized will soon fall, and throughout the year, the fungus will mature. R. acerinum spores are ready for infection in March and April, perfectly timed with the emergence of newly synthesized leaves. Wind picks up the mature sticky spores of R. acerinum from leaf litter (on my parents unraked lawn) which hopefully adhere to a suitable leaf. The lucky spores germinate and enter the leaf’s stoma, the pores associated with transpiration. Growing mycelia within the leaf causes small localized yellow spots that can be seen with the naked eye in June and July, just a few months after initial infection. Even still, this is such a small leaf perturbation, the vast majority of the leaf is still cranking out sugars through photosynthesis. These small yellow dots continue to grow and become darkly pigmented in the fall.
If you read this, now you know. Those black spots on maple leafs you are seeing, that’s a fungus. If you’re looking at a Norway maple and see tar spots, you can identify that fungus down to its species. R. acerinum is a really neat fungus that most people don’t know about. Ecologically, I gained a different perspective on parasitism by studying this species. This weak parasite doesn’t reduce the host plants fitness much at all. Because of this, there’s no reason for the tree species to evolve defenses overtime, unless the fungus becomes more degradative earlier in the growing season. Parasites that specialize in deciduous leaves that carry out their life cycle on the fringes of the plant's growing season are here to stay.