Being an arborist, I’m of course very mindful of complexion. Things like bruises and blemishes catch my eye, in addition to scabs, cuts, and even those out-of-place whiskers that appear out of nowhere. It sounds like a description of my aging skin, but I’m talking about blotches, warts and cuts that accumulate on tree leaves over the summer.
I suppose if we had to stand outside day and night all season, our skin would develop issues too. Those who work or play much outdoors need to be concerned about skin spots that suddenly show up. With tree leaves, that’s not the case – even the ugliest “skin” condition is generally no cause for concern.
One of the more alarming leaf disorders is called tar spot, whose symptoms are black blobs that often show up in late summer or early fall. Tar spot affects Norway, silver, red and sugar maples, in order of severity. The spots, which really do look like drips of roofing tar, seem to appear overnight, and sometimes cover much of the leaf. While it may look like a serious affliction, it’s really just a cosmetic issue (meaning if you’re good with cosmetics you can probably make your tree look pretty again).
Unlike the tar sands of northern Alberta in Canada, though, tar spots can’t be processed into crude oil. As disappointing as that may be, at least tar spot is not a problem. Spots are caused by several different species of fungi in the genus Rhytisma, which I mention because some of you play Scrabble.
If your tree’s leaves have sprouted tiny spindle-shaped structures that make it appear that the leaves need a shave, don’t worry. These are tiny galls, formed when a minute arachnid called an eriophyid (go for a triple score on that word) mite laid an egg, along with a dose of a plant hormone which directed the leaf to grow a little home for her young one.
Depending on the species of mite, these galls can be green, yellow, red or pink. Some are squat and thick, resembling a wart, but they are all completely harmless. Good thing, too, because galls shield the mites from anything you could spray on them anyway.
Believe it or not, scabs are caused by a disease called scab. I think it was late on a Friday when scab and tar spot were named, probably by a new intern who was later reprimanded for “making sense.” It affects apple, crabapple, hawthorn, juneberry and other trees in the rose family. Scab causes affected leaves to drop early, and is much worse in wet seasons when it can defoliate a tree by mid-summer.
It’s a serious problem for orchardists because it causes scab-like blemishes on fruit in addition to weakening the tree, so they routinely spray fungicides beginning at bud break. Other ways of managing scab include proper pruning, increasing air flow and access to sun, and buying scab-resistant varieties.
High winds, especially early-season events, can tatter leaves, a condition called “leaf tatter.” (Same intern, don’t you think?) Japanese beetles, caterpillars, sawflies and other insects chew on leaves over the summer, while leaf-cutter bees remove perfectly scribed circles. All in all, many trees look bedraggled by September. Should you be worried?
Here’s a secret: by late summer, trees don’t “need” their leaves any more. While this is true, it’s akin to saying you don’t need another five bucks at the end of the year. It would be nice, but it’s not going to change the big picture. A deciduous tree expends a huge amount of energy investing in new leaves each spring, and its leaves have to make enough sugar from sunlight to “repay” the tree, plus a little extra for rainy days. By early August, trees have recouped their investment along with a pile of interest.
Late-season disorders are superficial and no cause for concern. But if you’re embarrassed by your tree’s appearance come late summer, you can always try cosmetics.
Photo: Rhytisma acerinum fungus on a European sycamore maple. Courtesy Wikipedia user Annabel.
Unlike you and me a tree can’t run or move out of the way when something bad comes its way. But plants have evolved a very cool defense mechanism whereby cells being attacked by a pathogen (fungal, viral, bacterial etc.) will commit suicide (programmed cell death – PCD) to try and stop the spread of the bad guy to the rest of the plant. This is the reason you often see discolored spots on leaves. It can often be a sign of a good immune system in the plant that has stopped something bad from getting worse.
Paul, are the tar spots a sign of a PCD reaction to stop the spread of that fungus or is it from the fungus specifically?
The actual Rhytisma infection in leaves is subtle, just some discolored areas where the fungus is consuming tissue. When the fungus is robust enough it produces lack tarlike stromata that you see. These are the structures that will produce ascospores in the spring on fallen leaves. Simple raking and removal of affected leaves will help to minimize future tar spot infections.
I think that more significant than PCD are the antifungal agents trees produce around sites of infection. Trees are living organic chemistry labs, and also create compounds to repel insects, sometimes tailored to a specific pest. The healthier the tree, the stronger this type of response.