Dropped Leaves Don’t Always Mean Dead Trees

November 2nd, 2021

   Tree leaves that brown around the edges, develop spots, shrivel, and even drop prematurely worry a lot of gardeners.

Crabapples were a particular species hit hard by leaf disease in 2021.

   After all, healthy leaves are supposed to stay on our trees until they turn color in fall and drop naturally, right?

   When that doesn’t happen – as it did in a lot of cases this past warm and wet growing season – it’s no wonder gardeners fear the worst and think their tree has died.

   The good news is that disfigured and damaged leaves – and even total premature leaf drops – usually don’t mean a tree is doomed.

   This is a problem that looks and seems worse than it really is.

   So long as the roots and woody parts of the tree are intact, there’s a very good chance that your leaf-troubled tree will produce a new and healthy round of foliage next spring.

   Sometimes, denuded trees even grow a second set of leaves the same season.

   Sycamores are notorious for that after dropping their first set of leaves in early summer following all-too-common outbreaks of a fungal disease called anthracnose.

   Damaged and/or lost leaves can weaken a tree, but unless the problem is severe and repeated, trees usually grow through these setbacks.

   The main issue is that damaged or missing leaves short-change the tree of chlorophyll, which, if you remember from high school biology, is the green pigment that plants combine with water and sunlight to manufacture the fuel (sugars) that plants use to grow.

   Besides that loss of energy, growing two sets of leaves in one season is an energy drain.

   While that’s not good, it’s not nearly as threatening as root-level problems, such as soggy or compacted soil, root-rot diseases, boring insects (like birch borers and the dreaded emerald ash borer), and “operator errors” such as planting too deeply, over-mulching, and whiplashing bark with string trimmers.

   All of those are much more fatal than leaf issues, which are by and large cosmetic and temporary.

   So what can cause leaves to go bad and/or drop early?

   The leading reason is diseases, most of which are caused by fungi but sometimes viruses and bacteria.

   Most leaf diseases thrive in the warm, wet, and humid growing seasons like we had this year.

Fungal leaf spot is a disease that causes birch to lose their leaves prematurely.

   If we get a dry season next year, it’s entirely possible we won’t see a repeat of this year’s widespread leaf troubles – even if we do nothing at all.

   That’s because three factors must all come together (the “disease triangle”) for a disease to flourish – 1.) having the disease-causing organism around, 2.) having a species that’s prone to getting that disease, and 3.) having weather conducive for the pathogen to grow.

   Take any one of those away, and you stop or greatly diminish a disease.

   There’s not much we can do about the weather, and most people would rather not remove a plant to solve a disease problem. But one simple thing we can do this time of year is rake up and get rid of any fallen, diseased leaves.

   This bit of “sanitation” removes the spores on the leaves that otherwise would stick around to potentially reinfect the tree next year.

   Leaf cleanup alone isn’t a rock-solid disease-stopper, but it can help. And it’s at least simpler and less expensive than trying to spray the right fungicide at the right time to control a disease.

   Sprays aren’t even very helpful if you wait until after an infection is full-blown to apply them. Fungicides don’t “cure” diseases or undo damage that’s already occurred… they just protect growth that hasn’t yet been infected.

   A second common reason behind damaged and early-dropping leaves is bugs. Assorted caterpillars, Japanese beetles, leafminers, webworms, aphids, lace bugs, whiteflies, and more had banner years in 2021.

Fall webworm is an example of a bug pest that looks bad but really isn’t all that harmful to trees.

   Some of these chew leaf tissue. Others suck out the chlorophyll and cause discoloring or mottling.

   Either way, when damage is bad enough, trees just shed the now-useless appendages.

   The saving grace is that leaf-attacking bugs don’t harm the twigs, branches, trunks, and roots, meaning the infrastructure is still in place for the tree to manufacture new leaves.

   Weather is a third leaf-damaging factor.

   In particular, late-spring freezes can damage or kill tender young leaves that “thought” the coast was clear to open.

   Fortunately, most trees have a backup set of leaf buds that will grow to replace the damaged first set.

   Unfortunately, they don’t have a third set, which is a problem in that rare spring that warms early, then turns frigid, then warms again, then dishes out a second and very late freeze.

   That happened a few years ago to a young cutleaf Japanese maple I had planted the year before. An April freeze killed the first set of leaves just as they were opening. Then after the tree started pushing out a second set of leaves, along came another early-May freeze that killed the second set. Game over for that tree.

   In-season setbacks like hail and extremely windy storms also can damage leaves and/or tear them off limbs prematurely.

   In general, the later in the season any leaf trouble happens, the better trees are able to recover.

   I should point out that all of this applies only to leafy (i.e. “deciduous”) trees and not to needled evergreens (“conifers”) such as pines, spruces, firs, and hemlocks.

The dwarf conifer at right is showing poor color and is already likely dead.

   Those aren’t making and dropping whole new sets of leaves every year, and they don’t function quite the same way.

   Sometimes conifers will grow new sets of needles if they’re stressed by a sudden, erratic weather change, a planting/transplanting in too-hot weather, or an attack by a needle-chewing insect.

   But most of the time when you see a conifer go brown, it’s already dead – usually due to a systemic failure at the root level.

   Unlike leafy trees in which the leaves give you an early clue to threats such as bone-dry soil or heat stress, most conifers give you few or no obvious clues until it’s too late.

   Conifers can be dead for weeks until the needles finally brown, and the gardener notices that something doesn’t quite look right. Think about how a Christmas tree stays green inside even weeks after it’s been completely disconnected from the roots.

   By the time you see conifers browning, it’s almost always lights out. And all the fertilizer, water, and fungicide in the world won’t bring it back.

   Here’s hoping you’re not seeing any of that.

   But even if you are, wait until spring to give your “everbrown” a chance to redeem itself.

   It’s unlikely, but plants try their best to survive.