Lousy Fall Color? Maybe it's the Government

Residents of New England and southeastern Canada are dismayed by this year’s drab fall color display, an unfortunate result of funding cuts at USDA’s Plant-Pigmentation Program. Or at least I assume that’s what happened.

Actually, it’s largely due to the abnormally wet season we’ve had. Frequent rains kept foliage damp for long periods, which set the stage for normally weak, opportunistic fungal pathogens to cop an attitude, and set up housekeeping inside leaves. High temperatures and humidity helped speed things along.

Anthracnose sounds dire, but trust me – you can’t catch it. It’s a general term for widespread leaf infections like we saw this year, and many different fungal agents can be involved. Anthracnose causes tissue necrosis (death) of leaves, often in distinct zones, but sometimes diffuse. Leaves turn brown, either wholly or within the necrotic zones, and start to drop early.

While all deciduous trees can get anthracnose, the pathogens are often specific to “their” species or genus. In other words, walnut anthracnose won’t spread to maples. But if weather conditions favor these pathogens, many tree species will develop this ailment at the same time. A one-year infection probably will have only a minor effect on vigorous trees. If a tree is stressed, or if anthracnose returns year after year, then health could suffer.

But the dingy fall display is more than just a disappointment to us humans. It might indicate trees were not able to reclaim nutrients from their leaves before they dropped. In fall, deciduous trees make a waxy layer between leaf petiole bases and the twigs to which they’re attached, to protect twigs from losing moisture in winter.

As wax builds up, it starts plugging vessels that moved water and nutrients into, and sugars out of, leaves all summer. The increasing blockage leads to the loss of green chlorophyll molecules, which in turn reveals yellow and orange pigments already present in leaves. Before vascular tubes are entirely blocked, though, trees recover about half the nutrients from each leaf: nitrogen, potassium, iron, magnesium, manganese, and other essentials. This process is important to the nutrient budget of woody plants.

Noting that lots of leaves dropped early, and that most had little (or no) normal color, with some turning from green to brown, makes me wonder if the leaf-nutrient recycling program didn’t go forward as usual. I don’t know. In addition, this year’s lack of red anthocyanin, a “discretionary” pigment some trees make in the fall if they have enough energy, could be another clue about forest health.

Another outcome of this summer’s frequent and heavy rain events is the leaching of water-soluble nutrients from soils. This dearth of nitrogen further stressed trees, and may well have impacted leaf color. And finally, heavy rains can directly wash out plant nutrients from leaves, though this effect is not well-studied.

You can help your landscape trees by adding a two- to four-inch layer of compost or mulch out to the drip lines in place of grass. Don’t drive on, or add fill to, trees’ root zones (twice branch length), and water them in dry spells. Fertilizer can unduly stress marginal trees, and should only be applied if lab soil-test results show a need. If possible, stay out of the forest with heavy equipment when soils are moist and prone to compaction. But if you do have to take machinery into the woods in wet conditions, make sure operators know it’s critical they stay on skid trails.

Paul Hetzler has been an ISA Certified Arborist since 1996.