Diagnosing what your tree actually has

Symptoms are common. Causes are not.

A homeowner sees brown leaves. Brown leaves are the ending of a dozen different stories, and the whole skill in this work is running the story backwards. Where did the browning start, at the tips or between the veins or along the whole margin? Did it move inward or upward? Which species is it, and what time of year did it begin? Answer those four and the list of suspects usually drops from a dozen to two.

That’s the part a picture search on your phone can’t do. Online, every leaf photo matches everything. In the yard, the pattern is the evidence: an oak that browned from the outer canopy inward in July is telling a completely different story than an oak that browned from the bottom branches up in September. Same color, opposite conclusions, and the conclusions are worth thousands of dollars apart.

The Western PA roster, one suspect at a time

Oak wilt is the one that gets our full attention, because it’s the one that kills fast. On a red oak the leaves brown from the outer edge toward the midrib while still partly green, they drop while they still have color in them, and a tree can shed most of its canopy in a single summer. It moves through beetles drawn to fresh wounds and through root grafts between neighboring oaks, which is exactly why we don’t put a saw in an oak between April and July unless it’s already a hazard. A summer pruning cut on an oak in a neighborhood with wilt nearby is an open door.

Anthracnose is the opposite: ugly and mostly harmless. Sycamores and maples get it in a wet Pittsburgh spring, leaves develop dead blotches that follow the veins, whole shoots die back, and by the Fourth of July people are convinced the tree is finished. Then the tree pushes a second set of leaves and looks nearly normal by August. A sycamore that does this every damp spring has been doing it for forty years.

Verticillium wilt in maples has a signature you can almost see from the driveway: one whole scaffold limb dies while everything around it stays green, then a different limb goes the following year. It’s a soil fungus, it moves up through the vascular system, and cutting into an affected branch often shows an olive or greenish streak in the sapwood. Some maples live with it for years. Some die in two.

Bacterial leaf scorch fools people into thinking they’re watching drought. Leaf margins brown with a yellow band between the dead edge and the green center, it shows up late in the season, it comes back a little worse every year, and it favors pin oaks and red oaks. There’s no cure for it. There is a real difference between a tree in a slow decline you can manage and a tree that needs to come down, and pinning the name on it is what decides which.

Apple scab and cedar-apple rust hit the ornamentals: crabapples, hawthorns, serviceberries. Scab spots the leaves olive-black and strips a crabapple bare by August. Rust prints orange spots on the leaves and needs a juniper somewhere within a few hundred yards to complete its life cycle, which is why the crabapple next to the row of arborvitae is always the worst one on the street. Neither one kills the tree. Both make it look terrible every year, which is its own reason to deal with them.

Needle cast is the Pittsburgh blue spruce story, and every neighborhood has three of them. The fungus infects new needles, browns them out a year later, and works upward while the top of the tree still looks fine. A spruce that’s green up top and bare from the knees down is the most common tree call we get in the North Hills, and by the time it reaches halfway up the tree, the diagnosis and the outcome are the same sentence.

The four things that pretend to be disease

Half of what gets called disease around here isn’t. Drought stress browns leaves from the margins in, usually on the sunny side first, and it shows up on the youngest and the shallowest-rooted trees before anything else in the yard. Salt damage runs along driveways and road edges, kills whatever branches face the pavement, and stops in a straight line where the plow spray stops. Herbicide drift from a lawn treatment or a neighbor’s fence line makes leaves cup, twist, and strap out narrow, and it hits new growth hardest while the older leaves stay untouched. Girdling roots strangle a tree from the flare, produce a slow whole-canopy thinning with no spots and no lesions at all, and are only found by digging at the base until the flare shows itself.

None of those four are treated by anything sold as a disease treatment. Getting them named correctly saves you from paying for the wrong solution twice.