So I’ve been getting some calls about leaves turning yellow and dropping from mature oak trees in the landscape. There are quite a few things that can cause established deciduous trees to undergo premature leaf drop. We took a closer look at a few stressed-looking pin oaks (Quercus palustris).
Taking a closer look we noted discreet zones of necrosis, a.k.a. what looked like a leaf spot. We know from experience that oak phylloxera can also be confused with leaf spot. Patricia Thompson from Kelly Tree Care wrote a great article about Oak Phylloxera in ISA Ontario Arborist a few years ago
So we examined several of the symptomatic leaves, no signs of phylloxera.
We also examined several of the leaves that are just emerging (2nd flush) but again, no phylloxera (and very little leaf spot).
For reference, the images of Oak Phylloxera damage (left, from Andrea Battisti, Universita di Padova) and the insects themselves (right, from Jack Kelly Clark, University of California).
We did see a few orange Oak mites on leaf undersides (especially in the trichomes covering leaf vein junctures) but they weren’t at damaging levels and there was no sign of mite injury.
We did see new leaf mines on some of the leaves, causing very “minor” damage.
We also found rolled up leaves and inside were……moth larvae (Tortricidae) called “leafrollers”. The next generation of leafrollers is out and these little leps generally do low levels of damage. If they do get out of hand, usually an evening application of Bacilus thuringiensis will take care of them.
We also found characteristic Agromyzid midge feeding holes on the 2nd flush of leaves, which is not a concern. These midge adults would have caused the holes back when the leaves were still folded up inside buds, resulting in a “snowflake” like pattern of holes. They will deposit the odd egg as well so they might have something to do with the leafminer injury we saw.
CODIT, Compartmentalization of Decay in Trees (Alex Shigo). Many trees will form an abscision zone at the base of the petiole (or even around a leaf spot, like with “shot-hole” leaf spot) so that infected tissue is no longer connected to the host’s phloem and xylem. This reaction seems to be greater after periods of high humidity and high temperatures. We assume it is an effort to prevent disease spread throughout the tree. But in doing so, the tree sacrifices its precious energy-making equipment.
So if oak trees are looking a little sparse, it could very well be due to spring infections of fungal diseases because of frequent rains during leaf emergence. The good news is, many oaks are putting out a strong second flush of growth. The lesions on the older leaves did not look typical of anthracnose. We submitted our samples to the lab and the Pest Diagnostic Clinic came back with Tubakia leaf spot. Sinclair and Lyon say this “Quercus species are often affected by Tubakia leaf spot. They lesions tend to become large on leaves stressed by other factors such as nutrient shortage, as on the chlorotic Q. palustris.