A common sight in the Iowa landscape this summer has been patches of ~2-10 dead leaves (give or take) occurring randomly scattered throughout oak tree canopies (Fig. 1 and Fig. 2). Most leaves, however, remain unaffected. This condition (called flagging) does not discriminate between the various species of oak and has been reported on red, white, and bur oaks but seems most prominent on older oaks than young or newly planted trees.
In addition to seeing oak leaf flagging throughout the landscape, we’ve received samples and numerous questions about this problem throughout Iowa in the Plant & Insect Diagnostic Clinic (PIDC). Let’s discuss the possible causes of this oak leaf flagging, starting with the most likely. (Note: We can never rule out pathogenic causes of plant symptoms without having a physical sample to examine. More information on submitting samples to the PIDC can be found here.
Response to drought/environmental stress
Drought conditions in 2023 have been even worse than in 2022 (US Drought Monitor, Fig. 3), and we've recently experienced one of the hottest weeks in Iowa’s history. Suffice it to say, conditions out there are stressful for our landscape plants and are sometimes made worse by stressors inherent in the urban or managed landscape.
Interestingly, leaf flagging symptoms appear randomly throughout tree canopies and, once noticed, never seem to worsen. In other words, symptoms don’t progress as one might expect if this condition were caused by a plant pathogen. Therefore, given recent environmental stresses (lack of water, extreme heat, etc.), the coincidental appearance of leaf flagging on oak trees is likely to be a response to drought/environmental stressors. Trees are likely trying to conserve resources by aborting leaves on branch tips.
Again, we can never rule out plant disease or pests without examining a physical sample, so let’s go ahead and discuss possible plant diseases or insect pests that could cause symptoms like this in an oak tree canopy.
Bur oak trees in Iowa are most affected by Kermes scale, which can cause new growth to be deformed and stunted. Symptoms can also appear midsummer when small clumps of leaves or even single leaves near the end of a branch turn brown (Fig 4). Kermes scale can be seen as round protuberances coming out of twigs or petioles (Fig. 5). More on Kermes Scale here and here.
The incidence of Botryosphaeria canker on oaks often coincides with or follows a Kermes scale infestation. Symptoms of Botryosphaeria canker on oak trees include dead twigs with brown leaves. Twigs will have fungal cankers (sunken areas) that may have raised black bumps (where the fungus produces fruiting bodies) (Fig. 6).
Twig girdler/twig pruner
Twig girdler and twig pruner are species of longhorned beetles that can cause occasional issues on oaks in Iowa. Larvae of these beetles will feed on the wood inside twigs and eventually cut off circulation to terminal leaves and twigs. Foliage at the tips of branches begins to brown prematurely, and eventually, branch tips with brown leaves will fall to the ground (Fig. 7).
Symptoms of oak wilt do not show up with such an even distribution in the tree canopy as we are seeing this year. And the leaves usually show symptoms of actual wilt, which involves the entire leaf (including the veins), progressing from leaf tips/margins towards the petiole (Fig. 9). More on oak wilt here.
If you have any questions about problems with your oak trees or any other plants in your landscape, please don’t hesitate to reach out to us at the Plant & Insect Diagnostic Clinic by emailing email@example.com.
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