Gambel Oak

Gambel's oak leaves provide fall color.

One of the most common questions we receive at Coconino Cooperative Extension is “What’s wrong with my tree?” Sometimes the answer is obvious (i.e. you find lots of aphids on your tree), but other times it’s a puzzle. Here’s what we’ve been asked this summer.

Many homeowners have called with concern about brown blotches on the leaves of their Gambel oak. I have these spots on the stand of oaks in my own back yard. This is caused by an insect, likely an oak leafminer. While alarming and unattractive when you look at your tree from a distance, up close you will see that only small portions of each leaf are affected. This is usually not something to be concerned about as it rarely causes the tree to die. Why there’s so much of this damage this year, I don’t know. We don’t recommend spraying a pesticide for control as this will likely cause more damage than good and may affect natural predators of the leafminer.

Many fruit trees had loads of fruit last year but aren’t producing any fruit this year. A late spring frost is the main reason. It got down to 22 degrees on April 30, and that froze most blossoms. Last year’s abundance of fruit didn’t help production either as trees may have used most of their resources in 2018. Adequate watering and an application of fertilizer can help with this problem, though that won’t prevent of a cold snap. The good news is that many fruit trees that lose blossoms one year will produce a large crop the next year provided it doesn’t get below 28 degrees after they flower.

We always get lots of calls about aspen, and this year is no exception. More and more aspens are showing signs of disease, insect damage and drought stress. Warmer temperatures are playing a role in this. Having personally killed three aspens in my yard, I learned the hard way that aspen can be a troublesome tree in the home landscape.

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Aspen is affected by numerous insects such as aphids, oystershell scale, and clear wing moth. It gets even more diseases including numerous fungal cankers, rust, and several leaf spot diseases. A tree that is stressed due to lack of water or a too warm microclimate will be more susceptible to any number of these problems. Often, the only solution is to remove the tree. If you must have aspen, plant them on the north and east sides of buildings and only in the cooler neighborhoods. Warmer temperatures may mean that it’s time to stop planting aspen in the urban landscape and enjoy them at higher elevations.

By far the biggest tree concern has been due to drought. Even trees that are regularly irrigated may be suffering as we have experienced record temperatures and lack of any significant monsoonal moisture. Signs that a tree is drought stressed are brown edges on the leaves, thinning of the tree canopy, premature leaf drop, and yellowing of the leaves. And if you think irrigating a lawn will provide enough water for your trees, read on.

Most tree roots are in the top 12-24 inches of soil. Irrigating a lawn may not supply water deep enough for adequate tree health. For high value trees, I suggest deep watering trees out at the drip line. I like attaching a soaker hose to my garden hose and snaking the soaker hose around my tree. Turn on the water at a very slow drip and leave it on for a couple of hours. We want to be mindful of how much water we use, but we also don’t want to lose our valued trees.

Trees provide so many benefits such as cooling the landscape, supporting wildlife, bringing beauty to our landscapes, as well as taking in carbon dioxide. If this dry monsoon pattern continues, we may have to rethink what is important in our landscapes. I vote for trees.

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Hattie Braun was in the 1995 Coconino Master Gardener class. She now coordinates the program.

If you have a question about your trees, you can call the Master Gardener hotline at 928-773-6115 or email CoconinoMasterGardener@gmail.com. If it’s a problem we can’t handle, we can refer you to a certified arborist or tree worker.


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