This question of the week is one we have seen several times this year. As with most tree questions, we recommend you consult a tree company since they specialize in this field and will treat the pest/disease. We do not spray trees for diseases or pests. However, we would like to share this information since it has been more common than usual.
Q: My aspen trees aren’t looking as good as they did last year. The leaves are turning a pale yellow with some brown and falling off - what is going on?
A: Chlorosis (loss of green color of leaves) can be caused by a multitude of factors, such as lack of light, iron or macro-nutrient deficiencies, insects or disease.
In this particular case (see photos), it is due to spider mites. Spider mites are tiny (0.4-0.6 mm for an adult) creatures that suck the juices out of leaf cells The plants lose the ability to properly perform photosynthesis and the leaves eventually fall.
The biggest problem with spider mites is that it goes unoticed until it is too late. Telltale signs are webbing on the leaves. Any leaf damage caused will be left for the rest of the year even if every spider mite is killed. What starts as small yellow or brown dots on a leaf rapidly turn into the damaged, webbed leaves in the picture.
Spider mites reproduce incredibly fast and usually take several treatments. They are so tiny that a gust of wind is enough to blow them from tree to the plants below, where they will begin to damage those plants as well. Pests like the spider mites and a host of other diseases are more common this year as a warmer winter has failed to kill off populations as with previous, colder winters. If your aspen looks similar to the picture make sure you call a tree company to treat your infestation before it spreads to the rest of your garden!