Why Are The Leaves On My Trees So Sticky?

Courtesy PR Extension Office

My cottonwood trees are full of aphids and the accompanying honeydew. Lately, the leaves have turned gray and are dropping. What is happening?

According to Abi Saeed, MSU Extension Horticulture Associate Specialist, the sticky and gray leaves are covered with honeydew, and the resulting opportunistic fungus called sooty mold. Although sooty mold doesn't actively infect the plants themselves (and instead, is only found on plant surfaces), significant amounts of it can reduce photosynthesis and cause premature leaf drop.

Addressing the underlying problem of aphids can help to reduce this issue. Healthy and established trees can usually withstand some aphid feeding without many issues. In addition, vigorously growing and recently fertilized plants often have higher aphid populations. Aphids are often pretty sensitive insects, and depending on the severity of infestation, can be managed using a strong stream of water to physically knock them off the trees, which can reduce their numbers and help beneficial insects keep their populations under control. Using this strategy multiple times to reduce initial populations can help to control the infestation (though it may take some time and repeated water spraying).

If the water spray method doesn't work, contact insecticides such as insecticidal soaps can be another option, but these are more challenging to use in larger trees because they need to coat the surface of the aphid to work, and these insects are often protected by the foliage because they are found on the underside of leaves. In addition, unless professional application equipment is used, it is difficult to get the insecticide to reach fully within the trees. It may be beneficial to hire a professional for insecticide application if the water spray method doesn't produce results.

Systemic insecticides can be another option in situations where complete coverage with contact insecticides can be difficult (make sure to use products labeled for appropriate use).

More information on aphid management can be found at your local MSU Extension Office.