What’s that “White Fungus” on My Post Oaks?

As we get into August this year, the post oaks in Denton County are showing a lot of white on their leaves. 

This is not a fungus or serious disease but is “wool” that is produced by a small sucking insect called an Aphid.  The aphid produces the “cotton” or “wool” as  protective covering and as a wind sail for dispersal to other trees.  Even though they look bad, they are not a real threat to the health of the trees.  They appear in mid-later summer and cause some damage to the leaves, but the overall damage is negligible and restricted to this year’s leaves.  Aphids are sucking insects and produce a substance called “honeydew” that can make objects under the tree sticky.  Sooty mold often grows on the honeydew turning leaves, limbs, and other objects black.  Ants are also attracted to the sweet honeydew.

If the infestation is heavy, the best control is to just wash them from the leaves with a strong stream of water. Do not spray the tree with insecticides. It is difficult to get the leaves adequately covered with the insecticide spray and many beneficial insects will be killed along with the aphids. Nature has good ways of keeping things in balance and broad-spectrum insecticides destroy that balance.

On the leaves in the in pictures above, I found three different predators of the aphids: a lacewing larvae, an assassin bug nymph, and a spider.

Lacewing larvae.
Assassin Bug nymph.

The lacewing larvae are voracious eaters of aphids. This particular fellow is a “trash bug.” He camouflages himself with the “cotton” and other “trash” on his back and then crawls among the aphids consuming one after another. I was able to watch this particular assassin bug repair his camouflage and the go about eating aphids on this leaf.

The assassin bug pierces the aphids and sucks the juice leaving an empty carcass.

The spiders spin webbing around bunches of infested leaves and then enjoy their feast.

So, don’t be alarmed by the “white fungus” on your post oaks. It may be a little unsightly, but nature will take care of the problem. If you decide to control them, just use a hard stream of water to wash them off and leave the insecticides poisons on the shelf.

Scott Geer

Scott Geer

Scott Geer has a master's degree in forestry from Stephen F. Austin State University and is an ISA Board Certified Master Arborist.® He is also a graduate of the American Society of Consulting Arborists Academy.