Submitted by Krista Harding
The prolonged periods of rain and wet growing conditions is causing some issues in our area. The conditions we have had recently are ideal for many landscape fungi and diseases to appear.
One fungal disease happening to sycamore trees is anthracnose. Anthracnose is favored by cool, wet weather. The young leaves may wither and turn black. Older leaves will have a brown discoloration that follows the major veins of the leaves. In some cases, the petiole (leaf stem) is infected, which causes leaf drop. Although this is most common on sycamore trees, it can also affect birch, elm, walnut, oak and ash.
In severe anthracnose cases, the tree drops heavily infected leaves and may completely defoliate. However, healthy trees will leaf out again in just a few weeks. Anthracnose seldom causes significant damage to trees in Kansas, so chemical control is not necessary. Fungicides do not cure infected leaves, so applying fungicides now will not help.
Both my climbing and Knock Out roses look good this year and are blooming like crazy! But, I have my guard up and am on the lookout for rose slug infestations. Some people might think that the small green worm they find on roses is a caterpillar of some sort. However, it is not. It’s actually the larva of a sawfly. This insect skeletonizes rose leaves and young larvae will remove the green layer of leaf, leaving behind a clear material. As the larvae mature, they make holes in the leaf and eventually may consume all of the leaf but the major veins. Since these insects are not caterpillars, insecticides with BT will not be an effective treatment. Instead, try a strong jet of water to dislodge the slugs and make it more difficult for them to return to the plant. Other effective treatments include insecticidal soap, horticulture oils, spinosad and permethrin.
Another pesky landscape insect to be watching for is Euonymus scale. Euonymus scale looks like small, white cottony spots on affected foliage. The leaves will turn yellow and die as this insect continues to feed by sucking the plants juices. Heavy infestation happens on the undersides of leaves, twigs and stems. Labeled insecticides include malathion, permethrin and cyhalothrin. However, control is almost impossible for euonymus that has been heavily attacked and is in very poor health. In those instances, complete removal of those plants is recommended.
Finally, watch for rust on hollyhocks. This is the most common disease on hollyhock and can cause serious injury as leaves are progressively killed through the summer. Rust shows up as yellow spots on the surface of the leaves and orangish to brown pustules on the underside. The first line of defense in preventing rust is to remove all hollyhock stalks, leaves and other debris in the fall and destroy it. Remove any infected foliage you see now. A fungicide such as Immunox can be applied to protect healthy foliage. However, if you fight rust every year, I recommend a preventative fungicide application as the leaves emerge in the spring.
Should you have any horticulture questions this growing season, give me a call. As a reminder, I have office hours in each of the three locations within the Extension district. I am in the Erie office Monday, Wednesday and Friday; Iola on Tuesday and Fort Scott on Thursday.
Krista Harding is a K-State Research and Extension Agricultural agent assigned to Southwind District. She may be reached at 620-244- 3826 or [email protected].
K-State Research and Extension is an equal opportunity provider and employer.