"Help! My plant has curled leaves. What's wrong?" This is a question that I often get asked, but there is no easy answer. Curled or distorted leaves can be caused by more than one thing. Aphids, weed killers and plant viruses cause malformed leaves on plants.
You might think aphids would be the easiest of these to diagnose. You just have to look for these little plant suckers inside the curled leaves. Aphids are pear-shaped, soft-bodied insects that are fairly small, ranging in size from less than 1/16 inch to more than 1/8 of an inch long.
Aphids can be green, yellow, gray, pink-purple or even black. Typically, they are found in groups on tender new growth and buds. Their numbers can quickly build up because early in the season, all the aphids are females that give birth to live females that produce more females and so forth. They don't need males to reproduce.
Aphids feed on plants by sucking out sap with a piercing, sucking mouth, sort of like a straw in a juice box. When aphid numbers are small, they don't do much damage to plants, but large populations can stunt growth. Some aphids also inject a toxic saliva that causes distorted growth. These curled leaves then provide the aphids with protection from some predators and pesticide sprays.
Two aphids that often cause severe leaf curling are the green peach and the wooly ash. However, when gardeners uncurl malformed leaves to look for these aphids, they may not find them. That is because the aphids have departed their early spring hosts and moved to summer hosts before they return in the fall. The green peach aphid, which attacks plums, peaches and nectarines in early spring, spends the summer on weeds and vegetable crops.
The wooly ash aphid has a body covered with waxy secretions that makes it look "wooly." These aphids feed on new growth of ash trees in the spring and then spend the summer on the roots of the trees. They move back to the top to mate in the fall.
In the past, when gardeners encountered clusters of aphids on their plants, they would rely on chemicals to help manage the problem. Today, gardeners are encouraged to try nonchemical approaches first.
-- Avoid applying high levels of nitrogen fertilizer, which promote excessive vegetative or lush, soft growth that favors aphid feeding.
-- Knock aphids off a plant using a forceful stream of water, taking care not to harm the plant. The aphids will not climb back onto the plant.
-- Learn to recognize and encourage natural predators and parasites that feast on aphids.
-- Control ants feeding on the honeydew (sugary plant sap) secreted by the aphids. They actually protect aphids from predators.
If you decide to use a pesticide spray, avoid broad spectrum insecticides that will also kill aphid predators. Insecticidal soaps and horticultural oils will effectively control aphids present and visible on plant shoots and leaves. These are contact insecticides and must come in direct contact with the aphid bodies to be effective. Because many aphids feed on the bottom sides of leaves, be sure to get good coverage when using these materials.
Most aphids that feed on woody plants early in the season, such as the green peach aphid and the wooly ash aphid, are best controlled with delayed-dormant sprays in late winter just as the buds start to open.
-- Marianne C. Ophardt is a horticulturist for Washington State University Benton County Extension.