Some trees in Iowa have been losing leaves over the last few weeks. Foliar diseases, dry weather, and Japanese beetle defoliation are the primary reasons for the leaf drop.
Foliar diseases, such as apple scab on crabapples, cause spotting of the foliage and premature leaf drop. The premature leaf drop weakens crabapples, but does not kill the trees. The damage is mainly aesthetic. Raking and destroying the fallen leaves may reduce the severity of the disease next year.
The hot, dry weather in recent weeks has been stressful to trees. Some trees, like the river birch, cope with the hot, dry weather by shedding some of their leaves. Leaves in the interior of the river birch turn yellow and drop to the ground. Healthy, well-established trees should not be seriously damaged by the hot, dry weather of recent weeks. The hot, dry weather poses a more serious risk to trees planted in the last 3 to 5 years. Recently planted trees should be watered approximately every 7 to 10 days during dry weather.
Japanese beetles feed on more than 300 different plants. Some of their favorites are linden, elm, birch, pussywillow, cherry, and some crabapple varieties. Japanese beetles eat the leaf tissue between the veins, leaving lacy-looking or skeletonized leaves. The skeletonized leaves turn brown. Many of the damaged leaves drop to the ground. Defoliation of well established, healthy trees is usually not fatal. Defoliation is most harmful to recently planted trees (those planted in the last 3 to 5 years) and trees in poor health. Young, defoliated trees would likely benefit from a thorough watering every 7 to 10 days during dry weather.