Forest and Shade Trees of Iowa

Part 4: Iowa’s Forests

394 g r o w i n g t r e e s i n i o wa Japanese beetles also chew the foliage of some trees, especially lindens, plums, and paper birch. As this book was written, this new pest was spreading westward in Iowa. The beetles are green and brown in color, about 3/8 inch long, and often numerous. While they do not kill trees, they can cause significant defoliation and seriously mar trees’ appearance. Pesticides will reduce the damage but need to be reapplied, as new beetles will reinfest an area after others are killed. Galls are unusual outgrowths or deformations of the stems, leaves, and flowers, usually caused by insects or mites. They are seldom a serious threat, and no treatment is necessary. Chlorosis is an abnormal loss of the green pigment chlorophyll, causing leaves to be yellow or yellow-green in color. It can have a variety of causes, including drought, but high soil pH is often involved. Pin oak, red maple, and some other species grow best in acid soils, and when they are planted in alkaline soils, they cannot absorb certain nutrients they need. Trees that are severely chlorotic are best replaced by other species, but mild cases can be treated by acidifying the soil. Mulching and watering are also advised, as they will improve the environment for root growth. Fungi cause many kinds of leaf spots on Iowa’s trees. Most are not harmful. g r o w i n g t r e e s i n i o wa 395 Borers are the larvae of insects, usually beetles or moths, which tunnel inside the stems of trees. Symptoms include holes with sawdust or oozing sap, defoliation, dieback, and sometimes death. Some borers are primarily problems of weak or stressed trees, while others, such as the emerald ash borer, also attack healthy trees. Unfortunately, borers are not easily treated once they enter a tree. The best way to manage them is to avoid planting susceptible Chlorosis is an abnormal yellowing of leaves usually caused by nutrient deficiencies or drought. The larvae of various beetles and moths live as borers in the stems of trees, often causing significant damage. 396 g r o w i n g t r e e s i n i o wa species. Where a susceptible species is already planted, maintain tree vigor through good cultural practices. Cankers are fungal or bacterial diseases that attack twigs, branches, or the trunks of trees. Like borers, they are often stress-related and difficult to treat. Symptoms include dead or darkened bark, craterlike depressions in the stem, defoliation, and dieback. To minimize the likelihood that your trees will be infected, follow the cultural practices described above. Wilt diseases are caused by fungi, bacteria, or nematodes that infect branches or whole trees, causing wilting, defoliation, and death. They are usually spread by insects or by root grafts between adjacent trees. There is no cure once they establish in a tree, so prevention is the best protective measure. Infected trees should be promptly removed and destroyed. Decline is a gradual loss of vigor that results in dieback and often death of a tree. Poor growth, sparse foliage, undersize leaves, and dead upper limbs are signs that decline is occurring. Common causes include drought, construction damage, and soil compaction around the roots, which often do not show their effects for several years. In urban and residential areas, chronic chlorosis and girdling roots can also cause decline. When good care is provided, early-stage decline can sometimes be arrested. More often than not, however, the decline will be progressive. Borers are often attracted to declining trees and can hasten the death of those trees. Emerging Threats to Iowa’s Trees At the time this book was written, the gypsy moth and emerald ash borer were on the verge of entering Iowa. It is probably just a matter of time before these pests spread throughout the state. Gypsy moth damages trees in its caterpillar stage, when caterpillars eat the foliage of many kinds of trees, especially oaks. The caterpillars are of concern to foresters and homeowners because they feed early in the growing season and often occur in large numbers, completely defoliating trees. Early-season defoliation is very stressful to trees, and if repeated in subsequent years this can seriously weaken them, causing decline and attracting borers. The emerald ash borer is...