Yard and Garden: Trees


Trees are one of the most valuable assets in a landscape. All properties enjoy the benefits of having healthy and beautiful trees. In addition to the aesthetic appeal, a healthy tree can add value to a property. Home gardeners with questions about the management of other tree diseases may contact the experts by emailing or calling the ISU Extension horticulture hotline at hortline@iastate.edu or 515-294-3108.

My crabapple has begun to drop some of its leaves. Why? 

The leaf drop is probably due to apple scab. Apple scab is a fungal disease caused by the fungus Venturia inaequalis. Cool, wet weather in spring favors apple scab development. Crabapple varieties differ in their susceptibility to apple scab. Some varieties are very susceptible to the disease, while others are resistant to apple scab. 

Apple scab appears as velvety, olive-green to black spots on the crabapple leaves. Heavily infected leaves turn yellow and fall from the tree. Highly susceptible crabapple varieties may lose a large percentage of their leaves by mid-summer. Fortunately, apple scab does not kill affected trees. The damage is mainly aesthetic. 

Apple scab can be prevented by applying fungicides from bud break through mid-June. For most home gardeners, however, controlling apple scab with fungicides is not practical. Sanitation also plays a role in controlling apple scab. Raking and destroying the leaves as soon as they fall should help reduce the severity of the infection next season. However, the best way to prevent apple scab is to plant scab-resistant crabapple varieties. 

Why are the leaves on my pin oak yellow-green? 

In Iowa, the foliage of the pin oak (Quercus palustris) often turns a sickly yellow-green. The yellow-green foliage is due to a deficiency of iron. The problem is referred to as iron chlorosis. (A close examination of chlorotic leaves will show that while most of the leaf is yellow-green, the tissue around the major veins is a darker green.) Most soils in Iowa contain sufficient amounts of iron. However, in alkaline soils (those with a pH above 7.0), the pin oak is unable to absorb adequate amounts of iron because much of it is in an insoluble form. Since many soils in Iowa are alkaline, chlorotic pin oaks are common in Iowa. Wet soil conditions make absorption of iron even more difficult. 

Correcting an iron chlorosis problem is difficult. Applying additional iron to the soil usually doesn’t help. The soil already contains sufficient amounts of iron. Adding more iron doesn’t overcome the problem. Lowering the soil pH to 6.0 to 6.5 would allow the roots of the pin oak to more readily absorb iron in the soil. Unfortunately, lowering the soil pH is extremely difficult, if not impossible.  As a result, homeowner efforts to treat iron chlorosis are often unsuccessful. 

One strategy that sometimes works is to have an arborist or other tree care professional inject an iron containing compound directly into the trunks of chlorotic pin oak trees. The effects of a trunk injection may last three or four years. 

Why is my sycamore tree dropping its leaves? 

The leaf drop is likely due to anthracnose. Anthracnose is a common fungal disease of sycamore, ash, maple, oak and other trees. Anthracnose is most severe in years with cool, wet spring weather. While anthracnose may cause extensive defoliation, it does not cause serious harm to healthy, well-established trees. 

Symptoms of anthracnose on sycamores include brown blotches on the leaves, death of young buds and shoots, and leaf drop. In cool, wet springs, affected sycamores may lose most of their initial foliage. 

Fortunately, the sycamore trees will continue to produce additional leaves and shoots through early summer. Foliage that develops in late spring and early summer shouldn’t become infected as warmer, drier weather suppresses anthracnose.  Most sycamores should have a good canopy of leaves by late June or early July. 

Since anthracnose does not cause serious harm to sycamores, fungicide treatments are rarely warranted. 

The leaves on my peach tree are puckered and reddish in color. What is the problem? 

The symptoms are those of peach leaf curl. Peach leaf curl is a fungal disease. The disease is caused by the fungusTaphrina deformans. Infections occur as the peach tree buds begin to swell in spring.

A single fungicide application will control peach leaf curl. Fungicides, such as lime sulfur, Bordeaux mixture or chlorothalonil, should be applied in fall after leaf drop or in late March before the buds begin to swell. To achieve control, all branches and twigs must be thoroughly sprayed.