AMES, Iowa – Rainfall this spring has created ideal conditions for several tree diseases and foliage abnormalities. Horticulturists with Iowa State University Extension and Outreach are available to address homeowners’ tree concerns. Contact the ISU Hortline at firstname.lastname@example.org or 515-294-3108 to have additional questions answered. Submit a plant sample to the ISU Plant and Insect Diagnostic Clinic (http://www.ent.iastate.edu/pidc/) for a disease diagnosis; contact your county extension office for help correctly submitting a sample.
The ‘Spring Snow’ crabapple is widely planted as it is one of the few crabapple varieties (cultivars) that doesn’t bear fruit. Unfortunately, ‘Spring Snow’ is very susceptible to apple scab.
Apple scab is a fungal disease. It’s caused by the fungus Venturia inaequalis. Cool, wet weather in spring favors apple scab development.
Apple scab appears as velvety, olive-green to black spots on crabapple leaves. Heavily infected leaves turn yellow and fall from the tree. Highly susceptible crabapple cultivars may lose a large percentage of their leaves by mid-summer. Fortunately, apple scab does not kill affected trees. The damage is mainly aesthetic. Heavily defoliated crabapples are not very attractive.
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 simply not practical. Sanitation also plays a role in controlling apple scab. Raking and destroying the leaves as soon as they fall may reduce the severity of the infection next season. However, the best way to prevent apple scab is to plant scab resistant crabapple cultivars. (ISU Plant Diagnostic Clinic photo - branch of an apple tree with a case of apple scab)
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 the state. 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.
The bumps on the leaves are hackberry nipple galls. Galls are abnormal plant growths induced to form by small insects or mites. Hackberry psyllids (small aphid-like insects) are responsible for the galls on hackberry leaves. In early spring, adult hackberry psyllids emerge from their over-wintering locations and lay eggs on hackberry leaf buds. Eggs hatch into tiny nymphs that stimulate the raised swellings or galls in which the insect lives and feeds for the rest of summer. The psyllids complete their development in late summer and emerge from the galls. The adults spend the winter in cracks and crevices in tree bark and other sheltered locations.
Galls do not cause serious harm to healthy, well-established trees. Galls cannot be “cured” once they have formed. Preventative insecticide treatments are seldom warranted.