May 26, 1993

Apple IPM Update

Apple scab. Orchards in central and southwest Iowa are just past petal fall. Growers who are using the IRS reduced-spray program for scab control need to apply a tank mix of a SI fungicide (e.g., Nova or Rubigan) with a contact fungicide (e.g., mancozeb, polyram, captan) at petal fall and again at first cover (a week to 10 days after petal fall). This will bring you to the end of the high-risk period for primary infection by scab.

Resistance of Trees and Shrubs to Deer Damage

Damage caused by deer browsing on trees and shrubs can be a problem in some rural and suburban areas in Iowa. The amount of damage depends upon seasonal factors, weather conditions, availability of alternative foods, and plant palatability. Prevention of deer browsing is difficult. Home gardeners may be able to avoid some deer damage by careful plant selection. Horticulturists and wildlife specialists at Cornell University in Ithaca, New York, have identified trees and shrubs that are rarely or seldom, occasionally, and frequently damaged by deer.

Hollyhock Rust

Hollyhock rust is caused by the fungus Puccinia malvacearum. The rust fungus causes raised brown spots, usually pinhead-size or larger, on the undersides of leaves. The upper leaf surface shows orange to yellow spots. The raised spots may coalesce, killing larger areas of leaves.

Control: Remove diseased leaves. After flowering, cut the plant back to the base and destroy infected plants parts. Fungicides, such as chlorothalonil (Daconil 2787), are effective against rust but frequent applications are necessary to protect newly emerging leaves.


This spring's abundant rain has created the potential for an upsurge in mosquito activity that may be "felt" by many people over the next few weeks. The only reason we have not felt the problem before now is that our unseasonal cool temperatures have slowed the mosquito developmental rate and delayed adult emergence in many areas of the state. Regardless of whether record numbers of mosquitoes are produced this year, many people are now asking how to control these pests in and around their homes.

Black Flies--Springtime's Biting Gnats

Black flies are not very common in most of Iowa, though a few samples and calls are received every year. This year, we have experienced a marked increase in the number of calls about 'those !@#$%* gnats that bite like @#$!%*!'.

Black flies, also known as buffalo gnats and turkey gnats, are small, blood-sucking insects slightly less than 1/4 inch long with a stout-body and hump-backed appearance. They are most common along rivers and streams during late spring and early summer.

Mosquito Repellent Plants

Accompanying the recent interest in mosquitoes and mosquito control is a dose of healthy curiosity concerning the widely advertised mosquito repellent plants. The following article, reprinted from the July/August, 1992 issue of Organic Gardening magazine (page 18), provides one of the clearest summaries of the subject I have seen. Enjoy.--D. R. Lewis

Strawberry Leafspot

Strawberry leaf spot is caused by the fungus Mycosphaerella fragariae. Characteristic leaf symptoms make this disease easy to identify. At first, small purple spots develop on the upper leaf surface. As the spots enlarge, the centers turn a brown color and are surrounded by a dark, reddish border. These spots may coalesce as they enlarge, and cause death of leaves.

Because the fungus is spread primarily by splashing water, wet spring weather can lead to higher disease incidence.