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May 4, 2005

In general, landscape plants are looking very healthy. We had 2 weeks of warm, sunny, and relatively dry weather to start off the season. Before the recent cool, wet weather, I thought we might actually avoid anthracnose on trees this year. Anthracnose of shade trees requires cool, wet conditions in the 2 weeks following bud break. Succulent new plant growth is most susceptible. Older leaves and drier conditions later in the season usually retard repeating cycles of infection.

Although many of the buds on sycamores, oaks, maples, and ash trees susceptible to anthracnose opened in the recent warm, dry weather, they may not have escaped infection. Reports of ash anthracnose and sycamore anthracnose are coming from areas that saw more moisture in recent cool spells.

As a reminder, fungicides are not recommended for anthracnose of shade trees. Help the tree produce a new flush of foliage by providing water in periods of drought. Leaves that emerge in warmer temperatures are far less likely to be infected. Anthracnose causes water-soaked spots, leaf lesions from dark green to brown or black, and possibly some stem cankers on ash, oak, maple, and sycamore. Other trees are susceptible to varying degrees, but the trees listed are those on which we traditionally see anthracnose in Illinois. Dogwood anthracnose is much more severe but occurs later in the season. Look for dogwood anthracnose in June.

Rust galls of cedar-apple rust are now swollen with spores in central Illinois. Spores are most likely moving from cedars to their alternate hosts as this article goes to press. Keep this in mind if you are fighting cedar-apple, cedar-hawthorn, or cedar-quince rusts. This is the time to protect susceptible crabapples, hawthorn, and apples. To learn more about cedar-apple and related rusts, consult Report on Plant Disease, no. 802, “Rust Diseases of Apple, Crabapple, and Hawthorn.” This report is available in University of Illinois Extension offices and on the Web at http://www.ag.uiuc.edu/%7Evista/horticul.htm. Chemical options for sprays are available in the 2005 Commercial Landscape & Turfgrass Pest Management Handbook or the Home, Yard, and Garden Pest Guide.

General pruning note: Although the general recommendation is to prune trees and shrubs when dormant, many of us prune out any dead wood in trees and shrubs as it appears. In most cases, I would agree with this more practical schedule as long as conditions are dry. Now is a good time to remove dead wood because it is easy to find next to branches that are leafing out; and it is easy to get into a tree or shrub that is only beginning to leaf out. However, pruning in wet weather allows pathogens to survive and move on equipment. In addition, wetness allows the pathogen to remain alive or possibly germinate on the freshly cut wood. In areas where oak wilt is present, DO NOT prune oaks in the spring or early summer. Sap on fresh cuts attracts beetles that may be carrying the oak wilt fungus to your tree. Oaks should be pruned in late summer or the dormant season. Also keep in mind that pruning shrubs before they flower will remove this season’s flowers, so you may need to choose between pruning convenience and additional flowers.

Author: Nancy Pataky


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