ISSUE 5 June 3, 2004
PLANT DIAGNOSTIC LAB UPDATES
Just as in 2003 (see the Crop and Pest Report, Issue 5, 2003 article by Dr. Robert Stack), leaves are once again dropping off of ash trees. Most likely, defoliating ash trees are suffering from ash anthracnose, a fungal disease of green ash and other ash species. The cool wet weather of the past two weeks has especially favored development of the disease. Usually, this disease is aesthetic, and treatment with fungicide is not typically needed. Ash trees usually produce a new flush of leaves after early defoliation.
Typical symptoms of the disease include round to irregular blotches on leaves. These brownish blotches usually appear along margins and midribs of leaflets. Affected leaflets become deformed, and they appear scorched and curled. Small cankers and dieback may even occur on twigs of trees that have suffered from the disease for several consecutive years. Later in the season, when the environment isnít as favorable to disease development, numerous small, round lesions with gray centers and purple-brown margins (frog-eye leaf spots) may develop.
Cultural control measures include proper watering, mulching, and sanitation procedures. Good sanitation includes collecting and destroying fallen leaves, as well as pruning out dead or dying twigs and limbs.
If a tree is particularly stressed, from consecutive years of anthracnose infection, for example, or from root restrictions, drought, heavy insect infestation, or other factors, then the tree may experience a significant loss in vigor. In such cases, protectant fungicide sprays may be needed. Three applications work best, and timing of fungicide applications is critical for controlling disease development. The first application should be made as buds are beginning to swell, but before buds break. The second application should be made when the buds show green tips, and the third fungicide application should be made when the leaves are half grown.
For the current year, itís too late for this three-application fungicide regime, but spraying even after the first infection and subsequent defoliation can reduce late season infections. Several fungicides are registered for use against anthracnose. Be sure to read and follow all label instructions of the fungicide that you select to ensure that you use the fungicide in the safest and most effective manner possible.
For more information, you can view the following related NDSU extension bulletins on-line:
NDSU extension circular #PP697 "Deciduous Tree Diseases," by Robert Stack, available at:
NDSU extension circular #F1192 "Insect and Disease Management Guide for Woody Plants in North Dakota", by Marcus B. Jackson, Phillip A Glogoza, Janet J. Knodel, Cheryl L. Ruby, and James A. Walla, at:
Congratulations to Carrie Larson, as she leaves the Plant Diagnostic Lab and enters her new position as Plant Protection Specialist with the North Dakota Department of Agriculture. Ms. Larson has been a great asset to the Lab for nearly two years, and she will continue to serve the people of North Dakota well. She will be based on the NDSU campus in Fargo.
Plant Pest Diagnostician