Over the past week, I have had calls and e-mails regarding anthracnose top dieback in corn. First, a word of caution: Not all top dieback is caused by the anthracnose pathogen. Death of top leaves may be due to one or more of several factors that include hybrid characteristics, environmental stress, and corn borer damage. Bob Nielsen at Purdue University discusses these various factors in an article titled Top Leaf Death in Corn.
How to diagnose anthracnose top dieback
First, check the distribution of the problem in the field. I look for a yellowed, purple, or dead flag leaf on scattered plants across the field (Photo 1). Second, peel back the leaf sheaths at the top of affected plants and look for characteristic black anthracnose lesions on the outside of the stalk (Photo 2). If conditions are wet, you may see a pink jellylike substance on the stalk (Photo 3). These are the spores of the anthracnose fungus that are produced in a gelatinous matrix. If the stalk is split, the pith appears rotted or discolored in the upper internodes. In a few plants I examined, anthracnose stalk rot also was apparent at the base of the stem (black lesions and/or rotted pith).
Photo 1. Look for a yellowed, purple, or dead flag leaf. (Alison Robertson)
Photo 2. Peel back the leaf sheaths at the top and look for characteristic black anthracnose lesions on the outside of the stalk. (Alison Robertson)
Photo 3. In wet conditions, you may see a pink jellylike substance on the stalk. (Alison Robertson)
Do fungicides reduce anthracnose top dieback?
It is probable that infection occurred prior to fungicide applications. The anthracnose fungus (Colletotrichum graminicola) infects through leaf sheaths or the whorl early in the season and remains dormant in the stalks until late in the season when late-season stress triggers the development of the symptoms. A fungicide application applied at flowering could indirectly reduce anthracnose top dieback by reducing foliar disease severity and thus late-season plant stress. We are collecting data this growing season to verify this.
Anthracnose top dieback, like stalk rot, will result in premature plant death. Obviously, there is nothing that can be done now to prevent premature death. However, plants with stalk rot are very vulnerable to lodging. In fields in which anthracnose top dieback and/or stalk rot are prevalent, harvest should be scheduled a little early this year to prevent additional losses.
Alison Robertson is an assistant professor in plant pathology with research and extension responsibilities in crop diseases.
This article originally appeared on pages 262-263 of the IC-498(23) -- September 10, 2007 issue.