Remember to scout for corn stalk rots

Stalk rot occurs to some extent in every cornfield in Iowa each year because as corn stalks mature, they naturally decay. However, stalk rot can occur prior to physiological maturity and is considered a disease problem, which reduces yields in two ways. First, affected plants die prematurely (Fig. 1) and therefore produce lightweight ears with poorly filled kernels. Second, and this is more common, plants with stalk rot easily lodge (Fig. 2), which makes harvesting difficult, and many ears are left in the field.

Fig. 1. Corn plant killed by stalk rot.

Fig 2. A combination of poor roots and stalk rot resulted in severe lodging in some fields in 2001.

Stalk rot occurs because the plant is stressed. A stressed plant is unable to make enough carbohydrates to fill grain and sustain healthy stalk and root tissues; therefore, it becomes susceptible to infection by fungi that live in the soil or in crop residue. The most common stalk rot fungi in Iowa are Fusarium, Gibberella, and Colletotrichum (anthracnose), and Diplodia in the eastern part of the state. Stressful circumstances such as foliar diseases; hail, wind, and insect damage; drought; and overly wet conditions can each predispose corn plants to stalk rot.

Despite being caused by many different fungi, symptoms of stalk rot are very similar. Visible symptoms include grey-green, rolled up leaves, yellow or light brown stalks, and brown lesions on the stalk at the lower nodes. Splitting the stalks of suspected plants reveals the most characteristic symptoms of stalk rot: shredding and disintegration of the lower stalk tissue (Fig. 3). Diseased stalks are easily crushed when squeezed between the thumb and forefinger. They are also more susceptible to lodging during wind and heavy rainstorms. Anthracnose stalk rot, caused by Colletotrichum, can be easily distinguished from other stalk rots because it causes very dark, shiny lesions on the stalk rind (Fig. 4). Anthracnose can also cause top dieback in which the leaves of the stalk above the ear turn yellow or red and dry up (Fig. 5).

The most characteristic symptom of corn stalk rot is shredding and disintegration of the lower stalk tissue.

Fig. 4. Anthracnose symptoms on stalk rind.

Fig. 5. Anthracnose top dieback.

To reduce losses to stalk rot, scout fields before black layer (40-50 days after pollination). At least 100 plants, scattered throughout the field, should be assessed.

Look for visible symptoms (as described above) and test stalk firmness at the lower internodes with thumb and forefinger. If more than 15 percent of the stalks are rotted, schedule for the earliest possible harvest because significant lodging is possible. Scout different hybrids and fields with different tillage, rotation, or fertilization histories separately.

Future stalk rot problems can be avoided by rotating corn with other crops, planting disease (foliar and stalk rot) resistant hybrids, planting at hybrid appropriate plant populations, avoiding root and stalk injury, practicing insect and weed control, and ensuring adequate potassium fertilization. An Iowa State University Extension publication, Corn Stalk Rot in Iowa, IPM 50, which describes the symptoms and management of stalk rot in more detail, is available.

This article originally appeared on pages 109-110 of the IC-492(20) -- September 13, 2004 issue.

Updated 09/12/2004 - 1:00pm