Figure 1. Black lesions on the outside of stalks indicate anthracnose stalk rot.	Figure 2. Pink to salmon discoloration inside the stalk may indicate fusarium stalk rot.	Figure 3. White fungal growth at the node can occur with fusarium stalk rot.

 

September 25, 2009 In the last two weeks, stalk rot diseases have become evident in some Nebraska fields. Producers noticed some corn "drying down too fast or too early," and on further investigation, found that most of the plants had anthracnose (Figure 1) and/or fusarium stalk rot (Figures 2-3).   Now is the time to scout those fields at highest risk for stalk rot and to harvest affected fields first to minimize potential yield losses due to lodging. (See Risk Factors and Stress Conditions Favoring Stalk Rots)

Potential Losses Corn stalk rots occur to some extent every year, but when large areas are affected, it can complicate and hamper harvest and cut into yields. Survey your fields now to determine where you’re likely to have the most problems and then harvest those fields first. While fall storms have been limited so far, high winds and rains could quickly down a field of diseased corn.   On average, stalk rot diseases reduce yield by about 5% each year, although losses can be as high as 10%-20% and on rare occasions, 100%. Stalk rots reduce yield both directly and indirectly.   

Figure 4. Lodging caused by stalk rot can complicate harvest and reduce yields.

Plants with prematurely rotted stalks produce lightweight, poorly filled ears because of the plant’s limited access to carbohydrates during grain fill. Infected stalks are converted from sturdy, solid rods to weakened, hollow tubes that are prone to lodging, particularly if decay occurs below the ear. The problem tends to increase in high yielding hybrids that produce large, heavy ears that cannibalize carbohydrates from the stalk during times of stress or predispose the weakened stalk to lodging. Indirect losses occur from harvest complications and ear loss.   Stalk rot diseases are caused by common fungi and bacteria that opportunistically infect senescing, injured, or stressed plants. A vulnerable plant often may suffer from several different stalk rots at the same time. Stalk rots common to Nebraska include Anthracnose, Fusarium, Gibberella, Diplodia, Charcoal, and Bacterial. Each is favored by slightly different environmental conditions and will exhibit slightly different symptoms.

Common Symptoms While individual symptoms vary with the stalk rot disease, a few are common to most:

• Plant wilting is often the first indication.
• Leaves become discolored, turning gray or brown.
• Inside the stalk, decay causes discoloration of the inner pith tissue, which pulls away from the stalk rind into a weakened hollow tube.
• Lower internodes turn tan or brown.
• The root decays and in severe cases, can kill the plant in as little as two days.

Scouting This summer’s conditions were favorable for the development of stalk rot in many areas of the state, necessitating a careful field assessment before starting harvest. The most common method to scout for stalk rots is to use the Push or Pinch Test.   Walk through a field and randomly select a minimum of 100 plants representing a large portion of the field. To test for stalk rot:  

1. Push the plant tops approximately 30º from vertical. If plants fail to snap back to vertical, the stalk has been compromised by stalk rot.
2. Pinch or squeeze the plants at one of the lowest internodes above the brace roots (pinching the same internode on each plant). If the stalks crush easily by hand, their integrity has been reduced by stalk rot.

If more than 10% of plants exhibit stalk rot symptoms, harvest that field first to reduce the potential for plant lodging and yield loss.

Under severe stalk rot conditions, it may be more economical to harvest early at higher moisture and dry grain than to experience severe harvest losses. 

Resource For more information on the causal agents, symptoms, favorable conditions and management of each of the leading types of stalk rot found in Nebraska, see EC1898, Common Stalk Rot Diseases of Corn, released this week by UNL Extension.

Management No fungicides are labeled for the treatment of stalk rots; however, fungicides can be used to effectively treat other diseases which might weaken the plant and predispose it to stalk rot. Maintaining optimal soil fertility, particularly the balance between the macronutrients, nitrogen and potassium, can help limit plant stress and the potential for ears to drain carbohydrates and nutrients from the stalk.   Tamra Jackson Extension Plant Pathologist