Corn Stalk and Root Rots

Stalk rots are expressed as corn reaches maturity and tend to be a complex of several disease-causing fungi. Plants with rotted stalks usually have rotted roots as well caused by the same organism. In many cases, deterioration of stalk tissues is caused by carbohydrate stresses or other physiological factors with fungi strategically colonizing the tissue. Visual identification can be difficult. Deteriorated stalk tissue usually can be determined from pinch and push tests. Fields where stalk and root rots are developing should be harvested early to reduce yield losses.

Fusarium Stalk and Crown Rot infections are usually caused by Fusarium species, a pathogen commonly found in crop residue and soil (Figure 1). The combination of a wet spring followed by an extended hot and dry period can lead to crown rot infection. While infections have their beginnings in the early spring, they can persist much longer. The rotting of roots, crown, and lower internodes can lead to stalk lodging. Symptoms include whitish-pink to salmon discoloration of tissue at the nodes. Current corn products do not have resistance to Fusarium crown rot. Fungicide seed treatments can provide some protection, but treatment does not persist the entire growing season. Because crown rot is associated with various stresses in addition to a fungal infection, identifying and alleviating any stresses can help reduce losses.

 

Figure 1. Stalk rot in a crown caused by Fusarium.

Red Root Rot is a late season disease of corn caused by a complex of soil fungi that includes Phoma terrestris, Pythium, and Fusarium pathoghens. Symptoms typically appear just before senescence, with roots and basal stalk tissue (lower three internodes) having a reddish pink discoloration (Figure 2). Roots become deeper red as the disease progresses. It can be confused with Fusarium stalk rot, but red root has a darker red color. Infections can occur as early as mid-silking with high plant populations, high fertility, and irrigation as common causes. During late stages of ear filling, the disease can cause rapid, premature death of the plant. Above ground symptoms include a grayish green discoloration of leaves and stalks or a wilted appearance. The root mass of infected plants can be small, and the entire root ball may be pulled up during harvest.

 

Figure 2. Red root rot infection showing reddish pink to dark red root discoloration in left picture, and discoloration of basal stalk tissue (crown) in right picture. (Source: S. Koenning, North Carolina State University)

Management options for red root rot are limited, but crop rotation helps and the rate of disease development can vary between corn products. Environmental stresses during the season also contribute to disease infection and severity.

Stalk Cannibalization is a process that can make corn plants more susceptible to colonization by fungi and physiological stalk lodging. In response to stress, corn plants will mobilize sugars to fill the kernels thus resulting in reduced sugar content of stalks and disintegration of pith cells. While this is often called stalk rot, the fungi are primarily colonizing tissues that are predisposed due to any condition that reduces photosynthesis and the production of carbohydrates needed to fill grain and maintain stalk integrity. Physiological stalk lodging is favored by good growing conditions early in the season, followed by stress after pollination. Cloudy weather or smoky conditions that reduce photosynthesis can help to promote the process.

Fields should be scouted for discoloration of stalks or early drydown as these symptoms indicate stalk rot or stalk cannibalization. Walking through a field, test stalk firmness by squeezing or pinching each stalk a couple of nodes above the ground. Healthy stalks are firm and cannot be compressed. If a stalk feels soft, it may be rotted or physiologically weakened and more likely prone to lodging.

Different corn products and fields with different management practices should be evaluated separately. Identify which fields may develop lodging issues and target these for an early harvest to help prevent potential harvest losses. When selecting corn products for next season, it is important to note the ratings for stalk strength, lodging, and different diseases vary by product and can be influenced by management practices.​

​​