We have received questions concerning stalk rot of corn and whether it will be a major problem in Illinois this season. The extreme stresses put on corn this season in many parts of Illinois suggest that stalk rot may become a problem. Cornfields should be scouted now for stalk rot and decisions made regarding possible early harvest dates for fields at risk.|
Several types of corn stalk rot are in Illinois. These include diplodia stalk rot, anthracnose stalk rot, fusarium stalk rot, giberella stalk rot, and occasionally charcoal rot. These different types of stalk rot are all caused by different pathogens and are characterized by different symptoms. The fungal pathogens that cause the stalk rots tend to be widespread and opportunistic, taking advantage of plants weakened by various stresses. The pathogens survive most readily in infested stalk debris on or near the soil surface, hence no-till environments and continuous or short rotations out of corn can favor survival and infection by the stalk rot pathogens.
Diplodia stalk rot. (Photo courtesy of Don White.)
Giberella stalk rot. (Photo courtesy of Don White.)
Anthracnose stalk rot. (Photo courtesy of Don White.)
As stalk rot develops, often the first symptom is leaves changing to dull green or gray. Other symptoms are wilting, drooping of the ears, straw-colored lower stalks, and internal pith tissue that is decayed and discolored. Anthracnose stalk rot often appears earlier than other stalk rots, prior to normal senescence. Several internodes may rot, and a shiny black color develops on the outer stalk.
Scouting should be in progress for stalk rot. About 100 plants should be inspected with the "pinch" or "push" tests in each field. Ten plants should be tested in 10 different parts of a field. If stalk rot is well developed, the lower internode will easily compress when pinched firmly, and stalks will break or remain bent over when pushed 6 to 8 inches to the side at ear height. If 10 to 15 percent of plants in a field have stalk rot, then the potential for significant lodging is high and early harvest should be considered.
Stalk rot is favored by a combination of stress factors. Root or lower stalk damage may enhance infection by stalk rot pathogens, and stresses on the plant that reduce photosynthesis can result in depletion of carbohydrates in the lower stalk and enhance stalk rot. This season, drought stress, root damage such as that caused by rootworms, and dry, compacted soils were stresses to the corn crop in many parts of Illinois that may increase problems with stalk rot. In addition, a number of other stresses have been reported to be related to stalk rot, including high-yielding hybrids, leaf disease and insect damage to leaves, high plant populations, early maturation, high N fertility, low P and K levels, root and crown rot, and wind damage.
Stalk rots can often be reduced by avoiding or reducing as many stresses as possible, managing balanced fertility throughout the season, and harvesting early to minimize losses due to lodging. Hybrids with resistance to stalk rots and leaf diseases, and with good stalk strength ratings, should also reduce problems with stalk rot.--Dean Malvick