Conditions have occurred this summer that may result in increased corn fungal stalk rots in Illinois. Fields should be scouted to determine whether stalk rot is present, and early harvest should be considered if scouting efforts suggest stalk rot may be a significant problem. Stalk rots cause decay of the internal pith tissues of the stalk. They can affect yield by killing plants prior to maturity by causing lodging that results in decreased harvesting efficiency and by promoting increased ear rots after lodging occurs.
Why might stalk rots be a problem this year in many fields? Plants predisposed and weakened by a number of different stress factors are more susceptible to stalk rot. Some of the stresses and other factors that have been linked to increases in stalk rot include water stress, high-yielding hybrids, susceptible hybrids, root damage, leaf disease, conservation tillage, high plant populations, insect damage, early maturation, low N in mid- to late summer, high fertility (especially very high N), and low P and K.
This season, good growing conditions in much of Illinois resulting in large ears early in the season combined with drought conditions across much of Illinois in July and August may be pre-dominant stress factors that could enhance stalk rot. One scenario goes like this: Large ears with many kernels re-quire lots of energy and nutrients to fill the grain; however, drought conditions reduce photosynthesis and carbohydrate production by the plants. When photosynthesis is reduced, the main source of nutrients for the ears is also reduced, and the plant is forced to rob the stalk of nutrients, which makes the stalk more susceptible to stalk rots.
Four types of stalk rot in Illinois are anthracnose, diplodia, giberella, and fusarium stalk rots, all caused by pathogens that also cause ear rots. Charcoal rot is another type of stalk rot that occurs primarily under hot and dry conditions. The pathogens that cause stalk rots tend to be widespread and opportunistic fungi that take advantage of plants weakened by various stresses. The pathogens survive in infested stalk debris on or near the soil surface. Thus, no-till environments and continuous or short rotations out of corn can favor survival and infection by the stalk rot pathogens.
Corn stalk rot.
The first symptom of severe stalk rot is often seen as leaves changing to dull green or gray. This discoloration can be followed by 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.
Don't confuse irregularly shaped purple to brown discolored patches on and under leaf sheaths with stalk rot. This is called purple leaf sheath and is caused by growth of fungi, bacteria, and yeasts on pollen and other nutrients trapped between the leaf sheath and stalk. Purple leaf sheath does not infect the stalk and is not damaging.
Scouting should be done soon for stalk rot. Plants should be inspected with the "pinch" or "push" test in each field. Twenty plants should be tested in five different parts of a field. If stalk rot is well developed, the lower internode will easily compress when pinched firmly, and/or stalks will break or remain bent over when pushed 10 inches to the side at ear height. If 10% to 15% of plants in a field have stalk rot, then the potential for significant lodging is high, and early harvest at about 24% grain moisture should be considered.
Damage from stalk rots can be reduced by avoiding or reducing as many stresses as possible, managing balanced fertility throughout the season, and harvesting early to minimize losses from lodging. Stalk rot is a recurring problem that occurs in many Illinois cornfields each year.--Dean Malvick