Minnesota Crop News > 2001-2008 Archives
Stalk Rots in Minnesota This Year
Extension Plant Pathologist
Dave Nicolai, Regional Extension Educator-Crops
The corn crop generally looks good across Minnesota this
year, however, a problem may be lurking in many fields
that may reduce yields and set the stage for lodging and
harvest challenges. This problem is corn stalk rots, a
common problem that is increased by stress conditions in
the mid to late growing season. Corn stalk rot has been
reported this season in many areas of the state but particularly
in areas which suffered from drought stress and/or corn
rootworm damage earlier in the season. General information
about corn stalk rots and a perspective on this problem
for Minnesota are addressed in this article.
Corn leaves changing from to dull green or gray is often
an early symptom of severe stalk rot . Wilting, straw or
black-colored lower stalks, discoloration and decay of
internal pith tissue, and drooping of the ears may occur
as the stalk rot develops. Stalk rot shouldn’t be
confused with innocuous purple leaf sheath, which is seen
as irregular-shaped purple to brown discolored patches
on and under leaf sheaths caused by fungi and other microorganisms
growing on pollen and other nutrients trapped between the
stalk and leaf sheath.
Conditions have occurred in Minnesota this summer that
may result in increased corn fungal stalk rots. But why
may stalk rots be a problem this year in many fields? Plants
under stress are more susceptible to stalk rot. The pathogens
that cause most stalk rots tend to be widespread fungi
that take advantage of plants weakened by various stresses.
These may include water stress, 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.
In addition to these potential stress factors, high yield
environments can increase stalk rots. For example, large
ears require much energy and nutrients to fill the kernels,
however, stress conditions that reduce photosynthesis and
carbohydrate production by the plants may cause a shift
of nutrients from the stalk to the ears, resulting in stalks
more susceptible to stalk rots. No-till environments and
continuous or short rotations out of corn favor survival
and infection by the stalk rot pathogens because the pathogens
survive in corn residue on or near the soil surface.
Stalk rot is decay of internal stalk tissues caused primarily
by different types of fungi including Colletotrichum (Anthracnose), Giberella,
and Fusarium. Charcoal rot and Diplodia stalk
rots are important stalk rots that are common in states
south of Minnesota, but these as well as Pythium and bacterial
stalk rots could also occur occasionally in Minnesota.
Anthracnose stalk rot often appears prior to normal senescence,
which is earlier than most other common types of stalk
rots. It can cause a top die-back of the stalk which can
be confused with early dry-down. In this phase the plant
is killed from the top down as the fungus progressively
colonizes the stalk. Also, unlike most stalk rots, anthracnose
causes development of a black color on the outer surface
of the stalk and may cause rot of several internodes. The
black, blotchy lesion areas on the stalk surface cannot
be scraped away with a thumbnail.
Fusarium stalk rot often occurs late in the season. Yield
loss may be more related to corn lodging and broken stalks
than poor ear development. Fusarium stalk rot can be distinguished
by a whitish-pink to salmon discoloration of the internal
stalk tissues. Symptoms of Fusarium stalk rot can easily
be confused with Gibberella stalk rot, which produces a
pink to reddish discoloration of the internal stalk tissues.
Rotting commonly affects the roots, crown and lower internodes.
A diagnostic sign of Gibberella is the presence of small,
black specks (perithecia) on the surface of the stalk rind
and frequently clustered near the nodes. The perithecia
may be easily scraped away from the rind tissues with the
thumbnail. The fungus that causes Gibberella stalk and
ear rot also can cause head scab of wheat and barley.
Consider scouting fields for stalk rots. Test 20 plants
in five different parts of a field with the “pinch” or “push” test.
For the “pinch test”, the lower internode will
easily compress when pinched firmly. For the “push
test”, stalks will break or remain bent over when
pushed 10” to the side at ear height. If 10-15% of
plants in a field have stalk rot, then the potential for
significant lodging is high and early harvest should be
Stalk rot damage is difficult to manage but can be minimized
by reducing as many stresses on the corn crop as possible
throughout the growing season and by harvesting early to
minimize losses. Corn growers should select hybrids that
have stalk rot and leaf disease resistance, good standability,
and high yield potential. Balanced soil fertility, control
of corn borers and corn rootworms, and appropriated plant
populations, as may be suggested with particular hybrids,
are also important in reducing stalk rots.
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