Ear and Kernel Rots of Corn
By Laura Sweets
Corn harvest is beginning in many parts of the state, so it is a
little early to know which ear and kernel rots are causing problems in
specific areas of the state. Diplodia ear rot, Penicillium ear rot and
Gibb ear rot are common problems year in and year out but the severity
varies with weather conditions close to harvest. The Penicillium ear
rot and Gibb ear rot are particularly evident on the exposed tips of
ears, around insects tunnels and on ears that have remained upright.
If there are periods of wet weather before corn is harvested, some of
the corn plants that died prematurely from drought stress may show the
black discoloration caused by secondary fungi coming in on the
senescing plant tissues. Most corn fields in the state did get through
silking and pollination in relatively good condition and were exposed
to extended periods of hot, dry weather after pollination. Therefore,
the potential for Aspergillus flavus and aflatoxin may not be as high
as it was last year when much of the corn crop suffered from poor
pollination from dry conditions as the crop was silking and
pollinating.
Both Diplodia maydis and Diplodia macrospora can cause Diplodia ear
rot of corn. The ear leaf and husks on the ear may appear prematurely
bleached or straw-colored. When the husk is peeled back, dense white
to grayish-white mold growth will be matted between the kernels and
between the ear and the husks. Small, black fungal fruiting bodies may
be scattered on husks or embedded in cob tissues and kernels. The
entire ear may be grayish-brown, shrunken, very lightweight and
completely rotted. Diplodia ear rot is favored by wet weather just
after silking and is more severe when corn is planted following corn.
Penicillium rot is usually evident as discrete tufts or clumps of a
blue-green or gray-green mold erupting through the pericarp of
individual kernels or on broken kernels. Penicillium appears as small,
discrete colonies of mold growth with a dusty or powdery appearance.
The fungus may actually invade the kernel giving the embryo a blue
discoloration. Blue-eye is the term used for this blue discoloration
of the embryo.
Gibb ear rot (caused by Gibberella zeae) usually begins as a reddish
mold at the tip of the ear. Early infected ears may rot completely
with husks adhering tightly to the ear and a pinkish to reddish mold
growing between husks and ears. Although mold growth usually has a
pinkish to reddish color, it can appear yellow to yellow-orange or
yellow-red. Gibb ear rot typically begins at the tip of the ear but
under favorable conditions it can move down the ear causing extensive
damage. It may also develop around injuries from hail, birds or
insects.
Black corn occurs when any of a number of saprophytic or weakly
parasitic fungi grow on corn plants in the field. Alternaria,
Cladosporium, Aureobasidium and other species are frequently found on
these discolored or black plants. Since the affected plants may have a
sooty appearance these fungi are sometimes called sooty molds. These
sooty molds or secondary fungi tend to develop on plants when wet or
humid weather occurs as the crop is maturing or if harvest is delayed
because of wet weather. Typically these fungi come in on plants that
are shaded, undersized, weakened or prematurely ripened and on
senescing foliage. Plants that are lodged or that have been stressed
by nutrient deficiencies, plant diseases or environmental conditions
may be more severely affected. Although many of these fungi produce
dark or black mold growth, the color of the mold growth can range for
dark or black to olive green or even pink to white.
These secondary fungi tend to develop on senescing plant tissues,
primarily leaf, stalk and husk tissue, but under favorable conditions
can cause infection of the kernels. Infected kernels might show a
black discoloration.
It is possible that these sooty molds or secondary fungi could
contribute to stalk deterioration or stalk rot. Lodging could become a
problem in these fields, especially if there are high winds or strong
storms before harvest.
Grain from fields with high levels of sooty molds should be treated
with care if it is stored. Grain should be thoroughly cleaned to
remove lightweight, damaged or broken and moldy kernels. Grain should
be stored at the proper moisture content and temperature and checked
on a regular basis during storage.
Aspergillus flavus is evident as greenish-yellow to mustard yellow,
felt-like growth on or between kernels, especially adjacent to or in
insect damaged kernels. Aspergillus flavus is favored by high
temperatures and dry conditions, so Aspergillus ear rot is typically
associated with drought stress. The fungus survives in plant residues
and in the soil and spores are spread by wind or insects to corn silks
where the spores initiate infection.
These ear and kernel rots tend to be more severe on ears with insect,
bird, hail or other physical damage. Ears well covered by husks and
maturing in a downward position usually have less rot than ears with
open husks or ears maturing in an upright position. Both Penicillium
and Aspergillus can continue to develop on corn in storage if the
grain is not stored at proper moisture content and temperatures. These
two fungi can cause serious storage mold problems.
An additional concern with ear and kernel rots of corn is the
possibility of mycotoxin production. Mycotoxins are naturally produced
chemicals that in small amounts may be deleterious to animal or human
health. Aspergillus and Gibberella are most frequently involved in
cases of mycotoxin contamination in Missouri corn. The presence of
molds or their spores does not necessarily mean that mycotoxins will
be produced. Circumstances that favor mold growth may allow production
of mycotoxins in some situations, but frequently mold growth occurs
with little or no mycotoxin production. Once formed, mycotoxins are
stable and may remain in grain long after the fungus has died. In
general, swine and poultry are more susceptible than ruminants to
mycotoxin-induced health problems. In cases where mycotoxin problems
are suspected, a sample should be submitted to a qualified laboratory
for mycotoxin analysis.
Little can be done to prevent or reduce the invasion of corn by fungi
in the field. However, if ear and kernel rots developed in the field,
it is important to harvest the field in a timely manner and to store
the grain under the best possible conditions. Adjust harvest equipment
for minimum kernel damage and maximum cleaning. Before storing grain,
clean bins thoroughly to remove dirt, dust and any grain left in or
around bins. Thoroughly clean grain going into storage to remove
chaff, other foreign material and cracked or broken kernels. Dry grain
to 15 percent moisture as quickly as possible and monitor grain on a
regular basis throughout storage life to insure moisture and
temperature are maintained at correct levels. Protect grain from
insects.
Laura Sweets
573-884-7307
