Corn Diseases in 2000
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Several corn diseases made the summer of 2000 unique for corn production. Common rust, Diplodia ear rot, and stalk rot occurred with greater incidence and severity than in recent memory. Other diseases such as Stewart's wilt, gray leaf spot, eyespot, anthracnose leaf blight, and northern corn leaf blight were present. Stewart's wilt was not as damaging as had been expected following the warm winter that favored flea beetle survival, mostly due to good resistance in many dent corn hybrids. Eyespot was severe in northern parts of Illinois early in the growing season; however, it did not cause damage over large geographical areas. Gray leaf spot was not as severe as in previous years. Anthracnose leaf blight and northern corn leaf blight were present in most areas of the state but were not severe.
Common Rust
Puccinia sorghi, the fungus that causes common rust, does not overwinter in the midwestern United States. Primary inocula
for the disease in most of North America are spores blown northward from southern
areas where corn is growing during the winter months. Common rust was present
in central Illinois by the end of May, which is several weeks earlier than usual.
It is suspected that the early arrival of inoculum may have been due to the
greater acreage of corn planted in the southern United States and a severe epidemic
in Mexico in the spring of 2000. A new race of the fungus was described in 1999;
however, that is not believed to have been responsible for the early arrival
of inoculum.
Common rust is favored by moderate temperatures and high humidity. In most years
in central Illinois when temperatures reach into the 80s and 90s, the fungus
goes dormant and does not cause problems throughout much of the summer. Weather
conditions this past summer were relatively cool and wet most of the summer,
and the fungus continued to cause damage.
Diplodia Ear Rot
Diplodia ear rot, caused by Stenocarpella maydis (Syn, Diplodia maydis) caused
damage to corn from eastern Iowa to Ohio. Diplodia ear rot has been increasing
in both incidence and severity in the midwestern United States as the use of
conservation tillage has increased. Symptoms are evident several weeks after
flowering, when the husks turn from green to a bleach or straw color. In most cases, the fungus enters the base of the ear and moves up
to the tip. Entire ears are often shrunken
and grayish in color, with infected kernels glued to the husk by white mycelium
of the fungus. With some ears, only
a small amount of white growth occurs at the base of the ear. Black pycnidia
are evident late in the season on husks, cobs, and kernels. The fungus also causes a stalk rot disease, which also has become
more prevalent with conservation tillage.
The specific reasons for the severe epidemic of Diplodia ear rot are not clear.
Diplodia ear rot has been a problem in isolated geographical areas in the Midwest
for more than 10 years, but it has not occurred over large areas with the severity
as in the summer of 2000. Corn is the only host for S. maydis, and it overwinters
on corn stalks allowed to remain on the soil surface. Diplodia ear rot occurs when pycnidia of the fungus produce spores
that are spread to ear shoots by windblown rain one to three weeks after pollination.
It is possible that it has taken a number of years for Diplodia ear rot and
stalk rot to become widely established throughout the central Midwest, and this
year we had enough rain to spread conidia.
Ear rot was not uniform from field to field. This is possibly due to differences
in Diplodia stalk rot in previous years, planting date, and hybrid susceptibility.
The greatest damage occurred in fields where corn followed corn and stalk tissue
from the previous crop was on the soil surface. The disease also was common
in many fields where corn followed soybeans and corn stalks from a previous
crop were present on the soil surface. We do not know how far spores of S. maydis
can be spread by windblown rain, but several hundred yards would be very likely.
Therefore, inoculum could be spread to adjacent fields. Fortunately, there have
not been problems with feeding grain damaged by Diplodia ear rot to swine or
cattle.
Corn Stalk Rot
Premature death and lodging due to corn stalk rot occurred with greater severity
over large areas than at any time in the last 10 years. Premature death and
lodging were evident as early as late August, and the situation worsened as
corn matured. There are numerous factors that contributed to the stalk rot epidemic
of 2000. It is very likely that rust and other leaf blight diseases predisposed
plants to susceptibility. If leaves of the plant are damaged, the plant is not
capable of producing enough carbohydrates to fill kernels and maintain good
cell health in the root tissues. As cells in the root and lower stem die, stalk-rotting
fungi invade and rot those tissues. Damage to leaves of very young plants by
common rust may have had a very detrimental effect on root growth and general
plant health. Thus, common rust could have contributed significantly to the
stalk rot problems at the end of the growing season.
One contributing factor in many locations was the early fall application of
nitrogen when warm winter conditions were conducive for denitrification. Plants
that have adequate to high levels of N just after planting but are deficient
after flowering become much more susceptible to stalk rot. In some locations,
it also is likely that moisture stress was severe enough that photosynthesis
was reduced and plants could not produce enough carbohydrates to maintain the
health of lower stem tissues and also fill grain. Higher plant populations also
favor stalk rot. Seed quality for the 2000 growing season was excellent for
most hybrids, and conditions in the spring were good for stand establishment.
Those factors, in conjunction with the tendency to plant higher populations
for better yield, may have resulted in plant populations that were high enough
to lead to stalk rot.
Planning For the Future
It is difficult, if not impossible, to predict the future importance of corn
diseases. It is likely that common rust will not be a problem next summer if
temperatures are warmer than in the summer of 2000. Likewise, Diplodia ear rot
will not be a problem if the weather is dry just after flowering. If rainy weather
occurs shortly after flowering, it is likely that Diplodia ear rot will be serious
next summer, particularly on susceptible hybrids. This is likely to continue
into the future and consideration should be given to identifying resistant,
high-yielding hybrids. It is unlikely that stalk rot will be as big a problem
in 2001 as it was in 2000. More farmers delayed application of N, and this winter
has been colder, which will result in fewer problems with denitrification and
leaching. We also can hope that common rust, eyespot, and gray leaf spot will
not cause damage to leaves, thus predisposing hybrids to susceptibility.
Text prepared by Dr. Donald White, Professor of Plant Pathology, UIUC.