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Stalk Rot in Corn
Stalk rot is the most common
and widespread disease in corn and will be present to some extent in all
corn fields every year. Its presence becomes significant when infection
levels are severe enough to threaten yield losses or cause stalk lodging
which leads to harvest problems.
The occurrence of stalk rot
in corn and its distribution within a field or area is not always easy
to understand or explain. However, hybrid genetics combined with environmental
stresses account for its distribution within fields, among plants and
from filed to field.
Stalk Rot Development
A complex interaction between
hybrid (host), pathogen (fungi) and the environment form the "disease
triangle" influencing occurrence and distribution of stalk rot.
Hybrid (Host)
Corn produces plant sugars
by means of photosynthesis in the leaf area. Sugar, the energy source
of all cells, moves to growing points in the stems and roots during vegetative
growth, plus it accumulates in the stalk. After pollination the movement
of sugars shifts to the developing kernels. If growing conditions are
favorable, most of the sugar demand for grain development will be met
by photosynthesis in the active leaf area. Approximately 80 percent of
the sugar demand for grain development and cell maintenance in the leaf,
stalk and root is met by photosynthesis; the remaining 20 percent comes
from sugars stored in the stalk. However, if stress reduces photosynthetic
capabilities, accumulated sugars will be taken from the stalk to meet
grain-fill needs. Under stress, far more demand is placed on sugars stored
in the stalk.
Hybrids vary genetically in
their tendency toward stalk rot. Those that are most susceptible tend
to have a higher incidence of premature senescence or death of stalk and
root tissue. Healthy stalk and root tissue, which has adequate carbohydrates
available for cell maintenance, resists pathogens longer. If photosynthesis
is restricted and insufficient carbohydrates are produced, stalk and root
tissues will be weakened by cellular senescence. As the stress imbalance
for carbohydrate demand increases between grain fill and the lower stalk
and roots, the potential for stalk rot development increases.
Environment
Stresses associated with stalk
rot development are those that influence pathogen (fungi) development,
photosynthesis and the translocation of plant sugars. Any stress alone
or in combination with other stresses which reduce effective leaf area
lowers or removes photosynthetic capabilities, enhances stalk rot development.
Included among the stresses
that influence the development of stalk rot are leaf diseases (such as
gray leaf spot), extended cloudiness, high plant densities which limit
moisture or nutrient availability, insect damage (especially ECB), drought
stress, excessive moisture, low fertility, an imbalance in the potassium-nitrogen
ratio, hail and wind.
Pathogen
The fungi gibberella (Gibberella
zeae), anthracnose (colletotrichum graminicola), fusarium (Fusarium moniliforme),
diplodia (Diplodia maydis), and charcoal rot (Macrophomina phaseoli) are
the most common pathogens of stalk rot. They may occur alone, but frequently
occur in combination. These disease organisms survive from one growing
season to the next in the soil and plant residue. Each fungi is favored
by certain temperature, humidity and soil moisture conditions. Regardless
of the pathogens (fungi) present, stalk rot does not occur prematurely
unless the hybrid is adversely affected by stress or injury. They may
be able to invade root tissues or enter the stalks through small injuries
but are stopped by active healthy plant tissues.
How Stalk Rot Starts
When stress adversely affects
sugar production or the translocation of sugars, the lower stalk and roots
are weakened. Fungi can rapidly invade the plant and disrupt the ability
of vascular tissue to move water and nutrients to the upper portions of
the plant as well as the downward movement of sugars to the stalk and
roots.
Stalk rot symptoms will appear
as the roots and lower stalk deteriorate. The initial symptom of stalk
rot is a permanent wilting of leaves. When transpiration is greater than
the root's ability to supply water, wilting in the leaf area results.
Within one or two days, wilted leaves turn a grayish-green to milky color.
Ears will tend to droop with husks turning a tan to white color. The outer
rind of the lower stalk is yellow-green, turning yellow-brown within seven
to ten days. The pith tissue in the lower internodes rots separates and
pulls away from the rind. Only the vascular bundles or strands remain
intact. The lower stalk is greatly weakened and is easily crushed or collapsed.
Stalk rot symptoms usually
do not occur uniformly within a field. Rather wilted plants will be observed
adjacent to healthy ones. Plants with symptoms will have weakened or rotted
roots which are easily pulled from the soil.
Controlling Stalk Rot
It is very difficult to achieve
or expect complete control of stalk rot. However, there are selected management
or cultural factors which can be practiced to help reduce the severity
of stalk rot. Remember that timely harvest of fields where stalk rot may
be a problem can reduce or minimize risk of yield losses.
- Select and position hybrids
that are adapted for a given area. In addition to yield potential, consider
stalk and root strength, plant health, drought tolerance, insect resistance
-- especially ECB, and other yield influencing agronomic characteristics.
- Plant hybrids within their
adapted population ranges by location, soil type and productivity, available
or estimated water supply, fertility program and normal weather conditions.
- Base fertilizer applications
on realistic yield goals that consider soil type and related productivity.
- Keep nutrients in balance,
particularly potassium in relation to nitrogen.
- Scout and control insects
where feasible.
- Control weeds to eliminate
competition for light, space, moisture and nutrients.
- Improve soil conditions
(drainage, pH and organic matter) where possible. Reduce soil compaction.
- Crop rotation may help provide
some control by allowing crop residues to break down thus lowering pathogen
levels.
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