Early Season Leaf Spots And Blights Of Corn
COLUMBIA, MO.
There are several leaf spot and leaf blight
diseases which can develop on young corn
plantsanthracnose, holcus leaf spot and
Stewart’s bacterial wilt. There have been a few
questions about distinguishing between these
diseases so a review of their symptoms and disease
cycles seems appropriate.
Anthracnose leaf blight, caused by the fungus
Colletotrichum graminicola, usually occurs
early in the season on the lower leaves of young
corn plants. Anthracnose lesions tend to be
brown, oval to spindle-shaped lesions with yellow
to pinkish to reddish-brown borders. Lesions
may be 0.2 to 0.6 inch in length. Lesions
may merge or coalesce to kill larger areas of leaf
tissue. Concentric rings or zones are sometimes
apparent within the diseased areas of leaf tissue.
Lesions may be concentrated towards the
leaf tip (or portion of the leaf that was emerged
when rain occurred) giving the leaves a fired appearance
that might be mistaken for nutrient
deficiency or herbicide injury.
The fungus which causes anthracnose leaf
blight produces fruiting bodies in the dead leaf
tissue. Dark, hairlike structures called setae are
produced in association with the fruiting bodies.
It is possible to see the setae on infected
plant material in the field if a hand lens is used.
Anthracnose tends to be most common early
in the season on the lower leaves of young corn
plants. These leaves may be severely affected,
yellow and die prematurely. Generally the disease
stops at this point because of drier,
warmer weather conditions and is not considered
a significant problem. Under favorable
weather conditions, the fungus may move up
the plant causing foliage symptoms on higher
leaves. If favorable weather conditions occur
midseason (especially wet), anthracnose may
actually move up to the ear leaf. The anthracnose
fungus can also cause top dieback and
stalk rot later in the season. High temperatures
and extended periods of wet weather favor anthracnose.
Anthracnose leaf blight is more likely
to occur if corn is planted following corn.
In a normal year anthracnose leaf blight in
Missouri is not serious and would not warrant
a fungicide application. It is a little too early in
the season to know how severe anthracnose will
be or to know if it might spread beyond the very
lowest leaves on the plants. Following the
weather patterns over the next several weeks
and keeping an eye on disease development or
lack of development will be important.
Holcus leaf spot is caused by the bacterium
Pseudomonas syringae pv. syringae. Lesions are
usually oval to elliptical and range in size from
0.25 to 1.0 inch. Initially they are dark green
and watersoaked. Later they become dry and
turn light brown with a reddish margin.
The bacteria that cause holcus leaf spot are
spread by wind-driven rain or splashing rain, so
outbreaks frequently occur several days after a
rainstorm or storm with strong wind-driven
rains. Since holcus leaf spot is caused by a bacterium,
common corn fungicides will have little
effect on this disease. Holcus leaf spot might be
confused with herbicide injury such as that
caused by paraquat or other contact herbicides.
Holcus leaf spot might also be confused with
anthracnose leaf blight. Holcus leaf spot lesions
tend to be a little more oval to elliptical or even
circular in shape while anthracnose tends to be
oval to spindle-shaped or even diamondshaped.
Both types of lesions may have darker
borders but anthracnose tends to have larger
borders, lesions may coalesce to kill larger areas
of leaf tissue and discoloration surrounding the
lesions may be more extensive. Holcus leaf spot
tends to remain as discrete spots on the leaf
surface. Finally, the holcus leaf spot pathogen
does not produce fruiting bodies or the hairlike
setae which the anthracnose pathogen produces
in the dead leaf tissue of the lesions.
Checking the centers of the lesions with a hand
lens for the presence of fruiting bodies or setae
will help distinguish which pathogen is present.
On young corn plants the symptoms of Stewart’s
bacterial wilt include linear, pale green
to yellow streaks that tend to follow the veins of
leaves and originate from feeding marks of the
corn flea beetle. Lesions may extend the length
of the leaf. Plants may appear stunted or somewhat
distorted. If the bacteria become systemic
within the plant, the entire plant wilts and may
die prematurely. Cavities of a brown, soft rot
can develop in the stalk pith.
The variations in weather conditions this
spring have put stress on young corn plants. In
some fields seedlings have been showing yellowing
and/or stunting from cool, wet soils immediately
after planting and saturated soils
since planting. However, with the more recent
warm weather, corn in many parts of the state
has really taken off and is now 12 to 18 inches
tall. So symptoms of Stewart’s bacterial wilt are
beginning to develop on these rapidly growing
young corn plants.
On field corn the disease tends to be limited
to the leaf blight phase of the disease in which
foliage symptoms develop but the pathogen
does not become systemic within the plant.
With the leaf blight phase of Stewart’s bacterial
wilt, the linear, pale green to yellow lesions develop
on the leaves. These lesions tend to parallel
the leaf veins and to have wavy, irregular
margins. These streaks soon become dry and
brown.
The bacterium which causes Stewart’s bacterial
wilt overwinters in the guts of some species
of adult corn flea beetles. Adult beetles feeding
on corn seedlings in late spring and early summer
can contaminate the feeding wounds with
the causal bacterium. Flea beetles can continue
to spread the bacterium throughout the season
by feeding on infected plants and then healthy
plants. The potential for Stewart’s bacterial wilt
to develop on young corn plants is greater after
mild winters when higher levels of the corn flea
beetle may be present.
Most field corn hybrids have enough resistance
to Stewart’s bacterial wilt that additional
management is not necessary. Δ
DR. LAURA SWEETS: Extension Associate Professor,
Plant Pathology, University of Missouri