DISEASES OF GRAIN SORGHUM
P. Vincelli and D.E. Hershman
Like all crops, grain sorghum is subject
to infectious diseases which can sometimes limit production. Fortunately,
most grain sorghum diseases can be controlled. However, effective control
measures can only be implemented if the disease is first properly identified.
The following will aid in the identification and control of the most common
grain sorghum diseases in Kentucky.
Seed and Seedling Diseases
The death of grain sorghum in the first
weeks after planting is a problem in all Kentucky's sorghum producing areas.
Seed may be attacked by one or more seedborne or soilborne pathogens prior
to either germination or emergence. This usually occurs when conditions
are not optimum for plant development, such as in poorly-drained, cold,
wet soils, or in very dry, crusted soils. Ironically, these same conditions
are generally favorable both for pathogen activity and for disease development.
Sorghum seedlings are very delicate
during the emergence period and are slow to establish a permanent root
system. As a result, sorghum depends upon its primary, temporary root system
for a period longer than many other crops. Under less than optimal growing
conditions, this primary root system is extremely vulnerable to soilborne
pathogens. This is when damping-off in a field is most visible and damaging.
Look for poorly-growing, unthrifty
plants which may show yellowing, wilting and death of leaves. Carefully
dig some of these plants and look for discoloration and rotting of the
roots and extreme lower stem tissue. The deteriorating tissue may appear
anywhere from whitish-gray in color to pink to dark brown. In some instances,
especially after emergence in cooler soils, plants may show a purplish
coloration of the leaves. This problem is generally nutritional in nature
and is caused by a temporary phosphorus deficiency, rather than attack
by a soil pathogen.
Both seed and seedling diseases can
greatly reduce stand densities in the field. This failure to get good stands
is frequently the reason for excessively high planting rates in subsequent
seasons. High planting rates may also cause additional stress and stress-related
diseases at later stages of growth.
To control seed and seedling diseases of grain sorghum, observe the
•Use injury-free, high quality seed.
•Plant seed when soil is warm (above
65 F) and where moisture is not limiting.
•Use seeding rates and row spacing
best suited to the sorghum hybrid selected.
•Consider the use of seed protectant
•Maintain proper soil fertility levels.
•Strive for proper herbicide and fertilizer
•Avoid planting in low areas, or in
areas with poor drainage.
Grain sorghum root rot can be a considerable
problem in sorghum production. The extent of damage to the crop is largely
influenced by soil and environmental factors. Adverse growing conditions
such as excessive dryness, or cool, poorly-drained, infertile soils, tend
to encourage root rot. Periconia circinata and species of Pythium,
Rhizoctonia, and Fusarium are the fungi most frequently associated
with rotted sorghum roots in Kentucky.
The extent of damage from root rot
depends upon a soil pathogen's ability to attack sorghum roots and the
plant's ability to produce replacement roots. Generally, the plant's fine
feeder roots are the first to be attacked and destroyed. At this point,
vigorously-growing plants are quick to respond by replacing the diseased
roots with new ones. Plants growing under stressful conditions, however,
remain weak and are unable to produce new roots. Eventually, if enough
roots are killed, the plant's growth will be retarded or plants may die.
Control of grain sorghum root rot is
similar to those listed for seed and seedling diseases. Sound management
practices which stimulate plant vigor and development are important.
Stalk rots are common in Kentucky and
generally follow root rot or certain combinations of specific environmental
and plant developmental sequences which predispose plants to be attacked
by stalk rotting organisms. Stalk rots can greatly reduce yield either
directly through plant lodging, or indirectly through reduced head and
grain fill. Charcoal rot and stalk red rot are the most frequently encountered
stalk rot disease of sorghum in Kentucky.
Charcoal rot is incited by a soilborne
fungus, Macrophomina phaseolina, present wherever sorghum is grown.
The same organism causes a similar disease in corn and soybean. The disease
usually occurs during seed development as the crop is subjected to low
soil moisture and high temperatures. As a result, the disease is more widespread
in some years and localities than in others.
Because water stress is important for
disease development, the first plants affected by charcoal rot will be
those growing in soils that do not retain water well. In addition, prebloom
rainfall, tillage method, cropping sequence, plant spacing and soil fertility
also influence patterns of charcoal rot in the field. Under optimal conditions,
severe damage in a field can occur in a few days. If water stress conditions
are interrupted, however, the disease process may be retarded or stopped
Splitting the stalks of suspect plants
lengthwise easily identifies charcoal rot. If the disease is present, the
stalk's interior will be shredded, and very small, dark, fungal bodies
will be visible. These fungal structures give the split stalk a "peppered"
appearance and are what give the diseased tissue a charcoal color and the
disease its name.
The most practical way to control charcoal
rot is to grow grain sorghum varieties which are tolerant to charcoal rot
and/or predisposing stress factors. No resistant varieties are available.
The use of proper management practices will also help moderate disease
•Maintain balanced soil fertility;
avoid high levels of nitrogen and low levels of potassium.
•Avoid excessive plant populations;
space plants as far apart as is practical.
•If practical, irrigate during dry
periods after heading. This will help to eliminate predisposing water stress
•Rotate out of sorghum, corn, or soybeans
for two years. Sowing small grains and/or forages may reduce disease severity
when sorghum is once again grown. However, rotation will not eliminate
The other important stalk rot disease,
stalk red rot, caused by Colletotrichum graminicola, has two additional
disease phases. These are peduncle breakage and anthracnose, the latter
affecting the sorghum's leaves and head. All three phases may occur simultaneously,
or only one or two may be present. The fungus also attacks certain grasses.
Different strains of Colletotrichum infect sorghum than those that
On susceptible varieties, infected
peduncles and stalks develop brown sunken lesions with a distinct border.
When affected tissue is cut lengthwise, the internal portions will be soft
and will show a brick red discoloration. Diseased stalks frequently break
near the middle, while peduncles break just below the seed head. Severe
breakage and lodging can result in near total losses in highly susceptible
varieties. Diseased but unbroken stalks and peduncles usually produce small
heads, sometimes with abnormally small grain.
This disease is most prevalent when
daily periods of high humidity alternate with periods of dryness. Prolonged
periods of either high humidity or excessive dryness tend to check disease
The stalk red rot phase primarily attacks
stalks at any time after jointing. Infection occurs when spores of the
fungus, produced on the leaves and heads in the anthracnose phase, are
washed downward and infect stalk tissue. Peduncle breakage, on the other
hand, is not dependent upon this method of infection, and can occur without
presence of the anthracnose or stalk red rot disease development.
To control the stalk red rot, anthracnose
and peduncle breakage phases of the disease, use resistant varieties and
rotate to other crops when possible. Since the fungus survives in susceptible
weeds, it is best to maintain a good weed control program. Plowing to bury
infested crop residue can also be helpful, especially if crop rotation
is not practiced. The use of clean seed will also help to minimize the
incidence of disease.
Foliar Diseases Caused by Fungi
Numerous fungi can attack the foliage
of grain sorghum in Kentucky. Severe damage caused by these fungi is most
likely to occur under periods of extended humidity. Fortunately, grain
sorghum can sustain considerable damage to its foliage without incurring
significant yield losses. However, yield loss can occur if damage occurs
to the upper leaves of plants at the time the grain is filling. During
most years, disease development occurs from the whorl stage through maturity.
Table 1. Symptoms of common grain sorghum foliar diseases caused
|Northern leaf blight
||1" or more in length
||gray with tan to red borders
||very large, elongated spots
|Southern leaf blight
||small flecks to 1½" in length
||1/8" to 7/8" in length
||tan to red with distinct margins
||spore masses common in lesions
|Gray leaf spot
||elongate to round
||¼" in length or larger
||grayish when spores are being produced
|Zonate leaf spot
||irregular to semicircular
||patches of lesions run together
||alternating bands of dark and light tissue
||resembles a bull's eye target
||¼" by ½"
||grayish to yellow or purple
||rough to the touch
Symptoms of some of the common fungal
leaf diseases of sorghum in Kentucky are contrasted in Table 1. Each of
these has the potential to adversely affect sorghum production. Under normal
years, however, they play only a minor role in limiting sorghum yields.
Helminthosporium leaf blights
(i.e. northern leaf blight and southern leaf blight) are prominent on both
corn and sorghum in the humid regions of the United States. Both diseases
are much more damaging to corn than to grain sorghum; however, under proper
environmental conditions, they can severely damage certain sorghum varieties.
Generally, southern leaf blight is not as damaging to grain sorghum as
is northern leaf blight. The latter disease is known to predispose plants
to stalk rotting pathogens.
Anthracnose, the foliar phase of stalk
red rot, is not as much of a problem as it has been in previous years because
of the use of resistant varieties. On susceptible varieties, however, this
disease can be one of the most destructive diseases of grain sorghum in
Kentucky. It can result in total leaf destruction, usually beginning at
the jointing stage. The disease can rapidly progress up the plant so that
the foliage is completely destroyed by flowering time. On less susceptible
varieties, the time of total leaf destruction may be delayed until after
Gray leaf spot caused by the fungus
sorghi is a conspicuous, yet relatively unimportant, disease of grain
sorghum in Kentucky. Disease usually does not occur until late in the growing
season, after the crop is mature; as a result, yield losses are minimal.
If abnormally cool, humid weather and overcast days persist at midseason,
however, substantial damage may occur. Although all grain sorghum varieties
are susceptible to gray leaf spot, some varieties tolerate the disease
better than others.
Zonate leaf spot, caused by the fungus
sorghi is common on sorghum in all humid regions of the south. Severe
disease on sorghum seedlings may result in defoliation and even death of
affected plants. Abundant spotting on leaves of older plants may result
in poorly filled grain as a result of foliage destruction. Currently, there
are no varieties with a high level of resistance to zonate leaf spot.
Rough leaf spot, incited by the fungus
sorghina, is found only where sorghum is grown under very humid conditions.
Although losses due to this disease are generally minor, significant losses
can occur during extremely rainy periods in fields where sorghum is in
Generally, the best way to control
foliar diseases of grain sorghum is through a combination of resistant
varieties, crop rotation, and control of weeds which may serve as a source
of inoculum. Plowing of infested crop residue can also be helpful, especially
if crop rotation is not practiced.
The most common and only known important
sorghum virus disease in Kentucky is maize dwarf mosaic (MDM).
MDM is a widespread disease of corn
and sorghum in the United States. The causal virus readily infects johnsongrass
and numerous other grasses. Perennial weeds also serve as overwintering
hosts for the virus. In nature, the causal virus is transmitted from the
weed hosts to grain sorghum through the feeding activities of several species
of aphids. Presently, the corn leaf aphid and the greenbug (also an aphid)
are thought to be the most important vectors.
These vectors feed on infected weed
hosts in the spring and acquire the virus. Plants produced by johnsongrass
rhizomes are a common source of virus. The aphids then move to nearby crop
plants and transmit the virus to them. If weed hosts are controlled in
fields, but are populous along field perimeters, then border plants will
often be the first infected. Later, buildup and dispersal of aphids both
within and between fields results in extensive spread of the disease. Poor
weed control in fields, on the other hand, tends to foster a more random
distribution of early-infected plants; a border effect is seldom seen.
Infection and symptom expression of
MDM may occur at any time during the growing season; however, symptoms
of MDM generally first appear six to eight weeks after planting. At this
time, a distinct yellowing and mottling of leaves, especially young leaves,
will be noticeable. In late July to early August, the leaves will begin
to develop a reddish coloration and elongated stripes with necrotic centers
and reddish margins. In extreme cases, plants may die. Also, growth may
be stunted, flowering delayed, and/or plants may fail to head or set seed.
Generally, plants infected as seedlings will be much more severely affected
than plants infected at later stages of development.
MDM can be controlled by growing tolerant
sorghum varieties and by maintaining good johnsongrass control. Spraying
insecticides to reduce aphid populations is not an effective means of MDM
Bacterial stripe is the only prevalent
bacterial disease of sorghum !n Kentucky. Though conspicuous on plants
because of its striking symptoms, the disease generally does little damage
compared to the amount of leaf area affected.
Bacterial stripe produces long, narrow,
tan-brick-red to dark-purplish-red lesions, which first appear watersoaked
and then develop into dry strips of dead tissue. When viewed under proper
lighting, these dead areas appear to have a reflective glaze.
Bacterial stripe is most prevalent
from about mid-season on, especially during warm, humid weather. The causal
bacteria are carried over from season to season on seed, plant debris in
the soil, and on johnsongrass.
Control of bacterial stripe consists
of using clean seed, rotation to a non-host crop, and good weed control
measures. Plowing to bury infested crop residue can also be helpful, especially
if crop rotation is not practiced.
Certain head diseases of grain sorghum
can result in severe production problems in Kentucky. These diseases cause
a reduction of head and grain fill through direct attack of inflorescences
by disease organisms. Grain and silage quality can also be adversely affected.
Although a number of organisms can
cause head diseases in sorghum, the main diseases in Kentucky are Fusarium
head blight, the smuts and anthracnose. The latter disease is the head
and foliar phase of stalk red rot and has already been discussed. Other
"head molds" caused by species of Alternaria, Curvularia, and Cladosporium
do not usually result in yield reductions. They can, however, provide inoculum
for storage infections.
Fusarium head blight is characterized
by death of several to all of the florets in seed heads. When the disease
is severe, the entire seed head may be covered with cream to pink fungal
growth. This may be followed by infection of the peduncle, which may result
in a weak neck and stalk lodging. When this occurs, severe yield losses
can be sustained. Significant yield loss may also occur because of smaller
and lighter grain in the infected seed heads.
The head blight fungus is widely distributed
in nature and is capable of infecting sorghum heads at and soon after bloom.
disease is more likely to occur when high moisture conditions are present
near harvest time and when normal harvest is delayed.
Control of Fusarium head blight
is best achieved by timely harvesting of grain at appropriate moisture
levels. Attempts to develop resistant hybrids are being conducted, but
are in the early stages. Generally, however, sorghums with dense, compact
heads are more prone to attack than are varieties with loose, open heads.
The three smut diseases which affect
grain sorghum heads in Kentucky are covered kernel smut, loose kernel smut
and head smut. Of these three, only head smut is of economic importance.
Head smut becomes visible at heading when large galls appear in place of
the panicle and no grain is produced. The entire head then turns into a
mass of dark brown, powdery spores. The gall is first covered with a whitish
membrane, which rapidly breaks apart and allows the spores to scatter by
wind and rain to the soil.
Smut diseases are not as serious as
they once were because control measures are available. Covered smut and
loose kernel smut are effectively controlled by seed treatment with fungicides.
Control of head smut is primarily through the use of resistant varieties.
Sorghum downy mildew and crazy top
diseases, although not major problems in Kentucky, may be seen occasionally.
Plants affected by sorghum downy mildew
have light, yellow-green, striped leaves with a "downy" fungal growth visible
on the undersurface of the leaf. As the season progresses, infected leaves
become shredded and appear as though they have been damaged by hail. Affected
plants may be completely or partially sterile. Control of sorghum downy
mildew is through the use of resistant varieties. Crop rotations of two
or more years with wheat, soybeans, or a forage crop will also help. The
sorghum downy mildew fungus also infects corn, so rotation to corn is not
recommended for fields having problems with this disease.
Plants affected by crazy top have thickened,
twisted, puckered leaves with rough bumps and ridges along the leaf surface.
This, in addition to typical excessive tillering, gives plants a bunched
appearance. Infected plants usually survive, but do not produce grain.
Because high soil moisture levels and flooding are required for disease
development, disease will always be most noticeable in fields where overflow
occurs, or in low spots where water stands.
To control crazy top, provide adequate
field drainage and use tolerant varieties. Crop rotation is not useful
because the fungus infects many weed hosts.