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 following guidelines:
•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 fungicides.
•Maintain proper soil fertility levels.
•Strive for proper herbicide and fertilizer placement.
•Avoid planting in low areas, or in areas with poor drainage.

Root Rots
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
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 entirely.
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 development:
•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 conditions.
•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 problem.

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 infect corn.
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 development.
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 by fungi.
 

Disease name	Lesion shape	Lesion size	Lesion color	Other Characteristics
Northern leaf blight	irregular	1" or more in length	gray with tan to red borders	very large, elongated spots
Southern leaf blight	angular	small flecks to 1½" in length	tan	minor significance
Anthracnose	elliptical	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	dark purple	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
Rough leaf	broad, elliptical	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 seed maturation.
Gray leaf spot caused by the fungus Cercospora 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 Gleocercospora 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 Ascochyta 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 continuous production.
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.

Virus
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 control.

Bacterial
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.

Head Diseases
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. The 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.

Downy Mildews
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.