Soghum Pest Control

Insect Control

Description And Damage.
The sorghum midge is, at times, a severe pest of grain sorghum in Alabama. The adult midges are tiny, fragile-looking, orange-colored gnats or flies. Damage to sorghum results from larvae that hatch from eggs deposited by the female midge on the flowering grain head. The pinkish orange larva or maggot feeds on the developing seed. Those heads damaged by midges are often referred to as "blasted".
     Blasted heads may also result from drought, disease, high temperature, or infertility. When the sorghum midge causes this damage, tiny pupal skins can often be seen attached to the empty grain seed.

Field Scouting.
Since the adult midge lays eggs only during the time that sorghum is blooming (yellow anthers exposed on the seed head), that is the critical time to scout for midges.
     A single sorghum head requires about 9 days, beginning at the tip and progressing toward the base of the head, to completely pollinate. However, not all sorghum plants within a field will uniformly head-out at the same time; therefore, a field may have heads blooming for several weeks and will require scouting throughout that time.
     Fields should be scouted daily from early to mid morning to coincide with midge activity. Begin scouting when the field has reached 20-percent bloom and continue until 90-percent bloom. Randomly select and examine at least 100 heads across a field for the presence of midges on or around the grain head. Be careful not to disturb the plant.
     The plastic bag technique can also be used. Taking a half-gallon, clear plastic bag, walk through the field and select a sorghum head to test. Quickly place the bag over the head and close the bag. Shake the sorghum head vigorously into the bag. Then, carefully lift the bag and examine the contents for midges.

When To Treat.
The sorghum midge overwinters in the "seed husks" of host plants in the maggot, or larval, stage. Host plants, in addition to grain sorghum, include sweet sorghum, johnsongrass, sudangrass, broomcorn, and other wild or uncultivated grasses. Adult emergence occurs in spring from these hosts about the time that johnsongrass begins to bloom.
     The adult midges mate, and then the female lays up to ten eggs in each head. A female may deposit 250 eggs over her 1- to 2-day lifetime.
     Midge populations increase in the spring on wild hosts and move into early plantings of grain sorghum. These early plantings are not usually sufficiently damaged to warrant control, but they do contribute to the buildup of the midge population. The increased populations are then capable of heavily damaging successive grain sorghum plantings.
     Within a local area, early uniform planting is the best way to avoid severe midge damage. Once significant populations of midges are found in grain sorghum fields, there is little chance for the later-blooming fields to escape damage unless they are treated.
     Pesticide treatments should be applied when 20 to 30 percent of the field has begun to bloom and an average of one midge per head is found. If adults are still active 3 to 5 days later, a second treatment may be needed, as well as a third treatment 3 to 5 days later if the midge population is high and the yield potential is good.
     Remember, treatments applied after pollination of the grain head will be too late to prevent any damage from the midge.

Description And Damage.
Webworms are small, hairy caterpillars which reach a maximum length of 0.5 inch. They are greenish tan with well-marked darker stripes down the length of the body. The webworms feed on the developing seeds from the milky stage until seed maturity. Usually, several (5 to 15) will be found on a head. These pests will hollow out and consume many individual seeds in a head. The caterpillars will spin a web over the grain during feeding; however, these webs may not be conspicuous.
     Webworm populations are favored by cool, wet weather during late summer and early fall. Tight or close-headed varieties will usually have more webworms than the open-headed varieties. In some fields, damage from webworms may be unevenly distributed over the field with most significant damage occurring late in the season.

Field Scouting.
After sorghum begins blooming, heads should be examined twice weekly for the presence of these caterpillars. To sample an individual head, bend it over and shake it into a bucket.

When To Treat.
Apply insecticides for webworm control when you find an average of five or more small caterpillars per head in 25 percent of the heads examined.

Description And Damage.
Fall armyworms and corn earworms are the most common large caterpillars found feeding on grain sorghum in Alabama. They may be similar in shape and size, reaching 1.5 inches in length, but they vary widely in color.
     They can usually be separated based on the color and markings of the head capsule. The head capsule of the fall armyworm is usually dark with a cream to white inverted Y on its "face" while the corn earworm has a yellow to orange head capsule without the white inverted Y.
     Corn earworms and fall armyworms feed on the foliage of grain sorghum as well as the grain. Prior to head emergence, they feed in the whorl as "budworms. " This foliage damage may appear devastating but can be tolerated well by the plants. The most serious damage by these worms is caused when they feed on the head, consuming or damaging large numbers of individual seeds.
     Late-planted or double-cropped sorghum is usually more seriously damaged by these two insects.

Field Scouting.
Until head emergence, check weekly for evidence of budworms or damage. Although treatments are seldom warranted, excessive feeding on small sorghum plants may reduce stands or delay head emergence.
     Check for corn earworms or fall armyworms in the head the same way as for webworms.

When To Treat.
Insecticide treatments are seldom justified for budworms. When 75 percent or more of sorghum plants are being damaged or an average of one worm per whorl is found, treatments may be economically feasible. However, treatments must be applied by ground equipment, directing 25 to 30 gallons spray into the whorl, to obtain effective control.
     For corn earworms or fall armyworms feeding in the head, treat when an average of one worm per head is found.

Greenbugs And Other Aphids.
These soft-bodied insects are often called "plant lice". They cause damage by sucking plant juices. They may severely damage and stunt small seedlings or cause problems at harvest by secreting "honeydew" on the grain head. This causes grain to become sticky. A black fungus may grow on the honeydew, reducing the feed quality of the grain.
     Greenbugs or other aphids seldom require insecticidal control in Alabama. Generally, lady beetles, other predators, and certain parasites control aphid populations. However, if aphid feeding results in the death of two or more leaves per plant, insecticidal treatments may become necessary.

Lesser Cornstalk Borers.
These insect pests are particularly important on late or double-cropped plantings during the hot, dry period of mid summer. Dry, sandy soil enhances the possibility of damage from these pests.
     The active larvae are blue-green with prominent transverse brown bands. They bore into the stems of the tiny seedlings and attach silken tubes covered with soil particles to the plants from which they feed.
     Under ideal conditions for the borer, heavy stand losses may be sustained. Once infestations are discovered, attempts at control are usually ineffective. Treating at planting or as plants emerge gives best results.

Chinch Bugs.
The adult chinch bug is approximately 0.125 inch long and has a black body with silver-colored wings. The immature chinch bugs resemble the adults in shape but are reddish with a white band across the back.
     The adult and immature chinch bugs suck juices from sorghum plants, causing leaf reddening, wilting, and stunting of small plants. Heavy infestations may cause significant stand losses. Large numbers of chinch bugs may migrate in the spring from wheat or other hibernation sites to invade young sorghum fields nearby.
     Chinch bugs usually congregate at the base of plants; thus, insecticidal sprays should be directed to this area for effective control. Treat only the infested areas of the field.

European Corn Borer.
The European corn borer may sometimes feed in the whorl of sorghum, but control is usually not necessary.

Sugarcane Borer.
This insect may attack sorghum in the southernmost counties of Alabama. Injury tends to occur after injury from lesser cornstalk borer. There are several generations per year. Conventional at-planting treatments for lesser cornstalk borer can exacerbate sugarcane borer problems. In fields planted near sugarcane or that have had problems in the past with this insect, preserve natural enemies by avoiding at-planting treatments. The pesticides most effective against sugarcane borer are not registered on sorghum.

Disease and Nematode Control

     Diseases of grain sorghum reduce both seed quality and yield. Losses to disease have increased in recent years. Descriptions of the diseases found on grain sorghum are listed below.

Anthracnose can be a serious problem in fields where grain sorghum has been grown continuously. This disease attacks both foliage and stalks. It forms red to maroon lesions with tan centers along the mid rib of lower leaves. As the lesions increase in size and number, affected leaves become severely blighted and die. Rapid blighting can occur during wet, humid weather. Stalk rot can be identified by the appearance of red to purple tissue on the stalk's surface and in its pith just below the seed head. Anthracnose develops most rapidly after flowering as the grain matures.

Zonate Leaf Spot is a common disease of grain sorghum that occurs primarily on the lower leaves and leaf sheaths. Spots on the leaves are circular, reddish-purple bands separated by tan-colored areas which form a concentric or zonate pattern ranging up to several inches in diameter. This disease develops mainly during wet weather. It rarely has a significant effect on grain quality or yield.

Rough Spot develops as elliptical, reddish lesions with well defined margins on the leaves. Leaf spots are rough to the touch due to hard, raised, black fruiting bodies of the causal fungus. The spots merge, killing areas of the leaf. This disease is common but is rarely a serious problem.

Gray Leaf Spot is a late-season disease of grain sorghum. Small red spots on the leaves enlarge into dark red to purple rectangular areas bordered by leaf veins which may merge to form elongated stripes. The spots produce a gray cast as the causal fungus sporulates during moist, humid weather.

Maize Dwarf MosaicVirus (MDMV) is a disease of corn and grain sorghum spread by aphids. This disease forms a mosaic of fine, yellow-green and dark green streaks that are most apparent on the younger leaves in the whorl. Red leaf, characterized by red streaks, may appear on the foliage of some sorghum varieties exposed to cool night temperatures. The disease is most common on early grain sorghum planted in johnsongrass-infested fields.

Fusarium Head Mold occurs following periods of humid, hot weather at or shortly after bloom. The fungus invades the panicles and the rachis of the seed head and then the seed stalk. Red to purple spots are found on the diseased panicles and rachis. A white to pink cotton-like growth may be seen on infected seed heads. Tissues of the infected seed stalk are a solid dark red. Diseased stalks may lodge. Swine are highly sensitive to toxins produced by causal fungus on diseased grain.

Charcoal Rot develops during times of severe drought. Plants are infected at the soil line as the grain begins to mature. Soft tissues of the pith quickly disintegrate, leaving only the string-like vascular bundles. Extremely small black fungal bodies cover the vascular bundles, giving the pith its characteristic salt and pepper appearance. Diseased plants usually lodge.

Nematodes feed on roots of grain sorghum. Plants attacked by nematodes are chlorotic, slow growing, and stunted. Symptoms resemble those of nutritional or moisture stress. Roots damaged by nematodes are thickened, stubby, and discolored. Nematodes can be accurately diagnosed only by nematode analysis of soil samples from the affected area. See Circular ANR-114, "Collecting Soil And Root Samples For Nematode Analysis," for further information on sampling procedures.

This information was adapted from publication ANR-500A, 2002 Alabama Pest Management Handbook, chapter "Grain Sorghum Insect, Disease, Nematode, And Weed Control Recommendations".

General Control Practices

Crop Rotation.
Rotation is an effective practice for controlling anthracnose, head blight, and other foliar diseases of grain sorghum. After two consecutive grain sorghum crops, rotate to another field or forage crop for 2 to 3 years. Do not include sorghum-sudangrass crosses, sweet sorghum, or corn in rotation with grain sorghum.

Disease-Resistant Hybrids.
Resistance is one of the most economical and effective control measures for grain sorghum diseases. Using anthracnose- and/or virus-resistant hybrids is strongly recommended where these diseases are a problem.

Weed Control.
Johnsongrass is a reservoir for viral and fungal diseases of grain sorghum. Avoid planting grain sorghum in fields heavily infested with this weed. In lightly infested fields, suppress johnsongrass with herbicides and use a disease-resistant hybrid.

Tillage Practices.
Disease-causing fungi survive on crop debris. Deep turning sorghum crop residues can help control several foliar diseases of grain sorghum.

Plant Population.
High plant populations greatly reduce drought tolerance of grain sorghum and predispose plants to foliar and seed-head diseases such as anthracnose and Fusarium head mold. Current recommendations call for 40,000 to 80,000 plants per acre for dryland grain sorghum.

Fertility Practices.
Maintain recommended fertility levels and soil pH to promote rapid root development and to avoid some charcoal rot damage.

Planting Date.
Delay planting until soil temperatures reach 65°F at a 2-inch depth to prevent seed rots and seedling disease.

Seed Treatments.
Be sure that all seed is treated with a fungicide seed treatment for control of seed rots and seedling disease.

Nematicides.
Base your use of nematicides on the results of a nematode analysis of your soil samples. Nematicides are not economical treatments on grain sorghum.

Soghum Management Checklist

  The grain sorghum producers who get maximum returns from their investments pay special attention to certain key management practices. This grain sorghum checklist will improve your sorghum management system. If you can't mark off each of these points for your own farm, you may be missing out on potential income.

• Soil test for fertility and follow recommendations. Sample each field in the fall for fertility and lime needs. Liming soils to a pH above 6.0 helps produce healthy, uniform crop stands and high yields. Apply phosphorus and potassium according to recommendations. Generally, apply about 80 pounds of nitrogen per acre either at planting or as a split application with the second application applied before sorghum is 8 inches tall.
• Prepare a good seedbed. Mix previous crop residues into the top 2 to 4 inches of soil well ahead of planting. This reduces disease and soil insect pressure. Take care to avoid forming a "cloddy" seedbed. Consider in-row subsoiling if you have a hardpan or traffic pan in a field.
• Use a preemergence herbicide to control grass weeds. In fields with a history of grass weed problems, use a preemergence herbicide at the labeled rate for the soil type. This is the best opportunity to control grass weeds in this crop. You must use seed treated with herbicide "safeners" in areas where a preemergence herbicide is applied.
• Plant early when soil temperature is correct for optimum yields. Planting as early as possible usually gives best yields. As a general rule, plant grain sorghum as soon as the soil temperature at 2 inches warms to 65°F. Early planting usually allows for better growing conditions and good moisture, and the crop usually escapes most insect pressure and can be harvested sooner.
• Plant recommended varieties. Study the results of variety tests conducted by Auburn University and select varieties suited to your situation. Important factors to consider when selecting a hybrid are: yielding ability; susceptibility to lodging; maturity; head exsertion; head compactness; and damage from birds, insects, and diseases.
• Avoid high-density stands. Many problems are associated with excessive plant populations, such as increased disease problems, reduced drought tolerance, and lower yields. A plant population of approximately 60,000 plants per acre is most desirable. This will usually require about 5 pounds of seed per acre, but the weight of seed planted is not a good measure of plant population because seed size varies considerably with various hybrids. Carefully calibrate your planter to deliver the correct number of seeds per foot of row. For 30-inch rows, six seeds per foot of row will be adequate.
• Use residual postemergence herbicides to control problem broadleaf weeds. Apply postemergence residual herbicides over-the-top of grain sorghum when it is at least 3 inches tall and weeds are small. Weeds such as sicklepod, cocklebur, and morningglory can be controlled by the timely application of a herbicide such as atrazine. Follow label directions for the proper use rate based on soil type.
• Use post-directed herbicide if needed. Herbicides available for post-directed spray application are more effective on a wide range of weeds and relatively inexpensive. Grain sorghum must be at least 12 inches tall at the time of treatment, and only the lower 3 inches of the sorghum stem should be contacted by the spray. Directed sprays can give good burndown of small weeds and some grasses.
• Base insect management decisions on thorough field scouting. Scout fields and treat only where an economically damaging level of insects has been reached. General guides to economic treatment levels have been established for insects such as sorghum midge, corn earworms, and armyworms. Remember, good scouting is required in order to match the recommended insecticide to damaging insect(s).
• Apply insecticides, herbicides, and fungicides only as labeled and recommended. Calibrate sprayers and follow the recommended methods. A misapplication is costly; it results in a waste of expensive chemicals and/or damage to the crop.
• Maintain a field-by-field record or map of weed problems. In the late summer before harvest, prepare a field record or map of each field. Include a list of weeds present with their general location in the field, and estimate the size or magnitude of the different problem weeds present. Use these maps or records to plan for the next year's weed control program.
• Harvest when crop is ready. Start harvesting grain sorghum when the heads are mature and seed approach an average of 20- to 22-percent moisture. Drying sorghum seed will be necessary to reach a moisture percentage of 12 to 14 percent. Field drying of grain sorghum often results in significant harvesting losses, crop shattering, and increased disease problems.
• Develop marketing strategies. Consider contracts or other marketing methods for handling your crop well ahead of harvesttime. Don't get caught by the low cash prices available at harvesttime.