Anthracnose (fungus – Colletotrichum dematium var. truncatum): The fungus infects stems, petioles and pods of plants nearing maturity. Dark brown or reddish-brown areas may cover the surface of infected stems and pods. Lower branches die. Later, infected areas may become covered with the black fruiting bodies of the fungus. Seed in infected pods may be shriveled and moldy or show no external sign of the disease. The fungus is carried over on the seed. Germinating seed may be killed before they produce a seedling. Dark brown sunken cankers develop on the cotyledons of young seedlings. The fungus also overwinters on infected plant residue. Use of disease-free seed, crop rotation and burial of crop residue reduce disease incidence. Seed treatment improves stand, but will not eliminate the fungus. Foliar fungicides are recommended for control.
Brown Leaf Spot (fungus – Septoria glycines): This is the earliest foliar disease to appear on soybeans in the spring. Angular reddish-brown spots that vary in size from a pinpoint to 1/5 inch may appear on the lower leaves. Infected leaves turn yellow and fall prematurely. In severely infected fields, the lower half of the plant may lose all its leaves. The extent of defoliation depends on weather conditions following initial infection. Stems and young pods may also become infected. The disease may be seed-borne. The fungus overwinters on diseased stems and leaves. Warm, moist weather and poor drainage favor the spread of the disease. Control measures include use of disease-free seed, crop rotation, deep burial of crop residue and use of foliar fungicides.
Downy Mildew (fungus – Peronospora manshurica): First symptoms appear as indefinite yellowish-green areas on the upper leaf surface. Later, these areas become light to dark brown spots with yellow-green margins. Grayish downy tufts of mold growth appear on the lower surface. Severely infected leaves may defoliate prematurely. The fungus grows within pods covering the seed with a white crust of spores. The disease is spread with infected seed and is carried over on plant debris. Practice crop rotation, use disease-free seed, and plow under plant residue.
Purple Seed Stain (fungus – Cercospora kikuchii): A pink or light to dark purple discoloration of the mature seed coat provides easy identification. Size of the discoloration may vary from a small spot to the entire seed surface. Affected seed may be cracked, rough and dull. Seed quality is lowered. The causal organism attacks other plant parts and overwinters in diseased leaves and stems as well as in infected seed. Premature defoliation may occur when leaves are severely infected. When infected seeds are planted, the fungus grows from seed coats and infects seedlings. This serves as a primary source of inoculum. Wet weather during the growing season favors development of the disease. The fungus overwinters in diseased crop residue as well as on infested seed. Crop rotation, use of disease-free seed and burial of crop residue aids in holding the disease in check. Foliar fungicides are effective for controlling this disease.
Seed Decay and Seedling Diseases (fungi – Pythium sp., Phytophthora sp., Rhizoctonia sp., Diaporthe sp. are the most common): Poor seed quality, whether due to physical, physiological or pathological causes, predisposes seedlings to disease organisms. Poor seed quality is a major problem confronting soybean producers. Mechanical injury can rupture the seed coat allowing penetration of various pathogenic organisms or actually injury the embryo itself. Seed quality can be lowered by improper storage conditions, which include too high or too low moisture or temperatures. Both seed and soil-borne organisms can cause seed decay and seedling diseases. Seedlings infected with Pythium or Phytophthora usually develop a watery rot on roots and lower stems. Rhizoctonia causes a dry rot with reddish-brown lesions, typical of “sore-shin”. Cool, wet soil conditions enhance seedling diseases. In poorly drained soils, plant soybeans on a slightly raised bed when temperatures have warmed up enough for rapid germination. Plant only high quality soybean seed if available. If planting seed germination is below 80 percent, use a seed treatment fungicide to increase germination.
Target Spot (fungus – Alternaria sp.): Infection primarily on leaves, but may also occur on pods and stems. Spots on leaves are reddish-brown, circular in shape and vary in size from a pinpoint to more than 1/2 inch in diameter. Large spots are composed of concentric rings. The fungus is generally considered a weak parasite which attacks plants too late in the season to cause serious damage.
Bacterial Blight (bacterium – Pseudomonas syringae pv. glycinea): Usually, one of the first diseases to appear on young plants. Small, angular spots varying from yellow to dark brown develop on leaves of infected plants. The brown area is often surrounded by a water-soaked margin. Spots later dry and portions of the leaf may drop out. Leaves may have a torn, ragged appearance. The disease may occur on stems and pods. Cool, wet weather favors development of the disease. This disease is seed-borne and the bacteria may overwinter in crop residue. Plant disease-free seed. Bury crop residue and rotate with non-susceptible crops.
Charcoal Rot (fungus – Macrophomina phaseolina): Charcoal rot is usually found in mid-summer, mostly on sandy soils. This fungus is a weak pathogen and generally attacks young plants when their growth is retarded by drought. The fungus attacks the roots and lower stem. When the bark is peeled from the roots and stem base, tiny, black fungal bodies may be seen almost as if pepper had been sprinked on the tissue. The root and base of the stem exhibit streaks in the woody portion when split open. (See Charcoal Rot for additional information.)
Frogeye Leafspot (fungus – Cercospora sojina): This disease usually appears late in the growing season. The fungus infects leaves, stems and pods, but is most conspicuous on the leaf. On the leaf, it causes an “eyespot” lesion composed of a gray or tan central area surrounded by a narrow reddish border. Badly infected leaves fall prematurely. The fungus is seed-borne and causes weak seedlings. Control measures include planting disease-free seed, plowing under crop residue and rotation. Varieties vary in their resistance.
Rhizobium – Induced Chlorosis: A chlorosis or yellowing ranging from light green to nearly white may occur about 6-8 weeks after planting when the plants are making rapid growth. This chlorosis is more apparent in the top 2-3 leaves and is caused by lack of nitrogen. This condition is caused by an insufficient amount of nodule-forming bacteria. The chlorosis is usually temporary and plants appear normal by flowering. There is usually no yield reduction. Correct application of inoculum will prevent this situation.
Southern Blight (fungus – Sclerotium rolfsii): Occurrence of southern blight in a field is erratic and generally only individual plants are affected. However, in some instances large numbers of plants may be killed. Southern blight produces a rot of the stem at and below the soil line. Plants may be affected at any stage of growth. The first symptom is sudden wilting and subsequent death. The fungus produces a whitish fungal growth on the stem base. Sclerotia, fungal resting bodies about the size of mustard seed, are formed. The sclerotia are the resting stage of the fungus and will persist in the soil for years. The fungus occurs widely in many soils, and is capable of persisting on almost any type of organic matter.
Bud Blight(virus – tobacco ringspot virus): The most serious virus disease of soybeans. The first symptoms occur on young plants; a curling and browning of the terminal bud forms a crook. The bud becomes dry and brittle, and the leaf immediately below it shows a flecking of rusty brown specks. Later buds die and the plant is stunted and produces no pods or small, underdeveloped ones. These plants are known as “duds” and are easily recognized in the fall because they remain green after normal plants have matured. Plants infected later in the season may produce poorly filled pods that drop prematurely or they may be covered with purple blotches and remain on the plant. The disease usually appears first at the edge of a field and progresses inward, suggesting an insect vector. These is no known control measure.
Soybean Mosaic (virus): The most common virus disease occurring in Texas. Leaves of infected plants show a yellow vein clearing that develops in the small, branching veins of developing leaves. Infected leaves are narrower than normal with margins turned down. Margins later become ruffled and blister-like puckering occurs along the veins. Leaves become leathery in appearance, coarse and brittle. Symptoms become masked under higher summer temperatures. Infected plants produce misshapen pods and fewer seed than normal. The virus is carried in the seed and is transmitted by aphids. No control is known.
Bean Pod Mottle Virus (virus): Symptoms first occur in the primary leaf stage. The virus causes a yellow-green mottling that may fade and reoccur later in the growing season. Cool weather enhances disease development. The virus is transmitted by the bean leaf beetle. No control is known.
Bacterial Pustule (bacterium – Xanthomonas campestris pv. phaseoli): Primarily a disease of leaves, although it may infect pods. First symptoms are small, yellow-green spots with reddish-brown centers on the upper leaf surface. The central portion of each spot appears slightly raised and develops into a small pustule, especially on the underside of the leaf. Several infections on the same leaf produce a large, yellow to brown area with small, dark brown spots. The brown, dead areas on older leaves may break up and cause a ragged appearance. In later stages, the pustules rupture and dry. When rupturing and drying occur, it may be difficult to distinguish this disease from bacterial blight. Severe infections cause defoliation. The disease overwinters in infected plant debris and is carried over to some extent on infested seed. Crop rotation, resistant varieties and burial of crop residues are the most effective methods of disease control.
Cotton Root Rot (fungus – Phymatotrichum omnivorum): Plants die suddenly during the summer. Affected plants are easily pulled from the soil and have buff colored fungal strands on the lower stems and roots.
Pod and Stem Blight (fungus – Diaporthe phaseolorum var. sojae): The pod and stem blight fungus attacks and kills older plants nearing maturity. The disease is identified by the numerous small, black fruiting bodies (pycnidia) appearing on stems and pods of infected plants. The pycnidia are arranged in linear rows on the stems and scattered on the pods. The fungus infects seed and causes them to be shriveled, moldy and smaller than normal. Seed may be infected, but appear normal. Seed infection is the most serious phase of the disease. When infected seed are planted, the embryo is often killed before emergence or the seedlings are killed at an early stage. Delayed harvest results in an increased incidence of the disease, especially if rain or humid weather and warm conditions prevail. Prompt harvesting when weather conditions permit and foliar fungicides are the most effective means of controlling this disease.
Root-Knot Nematode (Meloidogyne spp.) Root-knot nematode infested plants are often stunted and have galls or knots on the roots. Plants may exhibit nutrient deficiencies and wilt during the heat of the day. These nematodes are more of a problem in sandy soil and seldom are damaging in heavy clay soils. Crop rotation and varietal resistance are effective in reducing losses to root-knot nematode.
Soybean Cyst Nematode Symptoms on Soybean (Heterodera glycines) The soybean cyst nematode (SCN) is the most serious pest of soybeans in the United States. To date, SCN has been observed in the state only in Northeast Texas (Bowie and Red River counties) and on the High Plains (Hale, Floyd and Swisher counties).
Symptoms of SCN can be easily confused with nutrient deficiencies, herbicide injury and other disorders. Plants are typically stunted in patches within an affected field. The degree of damage depends upon the initial numbers of nematodes present, the degree to which the environment favors reproduction within a season, and the susceptibility of the soybean variety grown. The female nematodes can often be seen as tiny white to yellow, pinhead size specks on the surface of infested soybean roots. Damage to soybeans from SCN can be managed using resistant varieties, nematicides, and rotation. If SCN is suspected, a soil sample collected from the damaged area should be submitted to the Plant Nematode Diagnostic Laboratory.
Stem Canker (fungus – Diaporthe phaseolorum var. caulivora): In Texas, this disease was first discovered in 1984. The disease is seed transmitted, but once it becomes established in a field, the fungus survives in infected residue (primarily undecomposed stems). Spores are released from the residue in late spring/early summer and infect young vegetatively growing plants (V-4 stage or about 10-12″ in height). The infection moves down the petiole and establishes a canker on the lower stem usually on the 2nd, 3rd, or 4th node of the plant. As the plant enters the reproductive stage (R4-R6), the canker enlarges and girdles the stem. This prevents pod-filling, may result in premature plant death, and yield loss on susceptible varieties can exceed 90%. The first indication of stem canker from a distance is the appearance of yellowing leaves (R6-R7) showing an interveinal chlorosis. Plants exhibiting this symptom will often show a slightly sunken, brown area on the stem or base of a branch or petiole. The disease is usually scattered throughout a field when the field first becomes affected but in subsequent seasons, 80-90% of the plants may show symptoms when a susceptible variety is planted.
Recommended control measures include the use of resistant varieties in fields where stem canker has become established. Crop rotation, delayed planting and plowing under crop residue may also reduce disease severity when planting moderately resistant or moderately susceptible varieties.