Soybean Diseases

Phytophthora Root Rot

  • Caused by a soil borne fungus, many different races, wet field conditions favor the onset of this, 3 different stages of impact—seed rot, seedling blight, and root or stem rot, can attack beans at any time during the season, favored by high potash applications, survives in the soil and the bean residues.
  • Impact: PRR can kill bean plants at any stage of growth, causes stand reduction and yield loss, plants will wilt and become brown and die off, seed infection causes seeds to be brown and mushy. Seedling blight is considered damping off with rapid decay of plant health, brown or black discolorations appear on the stem near the soil line, root and stem rot stage causes a brown discolored taproot, minimal nodulation, stunted and uneven plant stands, dark brown or reddish lesions progressing up the stem and tissues inside the stem will be brown and diseased.
  • Management: Look for varieties with stronger resistance on field tolerance, improve field drainage or plant in fields less prone to water logging, planting early may help in heavy soils or no till conditions.

White Mold

  • Fungus that can live in the soil for up to 10 years, favored by cool wet conditions, infection occurs at a water soaked lesion at a node that remains water soaked, disease can quickly spread by contact with other plants.
  • Impact: Yield loss, lodging, diminished seed quality, seeds will be shriveled and black.
  • Management: Select varieties that have a greater resistance since there is no absolute resistance to it, control weeds in the field that may be a host for the fungus, spraying fungicide has shown to reduce the severity of white mold. Some may find disadvantage in planting early and planting in narrow rows as it promotes reduced aeration through canopy and quick canopy closure all of which favor white mold. Rotation is important but due to the
    longevity of it in the soil, it is hard to rotate and completely take care of the disease.

Sudden Death Syndrome

  • Blue coloration on the outer surface of the roots may occur, the fungal colonies only take habitat at the crown of the plant, splitting the root will show a white pith but the outer layers will be a grayish/brown color, leaves will appear to have yellow spots- appearing on the upper canopy first, these will form chlorotic blotches between the leaf veins which will then turn brown and die, leaves will twist and curl and begin to fall off the plant as they die, flowers and pods will be aborted and those that remain will have small seeds, symptoms are very similar to that of brown stem rot—you really need to split the stems, cool and moist conditions favor this disease, especially heavily compacted areas of a field and if SCN is present the disease will cause a heavier impact.
  • Impact: Soybean leaves will die off and pods will abort, roots begin to deteriorate which reduces water and nutrient uptake. Yield and stand will be depleted.
  • Management: The fungus remains in the soil and survives on residues, it can enter the plant relatively early in the growing season but may not show effect until midseason, look at planting resistant varieties, reduce compaction in the field and other stresses to the crop, foliar fungicides will not protect the crop

Brown Stem Rot  

  • Split the stem of the plant, the pith will be brown and the surrounding tissues will remain green (opposite of SDS), commonly found between nodes and closer to the soil line. Infection occurs early in the season but the impact isn’t seen until later during reproduction. High temperatures inhibit the spread of BSR. Low pH favors the disease, moist soils also favor this disease due to restriction of nutrients leaves will become chlorotic, which causes yellow patches between leaf veins and browning of the edges until they fully become brown die off. Stems will also turn brown.
  • Impact: Infects roots and impacts uptake of nutrients and water, causing premature plant death and yield loss.
  • Management: Fungus survives in soybean residues. Soybeans are the only known host crop. BSR can continue to produce throughout the winter to infect early in spring, rotation is important, tillage may help bury infected residues.

Cercospora Blight and Purple Stain

  • Fungus favored by hot temperatures especially during pod fill and high humidity. It affects both the leaves and the seeds.
  • Impact: Cotyledons may become infected and die off, plant may be stunted, leaves may become infected and decreasing of leaf tissue and defoliation may occur. Purple staining on the seed may reduce the quality and ability of it to be sold to elevators. Seeds only become infected if the fungus invades the pods through the upper veins, but just because the leaves are infected doesn’t mean the seeds will be. The longer the plant stays in the field and is infected, the greater the chance of seed staining occurring. Leaves in the upper canopy will be those that are affected first, they will show a bronze or purple discoloration between the leaf veins. Severely infected leaves will become necrotic and may drop off the plant, but the remainder of the plant will stay green.
  • Management: Survives on soybean residues and infected seeds. Tillage and rotation are important if this was an issue the previous year. Seed treatments help protect against the seed and soil borne disease, some fungicides are also labeled for use if the infected warrants action to be taken. Timing is important in order to target the disease and reduce impact.

Downy Mildew

  • Seldom causes yield loss. Small yellow lesions appear on the leaves, shown on the youngest leaves first, lesions vary in size and can range from pale to bright yellow. The older the leaf the larger the spots may be, as they vary in size. Older lesions will turn necrotic and die off. Fuzzy growth on the underside of the leaf may also be an indicator during humid conditions. Favored by cool humid weather conditions.
  • Impact: Can affect the seed, and the seed quality but not usually yield. Seeds that are affected may appear to be white and dull with some cracks in the seed coat.
  • Management: Look for varieties that show resistance. Bury the crop residue and rotate crops.

Frogeye Leaf Spot

  • Small circular lesions begin on the upper surface of the canopy. Water soaked spots turn into brown lesions that have red/dark brown margins and may coalesce together and become very large irregular spots on the leaf. Tissues may die and the leaf may drop off the plant in heavily diseased areas, pods may develop lesions that are similar to those on leaves but will be more of a dark brown or reddish color as well as on the stems too.
  • Impact: On the pods the fungus may cause the seed coats to crack and open up leading to some seed discoloration, yield loss may occur.
  • Management: Overwinters in soybean residues and seeds, rain or wind spread. Dry weather limits the development of the disease, and it is favored by warm humid conditions, varieties may have some resistance to this disease, rotate crops and consider tilling the residue to break it up and inhibit overwintering, foliar fungicides might be helpful if threshold is reached.

Stem Canker

  • Reddish/brown lesions develop on the stem that resemble phytophthora but are higher up from the soil surface, turn from brown to black as they mature, leaves will die but remain attached to the stem, wet weather aids in the development of this fungus, depends on how much rain is received during early plant development as the rain will splash the spore from the debris onto the plant.
  • Impact: Lesions surround the stem and cut off water and nutrient flow to the plant, which can cause premature plant death.
  • Management: Stays on soybean debris so tillage may be an option, look for varieties that show some resistance to it. Rotate crops so the potential for overwintering is reduced.

Charcoal Rot

  • Usually start seeing symptoms during the reproductive stages of soybeans, may include smaller than normal leaves, reduced vigor, yellowing and wilting of the top leaves, on the lower stem a gray discoloration may appear, prefers soils with high temperatures and dry conditions, seeds may crack and shrivel up, pods can be dropped.
  • Impact: plants will die prematurely, reduction in yield, seed quality may be reduced, infection occurs when the spring is moist as it overwinters in the soil and soybean residues.
  • Management: rotation can help to a certain extent but will not eliminate the issue, there are no varieties that show resistance to this fungus, control weeds, make sure the soil is kept moist if possible, there is no chemical control for this, make sure the field is not nutrient deficient and the seeding rates are not excessive.