How Weather and Disease Can Affect Soybean Seed Quality

Purple Seed Stain (PSS) Caused by Cercospora species

Identification, Characteristics, and Diagnosis:

• Caused by the fungi Cercospora kikuchii, C. flagellaris, C. sigesbeckiae, and several more.
• Is not caused by the fungus Cercospora sojina, associated with Frogeye leaf spot.
• Warm and wet weather conditions favor infection.
• Infected pods can be purplish or discolored.
• Infected seeds can be partially or entirely “stained” purple (Figure 1).
• Cercospora leaf blight (CLB) can be identified by light- to dark-purple, red, or bronzed areas on sun-exposed leaves, which can eventually become leathery and/or blighted (Figure 2).^1,2
• Though CLB and PSS are caused by the same Cercospora species, it is not always correlated with PSS.

Management:

• Apply Delaro® 325 SC Fungicide or Delaro® Complete Fungicide at 8 to 11 fl oz/acre at the R3 growth stage. Additional applications at R5 growth stage may be considered if conditions are favorable and scouting shows high infection. To learn more about applying either of these fungicides, please click on the fungicide brands and contact your local retailer.
• Earlier-maturing soybean products may not be infected.
• Individual soybean products may have higher tolerance levels.
• Rotate to non-host crops.
• Residue management through tillage can help reduce pathogen survivability.^1,2

Soybean Diaporthe Seed Decay (DSD-also known as Phomopsis seed decay)

Identification, Characteristics, and Diagnosis:

• Caused by several species of the fungus Diaporthe, including D. longicolla and D. sojae.
• Infected seed can be shriveled, undersized, and have a white or chalky appearance (Figure 3).
• Infected seeds can have reduced germination, oil content, and test weight.
• The interior of pods can contain a white, cottony mold.
• Favors warm, wet weather during pod fill.
• Can infect seeds after physiological maturity.
• Early maturing soybean products may be more prone to infection.
• Moisture content levels below 19% retard infection.^2,3

Management:

• Do not plant infected seed.
• Seed treatments may help improve emergence.
• Select fuller-season (for the area to be grown) soybean products.
• Utilize tillage to help promote residue deterioration.
• Control weedy hosts such as velvetleaf.
• Harvest in a timely manner to reduce the risk of extended exposure of the mature crop to wet weather.^3,4
• An application of a labeled fungicide from R3 (pod set) to R6 (late pod) growth stage may help to lower the incidence of seed infection.⁵

Soybean Pod and Stem Blight

Identification, Characteristics, and Diagnosis:

• Caused by fungus Diaporthe sojae.
• Pathogens overwinter on infected seed and soybean residue.
• Linear rows of dark lesions (specks), called pycnidia, develop on stem nodes, pods, and petioles (Figure 4).
• The upper plant canopy turns yellow and dies.
• Pod infection can occur at flowering; however, symptoms do not appear until the R6 to R8 growth stages (full pod through harvest). Injury to pods by insects favors pod infection.
• Seeds may appear healthy, shriveled, cracked, or chalky because of coverage by white mycelium.
• Oil and flour quality can be reduced.
• Favored by wet weather during maturation growth stages and delayed harvest.^3,4

Management:

• Rotate crops to help reduce the amount of infected residue.
• Utilize tillage to promote decay of infected residue.
• Harvest in a timely manner to reduce the risk of extended exposure of the mature crop to wet weather.
• Utilize seed treatments to protect seed.^3,4
• One labeled fungicide application at the R6 growth stage has been found to help protect seed quality.⁵ Delaro® 325 SC Fungicide and Delaro® Complete Fungicide at 8 to 11 fl oz/acre are labeled for control of pod and stem blight. To learn more about applying either of these fungicides, please click on the brand names and contact your local retailer.

Anthracnose

Identification, Characteristics, and Diagnosis:

• Caused by the fungus Colletotrichum truncatum, which can overwinter on crop residue and be seedborne.
• Brown to black, irregularly shaped lesions on the stem, pods, and petioles (Figure 5).
• Premature defoliation can occur from petiole girdling.
• Infected pods may be filled with mycelium instead of seeds, or seeds may be fewer and/or smaller and can also be brown, moldy, shriveled, or normal in appearance.
• Microscopic dark spines/setae (acervuli) appear within the lesions (Figure 6).
• Leaves may roll and exhibit necrosis of minor veins between the major veins.
• Favors warm, wet, humid conditions.³
• Apply Delaro® 325 SC Fungicide and Delaro® Complete Fungicide at 8 to 11 fl oz/acre. To learn more about applying either of these fungicides, please visit click on the brand names and contact your local retailer.

Downy Mildew

Identification, Characteristics, and Diagnosis:

• Caused by a fungus-like organism, Peronospora manshurica.
• Infection occurs in the spring when oospores germinate and infect seedlings.
• Upper surfaces of young leaves develop pale green to light-yellow spots which enlarge into pale- to bright-yellow lesions (Figure 7).
• White to gray fungal tufts develop on the underside of the lesion.
• Oldest lesions become grayish brown to dark brown with yellowish-green margins.
• Masses of fungal-like growth can develop in pods and cause seeds to become dull white and covered with the pathogen.⁶

Management:

• Plant resistant soybean products.
• Rotate with a non-host crop for one year or more.
• Rarely affects yield potential; therefore, foliar fungicides are not recommended.
• If feasible, bury infected residue with tillage.
• Seed treatments can help protect seedlings from initial infection.⁶

Frogeye Leaf Spot

Identification, Characteristics, and Diagnosis:

• Caused by the fungus Cercospora sojina.
• Symptoms initially appear during reproductive growth stages as dark, water-soaked lesions on younger leaves with centers that become ash-gray to light brown in color.
• Later, the lesions become circular to angular with a purple to dark-brown margin around the tan to gray center (Figure 8).
• On leaf undersides, the center of the lesions may have a dark-black area where spores are being produced.
• Favored by warm (77 to 86 °F) temperatures and prolonged periods of dew or light rain.
• Lesions can grow through pods and may cause seeds to be dark, shriveled, and have cracked seed coats. Seeds can also have no symptoms. (Figure 9).⁷

Management:

• Plant resistant soybean products.
• Crop rotation and tillage to encourage residue decomposition can help reduce pathogen levels.⁷
• Apply Delaro® 325 SC Fungicide and Delaro® Complete Fungicide at 8 to 11 fl oz/acre at R3. To learn more about applying either of these fungicides, please click on fungicide brands and contact your local retailer.

Alternaria Leaf Spot

Identification, Characteristics, and Diagnosis:

• Caused by fungal species of Alternaria, a seedborne pathogen.
• Leaves may become reddish or yellowish in color.
• Dark-brown lesions, usually with concentric rings, ¼ to 1 inch in diameter usually appear on leaves and pods near soybean maturity throughout the canopy (Figure 10).
• Leaf lesions enlarge and merge together to produce large dead areas.
• Leaves eventually die and fall from the plant.
• Infected seeds are smaller, shriveled, and dark brown to black.
• Favored by warm, moist conditions late in the growing season.
• Usually, a secondary disease after mechanical or insect injury, or another disease.⁸

Management:

• Infection usually occurs very late; therefore, management is generally not necessary.
• Many soybean products have resistance.
• Seed treatments may offer protection.⁸
• Apply Delaro® 325 SC Fungicide and Delaro® Complete Fungicide at 8 to 11 fl oz/acre. To learn more about applying either of these fungicides, please click on fungicide brands and contact your local retailer.

Bean Pod Mottle Virus (BPMV)

Identification, Characteristics, and Diagnosis:

• Vectored by the bean leaf beetle, Cerotoma trifurcate Förster.
• Foliar symptoms range from mild chlorotic mottling on upper leaves to puckering and severe mosaic in lower leaves (Figure 11).
• Delayed maturity or green stems are often observed near harvest.
• Seed coat mottling may be present.
• Virus overwinters in bean leaf beetles and can infect seedlings as they feed.
• Plant infection by BPMV and soybean mosaic virus (SMV), vectored by soybean aphid, may cause severe dwarfing, foliar distortion, leaf necrosis, leaf mottling, and severe yield loss.⁹

Management:

• Managing emerging and first-generation bean leaf beetles in the spring with timely and labeled insecticides can reduce populations of the virus-laden insects.
• Controlling alternative BPMV hosts (cowpea (Vigna unguiculate), other bean species, and Demodium spp.) can help reduce the inoculum source.
• Delayed planting may increase early-season death of bean leaf beetles, reducing the vectoring population.⁹

Identification, Characteristics, and Diagnosis:

• Aphids are a primary vector.
• A green/yellow mosaic pattern is the most common initial symptom on leaves (Figure 12).
• More mature leaves may exhibit a yellow/brown mosaic pattern.
• Premature defoliation is common.
• Infected seeds exhibit a brown or black mottling (Figure 13).
• Spread from plant to plant by soybean aphid feeding.
• Plant infection by SMV and bean pod mottle virus (BPMV), vectored by bean leaf beetle (Cerotoma trifurcate Förster), may cause severe dwarfing, foliar distortion, leaf necrosis, leaf mottling, and yield loss.¹⁰

Management:

• Seeds should be virus-free.
• Plant resistant soybean products.
• Early planting may minimize aphid transmission at an early crop growth stage.
• Insecticide applications are not recommended because some insecticides may increase soybean aphid (Aphis glycines) movement in the field, increasing the dissemination of the virus.¹⁰