Effects of Weather and Disease on Commercial Soybean Seed

Identification, Characteristics, and Diagnosis:

• Caused by the fungus Cercospora kikuchii.

• Infected pods are likely to be purplish.

• Infected seeds can be partially or entirely “stained” purple (Figure 2).

• Initial foliar symptoms include light- to dark-purple areas on sun-exposed leaves that eventually become leathery, dark, reddish-purple, bronzed, and/or blighted (Figure 3).

Management:

• Apply Delaro® 325 SC Fungicide.  To learn more about applying Delaro® 325 SC Fungicide, please visit https://www.cropscience.bayer.us/products/fungicides/delaro and contact your 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.

Identification, Characteristics, and Diagnosis:

• Caused by the fungus Diaporthe longicolla.

• Infected seed can be shriveled, undersized, and have a white or chalky appearance (Figure 4).

• The interior of pods can contain a white, cottony mold.

• Favored at warm, wet weather during pod fill.

• Early-maturing soybean products may be more prone to infection.

• Moisture content levels below 19% retard infection.

Management:

• Do not plant infected seed.

• Seed treatments may help improve emergence.

• Plant resistant soybean products.

• 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.

Identification, Characteristics, and Diagnosis:

• Caused by various species of the fungi Diaporthe and Phomopsis. Diaporthe sojae is the preferred scientific name.

• Pathogens overwinter on infected seed and soybean residue.

• Linear rows of dark specks (fungal fruiting bodies) develop on stem nodes, pods, and petioles (Figure 5).

• The upper plant canopy turns yellow and dies.

• Pod infection can occur at flowering; however, most are infected around the R7 growth stage (beginning pod maturity). 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.

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.

• Apply Delaro® 325 SC Fungicide. To learn more about Delaro® 325 SC Fungicide, please visit https://www.cropscience.bayer.us/products/fungicides/delaro and contact your retailer.
  

Identification, Characteristics, and Diagnosis:

• Caused by the fungus Colletotrichum truncatum.

• Brown to black, irregularly-shaped lesions on stem, pods, and petioles.

• 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.

• Dark spines or setae stick out from the acervuli (fruiting bodies) within lesions (Figure 6).

• Leaves roll and exhibit necrosis of minor veins between the major veins.

• Favored by warm, wet, humid conditions.

Identification, Characteristics, and Diagnosis:

• Caused by a fungus-like organism, Peronospora manshurica.

• Overwinters in the soil and in infested crop debris.

• 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 seed to become dull white and covered with the pathogen.

• Favored by high humidity levels.

Management:

• Plant resistant soybean products.

• Rotate with a non-host crop for one year or more.

• Rarely affects yield; therefore, foliar fungicides are not recommended.

• Manage residues.

• Seed treatments can help protect seedlings from initial infection.

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.

Management:

• Plant resistant soybean products.

• Crop rotation and tillage to encourage residue decomposition can help reduce pathogen levels.

• Apply Delaro® 325 SC Fungicide. To learn more about Delaro® 325 SC Fungicide, please visit https://www.cropscience.bayer.us/products/fungicides/delaro and contact your retailer.

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 9).

• 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.

Management:

• Infection usually occurs very late; therefore, management is generally not necessary.

• Many soybean products have resistance.

• Seed treatments may offer protection.

• Apply Delaro® 325 SC Fungicide.  To learn more about Delaro® 325 SC Fungicide, please visit https://www.cropscience.bayer.us/products/fungicides/delaro and contact your retailer.

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 10).

• 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 the beetles feed.

• The virus can also overwinter in perennial weeds and infected seed.

• 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.

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 species) can help reduce the inoculum source.

• Delayed planting may increase early-season death of bean leaf beetles, reducing the vectoring population.

  
  

Identification, Characteristics, and Diagnosis:

• Aphids are a primary vector.

• A green/yellow mosaic pattern is the most common initial symptom on leaves (Figure 11).

• More mature leaves may exhibit a yellow/brown mosaic pattern.

• Premature defoliation is common.

• Infected seeds exhibit a brown or black mottling.

• Spread from plant to plant by soybean aphid feeding.

• Plant infection by SMV and bean pod mottle virus (BPMV), vectored by bean leaf beetle, may cause severe dwarfing, foliar distortion, leaf necrosis, leaf mottling, and yield loss.

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 movement in the field, increasing the dissemination of the virus.