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Diseases Affecting Soybean Seed Quality

10/09/2019
  • Several diseases can affect the quality of soybean seeds but diagnosing the exact cause can be difficult.
  • Seeds affected by purple seed stain are smaller in size, have reduced germination, and poor vigor.
  • Symptoms of Phomopsis seed decay include chalky, cracked, shriveled and lightweight soybeans.
  • Bean pod mottle virus is a disease caused by a virus transmitted by the bean leaf beetle and usually a concern when bean leaf beetle numbers are high in the spring.
  • By the time pod symptoms are observed late in the season, it is usually too late to do anything but fungal pathogens like Phomopsis can be more severe if harvest is delayed.

Several diseases can affect the quality of soybean seeds but diagnosing the exact cause can be difficult at harvest. Symptoms that include mottling, shriveling, blotching, chalky and lightweight seeds can not only affect the quality but also delay germination and reduce emergence if infected seeds are planted. Below is a brief description of some of the most common diseases affecting soybean seeds along with some of the symptomology to look for.

Purple seed stain: It is caused by the fungal pathogen Cercospora kikuchii, the same pathogen that causes Cercospora leaf blight. The pathogen overwinters in soybean debris and produces air-borne spores and it also overwinters in infected seed.Even though Cercospora and purple seed stain are both caused by the same pathogen, having the foliar symptoms, does not necessarily mean that purple seed stain will be present. Purple seed stain may be favored by warm and wet conditions during and after flowering. Purple-stained seeds are smaller in size, have reduced germination, and poor vigor. The purpling can appear as small spots or coat the entire seed (Image 1). Extensively discolored seed should not be saved for planting. Sometimes infected seeds show no signs of discoloration.

Image 1. Purple seed stain caused by Cercospora.(Source:GROWMARK, Inc.)

BeanQuality-Disease_cercospora_1.jpg

Phomopsis seed decay: Caused by the fungal pathogen Diaporthe longicolla, part of the Diaporthe disease complex associated with pod and stem blight and stem canker. When Diaporthe spp. is present, thin, irregular lines that look like a fine pen was used to draw them, can be observed on the lower portion of the inner stem when split lengthwise on mature soybeans (Image 2). Seed symptoms include chalky, cracked, shriveled and lightweight soybeans (Image 3). If these seeds are planted, germination will be delayed, emergence will be reduced, and a seedling blight may be observed. Phomopsis is favored by warm and wet weather from R3-R7 and the disease will continue to develop if harvest is delayed. 

Image 2. Zone lines on inner soybean stems. (Source. D. Mueller, ISU)

BeanQuality-Disease_zonelines_2.jpg

Image 3. Seed symptoms of Phomopsis seed decay. (Source: X.B. Yang, ISU)

BeanQuality-Disease_phomopsis_3.jpg

Bean pod mottle virus (BPMV): This is a disease caused by a virus transmitted primarily by the bean leaf beetle and usually a concern when bean leaf beetle numbers are high in the spring. Foliar symptoms of BPMV include yellowing, stunting and mottled leaves with a rugose surface (Image 4). Pods can also show symptoms of mottling and have fewer and/or reduced seed weight. Foliar symptoms are visible early in the season and are favored by cool weather. The virus can survive on some leguminous weeds and overwintering beetles that feed on these infected weeds will carry the virus to soybeans. First and second-generation beetles feeding on infected plants will continue to spread the virus. Additionally, plants infected with BPMV may be more susceptible to Phomopsis seed decay.

Image 4. Foliar and pod symptoms of bean pod mottle virus.

BeanQuality-Disease_4.jpg

Soybean Mosaic Virus (SMV): It is caused by a virus that can be seed transmitted or vectored by aphids. Foliar symptoms include a mosaic pattern of light and dark green areas, yellowing, curl leaves with a rugose surface (Image 5). Plants can be stunted, have fewer or smaller pods and seeds can be mottled and smaller in size. The symptoms of SMV can look very similar to those of BPMV and laboratory diagnosis is necessary to accurately distinguish one from the other. Symptomatic plants may occur in patches or singly throughout the field.

Image 5. Foliar symptoms of soybean mosaic virus.

BeanQuality-Disease_mosaicviruse_5.jpg

Management

By the time pod symptoms are observed late in the season, it is usually too late to do anything. However, keep in mind that fungal pathogens like Phomopsis can be more severe if harvest is delayed. When Cercospora or Phomopsis are present, late fungicide applications (around R5) can help protect the seed quality and may be a helpful tool, especially on seed acres. Foliar insecticides targeting bean leaf beetles can help reduce the incidence of BPMV and when mild winters result in high survival of bean leaf beetles, insecticide seed treatments may be useful. Because most of these diseases can be seed transmitted to some extent, planting high quality, certified seed will reduce or avoid seed transmission.

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