CPN 1006 — May 2015
Stem canker is a disease of soybean in the United States and Canada where infections occur primarily on the lower portion of the stem. Multiple fungi in the genus Diaporthe cause the disease. However, identifying and managing stem canker is similar regardless of which fungus is involved.
Often, the first symptoms of stem canker are dead plants with dried leaves still attached to petioles late in the season. Diseased plants usually occur in patches within fields (Figure 1). Initial symptoms typically appear on the lower third of the stem shortly after flowering (growth stage R1).
The lesions start as small, reddish-brown spots at the base of a branch or leaf petiole and then expand to form slightly sunken cankers that are reddish-brown with reddish margins (Figure 2). Cankers may remain on one side of the stem and can extend over several nodes or girdle the stem, killing the plant (Figure 3). In some cases, Diaporthe fungi cause top dieback by forming a dark brown canker on the upper four to six internodes, which kills only the top of the plant (Figure 4).
Diaporthe fungi may also be associated with gray streaking in the lower stem and taproots. Interveinal foliar chlorosis and necrosis may occur as a result of a toxin the fungus produces, but it is difficult to distinguish foliar symptoms from several other soybean diseases that cause similar symptoms (see Diseases with Similar Symptoms, page 4).
The fungi that cause stem canker survive in residue or in the soil for several years. Seed can be infected, but fungal spores from infested residue are usually the primary source of the fungus. Several weed species also serve as hosts, including black nightshade, curly dock, morningglory, and others. However, many of these weed hosts do not show symptoms of stem canker.
The fungus produces spores during rainy weather, which then splash onto plant tissue. Infection occurs during the early vegetative stages of soybean growth, although cankers are not visible until the plant enters reproductive stages. Secondary spore production on infected plant tissue can occur, but later infections will not have as great an impact on disease development.
Infection can occur over a wide range of temperatures, but the fungus requires extended moist periods to infect. Disease can develop to epidemic levels when rainy weather persists during the early vegetative stages of soybean growth.
Several other soybean diseases can cause similar symptoms, which can make diagnosis difficult (see Diseases with Similar Symptoms, page 4). Cankers may be isolated between nodes, or extend from a node to the soil line (Figure 5). Plants infected with the fungus that causes stem canker typically do not have interior taproot discoloration. You may need a laboratory diagnosis to distinguish stem canker from other diseases.
Yield loss from stem canker can approach 50 percent on susceptible cultivars under favorable conditions. Stem canker can cause premature death of soybeans in large areas of the field (Figure 6). These plants often have fewer and smaller seeds.
The impact on yield is greatest when plants are infected early in the vegetative stages and weather is conducive for disease development. Infections that occur during reproductive stages often affect yield less. Soybeans with partial resistance to stem canker must be infected very early in the season for extensive yield loss to occur.
The best way to manage stem canker is to plant resistant soybean varieties. Consult your seed dealer to obtain current information about varieties with stem canker resistance.
Rotating crops to a nonhost may reduce the amount of inoculum available to infect the next soybean crop. Soybean rotations to nonhosts such as corn, wheat, and sorghum are recommended for at least two years after a severe disease infestation (Figure 7). If stem canker is severe, avoid rotating the field with alfalfa, which is also a host.
Evaluations for stem canker control indicate that fungicides may not be effective when susceptible cultivars are used. However, fungicides can manage stem canker in cultivars with moderate resistance. Apply sprays during early vegetative stages.
Fields under minimum or no-till production are at higher risk for stem canker development because they have more crop residue. Incorporating infested crop residue into the soil will reduce the survival rate of the fungus and the amount of fungus available to infect the next soybean crop. You can further reduce disease development by planting fields with a history of stem canker last.
Fields high in soil organic matter or with high fertility are also at increased risk for disease. Maintain adequate fertility to reduce disease impact.
The foliar symptoms of brown stem rot (BSR) include yellowing and necrosis between veins. The stem symptoms include brown, discolored pith tissue, especially near the nodes of soybean stems (Figure 8).
Stem canker’s foliar symptoms may be similar to those of BSR, so the real difference is visible in the stem. Split the plant stems to confirm the discolored piths that are characteristic of BSR-infected plants.
Fusarium wilt-infected leaves turn yellow, die, and remain attached to the stem (Figure 9). Plants affected by Fusarium have brown vascular tissue in the roots and stems, which cause plants to eventually wilt and possibly die (Figure 10).
Plants with Fusarium wilt have brown vascular tissue in the roots and stems — plants with stem canker do not. Also, plants with Fusarium wilt do not show external decay or stem lesions above the soil line.
Phytophthora root and stem rot (PRR) occurs in wet, waterlogged, compacted soils. Symptoms of this disease generally appear during or shortly after waterlogged soil conditions.
Stems of Phytophthora-infected plants have characteristic dark brown lesions visible on the outer stem tissue that are continuous from the roots and up the lower stem.
Sudden death syndrome (SDS) occurs in wet, compacted soils. SDS symptoms are expressed as yellowing and necrosis between the veins of leaflets during the soybean plant’s mid- to late reproductive stages. The veins of symptomatic leaves will remain green.
SDS-infected plants will not have a stem lesion as stem canker-infected plants do. As the foliar symptoms of SDS progress, the leaflets will eventually curl or shrivel and drop off with only the petiole remaining (Figure 12), instead of remaining attached and dead as they do with stem canker.
Tobacco streak virus (TSV) causes bud blight, leaf and flower bud proliferation (Figure 13), green stem disorder, and sometimes a lesion or blotch near the nodes (for example, red node). The internal stem tissue of infected plants also can be brown near the nodes.
Stem canker will not cause bud proliferation. TSV lesions near the node are much smaller than stem canker lesions.
White mold (Sclerotinia stem rot) is typically more of a problem in years when conditions are rainy and cool during flowering. Lesions develop on the nodes and expand up the stems. Infected leaves often die and remain attached to the stem.
Sclerotinia-infected plants can be identified by the presence of white fungal growth on the outside of the stems (Figure 14). In addition, the white mold fungus produces sclerotia that are hard and black.
To learn more about stem canker, visit the visit the NCSRP Soybean Research Information and Initiatve (SRII) website (www.soybeanresearchinfo.com) or consult your land-grant institution. Other publications in the Soybean Disease Management series are available by visiting the SRII website or your land-grant institution’s website.
Kiersten Wise, Purdue University
Carl Bradley, University of Illinois
Martin Chilvers, Michigan State University
Loren Giesler, University of Nebraska
Febina Mathew, South Dakota State University
Daren Mueller, Iowa State University
Damon Smith, University of Wisconsin
Albert Tenuta, Ontario Ministry of Agriculture and Food
Emmanuel Byamukama, South Dakota State University
Anne Dorrance, Ohio State University
Doug Jardine, Kansas State University
Dean Malvick, University of Minnesota
Sam Markell, North Dakota State University
Adam Sisson, Iowa State University
Laura Sweets, University of Missouri
All photos were provided by and are the property of the authors and contributors except the cover photo and Figures 4, 7, and 8 by Craig Grau, University of Minnesota; Figure 10 by John Kennicker, Iowa State University; Figure 13 by Gary Munkvold, Iowa State University; and Figure 9 by Alison Robertson, Iowa State University.
The Soybean Disease Management series is a multi-state collaboration sponsored by the North Central Soybean Research Program (NCSRP). Learn more about the NCSRP at www.ncsrp.com.
This publication was developed by the Crop Protection Network, a multi-state and international collaboration of university/provincial extension specialists and public/ private professionals that provides unbiased, research-based information to farmers and agricultural personnel.
This project was funded in part through Growing Forward 2 (GF2), a federal-provincial territorial initiative. The Agricultural Adaption Council assists in the delivery of GF2 in Ontario.
The authors thank the United Soybean Board and Grain Farmers of Ontario for their support.
Design and production by Purdue Agricultural Communication.
This information in this publication is only a guide, and the authors assume no liability for practices implemented based on this information. Reference to products in this publication is not intended to be an endorsement to the exclusion of others that may be similar. Individuals using such products assume responsibility for their use in accordance with current directions of the manufacturer.
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