Brown Stem Rot of Soybeans
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Key Points
- Brown stem rot infects roots early in the season, but symptoms of vascular system damage usually appear in mid-summer.
- Development is favored by moderate temperatures (60 to 80 ºF) and soil moisture is near field capacity.
- Infection causes vascular and pith tissues to turn brown to reddish-brown.
- Pioneer® brand soybean varieties are rated for genetic resistance to brown stem rot.
Disease Facts
- Caused by Phialophora gregata, a fungus that survives in soybean residue.
- Widely established throughout the North Central U.S., where soybeans are its only host.
- There are two known strains – strain A is highly aggressive, and strain B is mild.
- Fungus infects roots early in the season, but symptoms of vascular system damage usually appear in mid-summer, during reproductive development.
- Fungus is not carried with seed, and minimal amounts of inoculum are carried with soil adhering to farm equipment.
Image 1 – Mature soybean stem infected by brown stem rot
Conditions Favoring Disease Development
- Brown stem rot (BSR) development is greatest between 60 and 80 ºF (16-27 ºC).
- Higher temperatures inhibit BSR – little or no disease develops above 90 ºF (32 ºC).
- Severity and incidence of BSR is greatest when soil moisture is near field capacity, which is also optimal for crop development.
- Symptoms worsen if disease development is followed by drought stress during pod fill.
- Severity increases when soil pH is near 6.0 and is less severe at pH of 7.0 or greater.
- BSR may be more severe in fields where SCN is also a problem.
Phialophora gregata Disease Cycle:
- Survives in infected soybean residue left on soil surface.
- Pathogen does not produce survival structures – survival is totally dependent on soybean residue.
- Fungus can continue to reproduce throughout the winter, influencing inoculum levels in the spring.
- Conidia (spores) are produced in the spring.
- Infection of new soybean crop occurs through roots (by growth stage V3) and progresses to stems.
- Infected stems become inoculum source for next disease cycle.
Image 2 – Susceptible variety on left, resistant variety on right.
Impact on Crop
- BSR infection progresses from roots to vascular system (water and food-conducting system) of soybean plants.
- Infection causes a gradual disruption of the vascular system.
- Premature plant death may occur, especially if heat and drought stress impacts badly diseased plants.
- Degree of yield loss depends on environmental conditions, variety, and fungal strain.
- BSR strain A causes more damage and may reduce yields up to 40% when severe.
Image 3 – Foliar symptoms of brown stem rot.
Stem Symptoms
- BSR infection causes vascular and pith tissues to turn brown to reddish-brown (pith discoloration is a characteristic/ distinguishing symptom).
- Split stems longitudinally to inspect for BSR.
- Check at and between nodes near the soil line.
- The height of internal stem discoloration is a measure of BSR severity.
- When disease is severe, discoloration is continuous from the base of the plant upwards.
- When disease is less severe, discoloration only occurs at nodes, with healthy, white tissue between nodes.
Image 4 – Split soybean stem showing BSR infection.
Management
- Select Resistant Varieties: Pioneer® brand soybean varieties have been continually improved for resistance to brown stem rot.
- Screening potential new varieties in areas of high BSR incidence is key to variety improvement.
- Corteva Agriscience rates its varieties and makes ratings available to customers.
- For fields with a history of BSR problems, use a variety with a rating of 6 or higher.
- Your Pioneer sales representative can help you select varieties with appropriate BSR resistance and other important traits.
- Crop Rotation: Effective in reducing disease inoculum – two years away from soybeans is more effective than one.
- Tillage: Some tillage may be necessary to bury infected residue – the rate of inoculum decline is directly related to the rate of soybean residue decomposition.
- Manage SCN: Plant varieties resistant to both sudden death syndrome and soybean cyst nematode.
- Sanitation is not needed: Minimal amounts of inoculum are carried with soil adhering to farm equipment.
Image 5 – Left: Variety susceptible to BSR Right: BSR resistant variety