South Dakota Soybeans: Watch for Brown Spot, Phytophthora Root Rot

Brown Spot

Figure 1. Brown spot symptoms on soybean found in lower canopy.

A few soybean fields scouted had between low to moderate levels of brown spot (also known as Septoria leaf spot). Soybean planted into soybean stubble had elevated levels of brown spot (Figure 1). Brown spot may cause early senescence of lower leaves. Infection can start as early as VC (cotyledons and unifoliate leaves) growth stage and can serve as the source of inoculum for upper leaves. Yield losses due to brown spot of up to 15% have been reported in other states.

The pathogen that causes brown spot (Septoria glycines) survives on soybean stubble and infection is favored by warm, moist weather. Brown spot severity increases with increasing leaf wetness periods of 6-36 hours and with a temperature range of 60-86 oF, especially in narrow-row spaced soybeans. The spores of the fungus are distributed by rain splash and wind.

For soybeans on soybeans and with narrow-row spacing, scouting to assess the level of brown spot and applying a fungicide at R3 may be beneficial. Some brown spot symptoms may be seen in rotated soybeans and with wide row spacing (>20 inches), but rarely does the level of brown spot in such conditions develop to cause significant yield losses.

Phytophthora Root Rot

Figure 2. A soybean field with a large portion of plants killed by Phytophthora root rot near Salem, SD on July 16, 2015. Photo: Paul Luetjen

Phytophthora root and stem rot (PRR) was observed at moderate levels in some fields (Figure 2). Plant stress caused by moisture, compaction, or nutrient stress may exacerbate PRR symptoms. It can be distinguished from other root and stem rots by an extended brown lesion above the soil line (Figure 3). Plants may be killed in succession depending on the severity of the root rot and stress level.

The pathogen that causes PRR (Phytophthora sojae) overwinters in the soil and on infected soybean residue as oospores (thick-walled fungal spore). Oospores produce zoospores which swim in a film of water and are attracted to and infect germinating soybean seeds or roots.

Infection can take place before seedling emergence causing pre-emergence damping off or after seedling emergence and later in the season. Plants are most vulnerable to Phytophthora infection within the first three weeks of planting, however, susceptible cultivars can get infected even in mid-season.

Figure 3. Phytophthora root and stem rot symptoms. Notice the extended dark brown lesion on the stem.

PRR is most common in heavy, compacted clay soils that are subject to saturation and flooding, which is why symptoms are common in low spots and entry areas of the field. Also no-till soybean on soybean fields may have an increased risk for PRR because such fields remain wet for an extended period of time.

PRR can be managed through integration of several practices including planting a resistant cultivar, drainage, seed treatment (with products containing active ingredients mefenoxam or metalaxyl), and crop rotation. Crop rotation may not necessarily suppress PRR because oospores can survive in soil for more than one year, but it will reduce the level of inoculum.

PRR Survey

For soybean fields planted with a PRR resistant cultivar and yet have plants killed by PRR, this may indicate shift in PRR race that can overcome the deployed resistance gene. We are conducting a survey to determine most common PRR races in South Dakota. If you wish to know the PRR race in your field for free, contact Emmanuel Byamukama for us to pick symptomatic plants.

Symptomatic plants can also be mailed to the SDSU Plant Diagnostic Clinic at:

South Dakota State University
153 Plant Diagnostic Clinic (SPSB), Box 2108
Brookings, SD 57007
Phone: 605.688.5545
Fax: 605.688.4024

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