What I found while taking a walk in soybean fields in NW MN

This article was written by Angie Peltier, UMN Extension crops educator.

In recent days I have had the chance to drive around the region to see what there is to see in soybeans.

In the small blessings department, it looks as if recent rains and the fungal pathogens that they favor, natural enemies of the soybean aphid and/or shortening daylength and declining food quality of soybeans has resulted in soybean aphid populations dropping far below treatment thresholds. If one does find a soybean population at treatment threshold (an average of 250 aphids per plant, more than 80% of plants infested, populations increasing), double check soybean growth stage by opening up and taking a look at the seeds in the pods at the top four leaf axils. If the seeds within the pods are nearly filling the seed cavity, the crop is approaching the full seed or R6 growth stage and will soon begin losing leaves, pods will begin to reach a mature color and seeds dry down. It is the rare situation (very high population densities) where one should consider an insecticide application in R6. If your plants aren’t that far along in growth and development, it means that your scouting season isn’t yet over, and you should continue to scout until your crop reaches R6.

Stem canker.

In a couple of NW MN fields plants were suffering from symptoms of stem canker. Stem canker can be caused by multiple species of fungi in the Diaporthe genus. These fungi spend the winter in residue from previously infected soybean crops or in the soil itself. Because spores that cause infection are disseminated by rain-splash, infection is favored by wet weather early in the growing season (sound familiar?). Soybeans are thought to be infected during early vegetative growth stages despite more conspicuous evidence of infections only occurring at and after flowering.

Symptoms of stem canker can include yellow or brown tissue between leaflet veins. These symptoms can have multiple causes and so are not diagnostic of stem canker. A careful look at stem and taproot tissue inside and out can provide a pretty reliable diagnosis.

Stem canker lesions appear to originate at leaf nodes on the main stem (Figure 1). Lesions do not need to girdle the entire stem and may not result in much visible damage to the internal stem tissue despite leading to wilt or plant death.

There are two primary means of controlling stem canker in soybeans: crop rotation and varietal resistance. Few (if any) seed companies list stem canker ratings in their seed catalogue. Ask your seed salesperson for any internal data they may have regarding resistance while communicating your need to purchase stem canker resistant varieties going forward.

Figure 1. Stem canker lesion, photo: Angie Peltier. Note that the lesion begins above the soil line at a leaf node and can have a brown to reddish-brown color.

 

Cercospora leaf blight.

Those that grow sugarbeets are no strangers to diseases caused by Cercospora, but Cerspospora leaf blight (CLB) in soybeans is caused by a different cercospora species: Cerspospora kikuchii. Symptoms of CLB include reddish-bronze discoloration of the leaves toward the top of the plant (Figure 2). Those leaves that tend to have the most direct sunlight tend to be the most symptomatic as the fungus that causes it produces a phytotoxin that is active in direct sunlight. Affected leaves can often feel leathery and ruggose, unlike unaffected leaves that tend to feel more soft.  Petioles (the small stems that attach the leaf blade to the main stem) can also have the same reddish-bronze discoloration as can pods. In severe epidemics that occur in the southern US, leaf lesions can eventually kill leaf tissue, leading to leaf loss.

Figure 2. Cercospora leaf blight symptoms on upper soybean leaves, photo: Angie Peltier.

Sometimes seeds can also become infected by C. kikuchii leading to a symptom called purple seed stain. This can result in discounts at the elevator and if soybeans are being grown for seed and can necessitate the use of a seed treatment fungicide the following growing season.

Symptoms of CLB tend to appear in reproductive growth stages and are favored by conditions that include warm temperatures and high relative humidity. We have had periods of both warm temperatures and high relative humidity in NW MN in 2024.

Managing CLB has been a necessity for soybean producers in southern states, but growers in northern states have not often had to actively manage this disease, although some fungicide active ingredients have shown good efficacy.  If you have had CLB that you suspect has resulted in yield loss, be sure to rotate away from soybean as the pathogen will begin to degrade as infected tissue begins to degrade. Soybean varieties vary in their susceptibility to CLB. While seed companies in MN don’t tend to have a column in their seed catalogues dedicated to CLB resistance/susceptibility, if you have had a CLB epidemic that you suspect has led to yield loss, ask your seed salesperson whether they have any information about CLB susceptibility of their varieties as you are in need of resistant varieties.

White mold.

If you start to see wilted plants with leaves remaining attached to the main stem, it warrants further investigation as these symptoms can have multiple causes. If you pull back the canopy and see bleached stem lesions on the wilted plants, you very likely are seeing the aftermath of a white mold infection (Figure 3). Caused by a long-lived, soil-borne fungus called Sclerotinia sclerotiorum, many of the broadleaf crops in NW MN (soybean, dry bean, sunflower, potato, sugar beet, canola) are susceptible to this pathogen.

Figure 3. Bleached stems of soybean plants with white mold (blue arrows) and an uninfected stem (orange arrow), photo: Angie Peltier.

Infection takes place at leaf internodes when spores land on dying flower tissue during the early reproductive growth stages. Seeing the symptoms now can only empower management decisions in future soybean crops.

Foliar fungicides have different efficacy against this pathogen and must be applied before infection takes place (at beginning or full flowering, or R1 to R2, respectively). A nearly closed or closed canopy, moderate temperatures and heavy dews or rainy weather favors infection. Manure applications can produce a very thick canopy and so also favor infection.

As the pathogen produces long-lived survival structures and it is unrealistic to only grow grass crops, crop rotation is impractical. However, the longer one can go between planting susceptible crops, the better the chance that some survival structures have lost viability.

Partial resistance is available in some soybean varieties and so one should plan to select a variety with a good resistance score. In a year that favors white mold however, some yield loss is likely to take place as varietal resistance still allows for some infection.

Widening row spacing and reducing seeding rates can reduce white mold pressure but should only be reserved for ‘trainwreck’ fields as these practices can also have a negative impact on crop yield potential and not each year is favorable for white mold to develop. In the central sands region of the state, white mold epidemics have sometimes resulted in average yields of 2-5 bushels/acre. It is in these sorts of extreme circumstances that one should consider adjusting seeding rate and row spacing.

Septoria brown spot.

Most soybean fields will have symptoms of Septoria brown spot occur by the end of the growing season. Septoria brown spot (SBS) lesions tend to be small and brown (hence the disease name) surrounded by yellow tissue (Figure 4). It is only in years in which there are long periods of leaf wetness in which the disease can occur in the upper canopy, where some loss to yield potential may be possible.

While there are foliar fungicides labeled for SBS, it is the rare growing season that a beginning pod (R3) application is warranted to protect leaf tissue that has not yet been infected.

Figure 4. Septoria brown spot lesions are brown surrounded by yellow leaf tissue and first occur in the lower canopy, photo: Angie Peltier.

Downy mildew.

Under periods of high relative humidity both symptoms and signs of a downy mildew infection may be present. Briefly, on the upper leaf surface, small neon yellow-green lesions dramatically contrast with the rest of the dark green leaf tissue (Figure 5). When it has been very humid, one can also see signs of the pathogen itself as the lesions begin to sporulate on the underside of the leaf. The fungal growth can often be observed with the naked eye, but some may need to use a hand lens (Figure 6).

This disease is favored by moderate temperatures and periods of high relative humidity and is caused by an oomycete fungus-like organism. Seed treatments specific to fungal-like organisms (oomycetes, or water molds) can decrease disease risk as can rotating to another crop beside soybean or bean and managing residue from previously infected crops.   

Figure 5. Small, pale, yellowish-green lesions of downy mildew are observed on the upper leaf surface, photo: Angie Peltier.

Figure 6. Signs of downy mildew infections can be observed as pale, fluffy fungal growth on the underside of lesions on the lower leaf surface, photo: Angie Peltier.