A tour of NW MN soybeans reveals disease & pest issues

Traveling throughout Kittson, Mahnomen, Marshall, Norman, Pennington, Polk, Red Lake and Roseau Counties in northwest Minnesota this week to survey soybeans, revealed various pest and disease issues. 

Soybean aphid. 

Figure 1. Soybean aphid. Courtesy of: Christina DiFonzo, Michigan State University, Bugwood.org.

Soybean aphid (Figure 1) population densities remain either undetectable or very low, with most plants housing fewer than 20 aphids. With so few soybean aphids present, I was also unable to find Asian lady beetles or their larvae or aphid “mummies” that had been parasitized by the Asian parasitic wasp, two of the many insects that provide biological control of the soybean aphid.

Spider mites. 

Two-spotted spider mites were widespread throughout the lower and middle canopies at all except northernmost locations.

One may notice that infested leaflets have a duller appearance than those that aren’t infested due to the dull-yellow ‘stippling’ symptoms characteristic of mite feeding injury (Figure 2). If you have a hand lens (or just very good eyesight) you may be able to see the mites themselves (Figure 3) or the webbing that the mites produce on lower leaf surfaces.

Figure 2. Stippling injury caused by two-spotted spider mites gives soybean leaves a dull appearance from a difference (Photo: Angie Peltier).

Figure 3. Magnified image of a leaf with blue arrows pointing to two-spotted spider mites (Photo: Angie Peltier).

Ken Ostlie and Bruce Potter, entomologists with UMN Extension, developed a 1-5 rating scale to determine whether or not a spider mite infestation warrants treatment. The scale is based on the presence or absence of the mite, where in the canopy injury symptoms and leaf loss has occurred and the crop’s growth stage. Three facts may mean that treatment may no longer be warranted this year, including: plants are at or are soon nearing the full seed (R6) growth stage, mite populations will not grow as quickly now that temperatures have dropped and the continued wet weather conditions favor the growth of fungi that parasitize mites.

Downy mildew. 

In growing seasons in which wet, humid prevail and temperatures are cooler, symptoms and signs of downy mildew may be present in your soybeans. Symptoms are angular, lime-green to yellow spots on the upper leaf surface (Figure 4). Signs of the pathogen are tan, fluffy fungus-like growth on lower leaf surfaces (Figure 5). This disease is typically considered to be minor and not actively managed as yield loss due to downy mildew is rare.

 

 

Figure 4. Symptoms of soybean downy mildew include angular lime-green to yellow lesions on the top leaf surface (Photo: Angie Peltier).

Figure 5. Signs of the pathogen that causes soybean downy mildew include a fluffy, tan growth on the bottom side of leaflets (Photo: Angie Peltier).

White mold. 

Unless one ventures into a field, individual plants suffering from white mold may not be easy to observe as they tend to wilt and drop below the canopy created by their still-healthy neighbors. While I surveyed many fields this week, despite the wetter than normal conditions this growing season and the (sometimes) cooler temperature and heavy dews, I found white mold symptoms (Figure 6) in only one field. 

Figure 6. Symptoms of white mold include bleached, stringy stem lesions, wilting and premature plant death (Photo: Angie Peltier).

Unsurprisingly, this field also happened to have the tallest, most lush soybeans of any of the fields I visited, with waist-high plants, large, turgid leaves and fully-closed rows. If there is a period of cooler and wet or humid weather at flowering, these are the sorts of conditions under which both the pathogen is able to produce mushrooms and spores and plant infection is favored to occur. 

While there is nothing that can now be done to manage this disease in 2020, know that the white mold pathogen produces long-lived survival structures that can drop to the ground during harvest and cause problems when next growing susceptible broad-leaf crops including soybeans, dry edible beans, canola, sunflower, etc. 

Septoria brown spot.

 

Figure 7. Irregular, light-brown lesions on a distal trifoliolate leaf, characteristic of brown spot (Septoria glycines). Courtesy G. L. Hartman; Reproduced from Hartman, G. L., et al., eds. 2015. Compendium of Soybean Diseases and Pests. 5th ed. American Phytopathological Society, St. Paul, MN.

Symptoms of a fungal disease called Septoria brown spot (Figure 7) were also widespread on the lower leaves of plants throughout the region. This is not particularly surprising as it is the rare year when one cannot find Septoria brown spot symptoms in the lower canopy at some point in the growing season. While there are effective fungicides labeled for brown spot management, it is rare that managing this disease is either necessary or economical.