Some late-season soybean diseases to watch for, especially in later-planted fields, include Frogeye leaf spot, Phytophthora, and Cercospora leaf blight.

Frogeye leaf spot can develop under the same conditions as gray leaf spot. It can affect not only leaves but also stems, pods and seeds. Yield losses can be significant if conditions are warm, rainy or humid, the variety is susceptible, and the infection begins early. A fungicide application at R3 may be warranted if disease is present, but some populations of Cercospora sojina, which causes frogeye leaf spot, have developed resistance to Qol (strobilurin) fungicides. Growers can consult the Fungicide Efficacy Tool to find the best fungicide choices for this disease.

For soybeans, an R3 fungicide application will not control late-season Phytophthora, which also can be worse in later-planted fields. This soilborne disease has been appearing more frequently in Illinois after heavy rains on poorly drained or compacted soils. The key is to plant a soybean variety with a good Phytophthora rating, but even those varieties may be susceptible because there has been a loss of efficacy of specific Rps genes used for disease management. Choosing a soybean variety that includes the Rps-gene (Rps3a) along with high levels of partial resistance, or field tolerance, is recommended.

Cercospora leaf blight and purple seed stain (Cercospora kikuchii) are diseases that often appear later in the season during warm, humid summers. It affects the upper canopy, looks purple, and can be mistaken for sunburn. The fungus also infects seeds and can lead to dockage at the elevator. Fungicide applications at R3 might give you some control, but not great control. There are some fungicides that perform better against Cercospora leaf blight, and you can find them on the Fungicide Efficacy for Control of Soybean Foliar Diseases chart.

Soybean Insects:

Some examples of insects that might plague later-planted soybeans fields are Japanese beetles, soybean aphids, bean leaf beetles and stink bugs. These insects may appear later in the season and might not be controlled with a R3 growth stage insecticide application.

Be on the lookout for Japanese beetles feeding on later-planted soybeans. Not only do they skeletonize leaves, but they also will feed on pods. They often appear in clusters on field edges in the upper canopy, so it is recommended to check the entire field (if possible), or spray field edges. During pod fill (R3 – R4), the threshold is 20%-25% defoliation on the entire canopy.

Not only do bean leaf beetles like to chow on early planted soybean fields, their second generation can emerge in late July to early August and feed on pods, especially on later-planted soybean fields. Watch for the insects and the small round holes they create on pods and treat if populations are high – 2 to 4 beetles per plant or 10 to 15 beetles per sweep.

Illinois has not had a major soybean aphid outbreak in 10 to 15 years, though a few isolated cases were reported in the northern part of the state last year. Soybean aphids are a pest that can become worse in later-planted soybeans, especially in cool, dry weather conditions. Randomly check the underside of soybean leaves across a field. Count aphids per plant and not just on the leaf. If you find 250 aphids per plant on 80% of plants across a field, you will want to treat them if populations are increasing with no natural enemies present. Both soybean aphids and bean leaf beetles can be a vector for viruses.

Several different species of stink bugs are present in Illinois, and some have already appeared in some southern Illinois fields. Continue monitoring them the rest of the season, especially in later- planted soybean fields. Stink bugs are often more noticeable along field edges at the R5-R6 growth stages. They feed by piercing pods and seeds, which can result in shriveled, discolored or deformed beans, and can reduce seed quality and oil content. The treatment threshold is 9 stink bugs per 25 sweeps or 1 per row foot.

Remember, to stay vigilant and to continue scouting soybean fields throughout August!