Soybean growers scouting their fields might be seeing some yellowing plants at this time of year. Plant pathologist Emmanuel Byamukama from South Dakota State University Extension covers potential causes for the condition:
Potassium deficiency. Potassium deficiency symptoms are frequently seen on soybeans at early stages of development (as early as V3), but the condition can develop throughout the growing season. Potassium deficiency symptoms include yellowing of leaf tissue starting at the leaf tip and moving down the leaf margin (outer edge of leaf). Severe K deficiency can lead to premature leaf fall of the older leaves.
Potassium deficiency is often more severe and shows up in fields with adequate soil test K levels or fertilization in drought years like this year. This occurs because the dry soil conditions slow down root growth and the movement of potassium to plant roots.
Also, potassium gets temporarily trapped in some clay types as soils become drier. The best way to alleviate this type of K deficiency is to test your soils and fertilize them according to guidelines in SDSU Extension’s Fertilizer Recommendation Guide. These deficiency symptoms often decrease or disappear once adequate precipitation occurs, because root growth will increase along with K movement in the soil to the roots.
Iron deficiency chlorosis. This condition is most common in soils with high pH, elevated salts, and elevated levels of nitrates and free lime, as well as poorly drained areas of a field. Plants tend to be stunted, and yellowing occurs between the leaf veins but the veins remain green (interveinal chlorosis). Leaves may also develop brown and necrotic spots in leaf margins. Iron is an immobile nutrient, so these symptoms most often occur first on newer, upper leaves.
Soybean cyst nematode. SCN at high population density in the field can cause soybean plants to be stunted and yellow. The top leaves tend to be affected more than older leaves. Plants likely to be infected are those in low areas, at field entrances and along the fence line, where SCN is likely to be spread first. At a low SCN population density, no obvious symptoms are observed on soybean plants, but yield loss can still occur even.
Lightening injury. Soybeans can be injured by lightning when a heavy storm occurs and stormwater collects in portions of the field. Plants injured by lightening tend to be found in a circular pattern and have uniform sudden wilting of leaves. Despite being dry for most of this season, a few thunderstorms came through, and at least one field has been found with lightning injury to the soybeans.
While other causes for yellowing leaves include common diseases such as brown stem rot and soybean sudden death syndrome, these have not been found on soybean fields so far. If symptoms being observed are due to a nutrient deficiency, a soil fertility test would be recommended for the future growing season.
Tissue sampling can also be done to determine nutrient levels in the plant tissues. Plant tissues for nutrient analysis should be sampled from a good area and the affected area separately, and sent to a lab.
Find more information on soybean management at SDSU Extension.Source: South Dakota State University Extension, which is responsible for the information provided and is wholly owned by the source. Informa Business Media and its subsidiaries aren't responsible for any of the content contained in this information asset.