By Bill Mullen
Jr., Director of Agronomic Services for Seed Consultants Inc.
Until we park the truck and walk our fields, we will never fully know the issues affecting crop development today. Walking fields of soybeans now will give us information on how the crop is actually handling the stress. There are various disease and insect issues to be aware of while walking fields, such as soybean cyst nematode, soybean aphids, sudden death syndrome, frogeye leaf spot, and white mold.
Now is the time to be scouting for soybean cyst nematode (SCN) in your fields. SCN injures soybean roots, and leads to stunted plants. There are not good above ground symptoms to indicate the presence of SCN in a field, especially when moisture is adequate. The cyst is found on the roots are filled with eggs which penetrate the roots and develop into adults in 14 to 21 days. As they develop, the cysts rupture the root, later die and then fall off into the soil. Schedule soil sampling this fall for any questionable fields. Fields with high egg counts may be best suited for corn-on-corn or other practices that avoid soybeans for several years.
SCN may also be present in fields with sudden death syndrome (SDS). SDS first appears as small, yellowish spots generally found in the middle and upper canopy. Leaf veins, petioles, and stems stay green while all other areas turn brown and die. Pith inside the stem is healthy white but roots will eventually die. The SDS fungus infects plants before flowering in saturated soils and is visible shortly after flowering.
It is also time to be watching for soybean aphids that usually occur at R1 through R5 stage. Having at least 250 aphids per soybean plant and populations rising, one needs to consider an insecticide treatment.
Phytophthora Root and Stem Rot occurs in areas with heavy rains over several weeks. Plants throughout these saturated areas have on the stem a chocolate brown canker going up the plant. This stem only appears on plants not having an effective specific Rps gene or carrying low field tolerance to Phytophthora root and stem rot. If infected, whole plants will wither and die.
Frogeye leaf spot is found during warm, humid weather conditions especially in the southern areas of the state below I-70. This fungus comes from the soil as well as from rain and wind. It is characterized by small reddish brown margins around circular spots or angular areas with light brown centers found on upper leaf surfaces. Lesions will enlarge and, when numerous, cause leaves to wither and drop prematurely, plants die, and yield loss occurs. The fungus can overwinter in crop residue as disease inoculum builds up from previous year’s infestation.
Producers need to look at their soybean fields at the R1 and R2 stage. If the pathogen is wind blown, it is often found at R5 to R6 stage. Certain soybean varieties lines are more susceptible to Frogeye leaf spot , while other lines have better tolerance to this disease. Just like corn, a fungicide application is worth the cost of application and product in certain soybean varieties that don’t have good disease tolerance. Many soybean varieties planted today, do have a good disease package and do not require a fungicide application. Keep in mind, a fungicide application normally delays the harvest 10 to 14 days.
White mold in soybeans, also known as Sclerotina stem rot, appears as a prematurely dying plant with withered leaves. Affected plants remain erect and rapidly turn brown. Look for white, moldy growth on the stems and black Sclerotia on the outside as well as inside the stem. White mold is more severe in narrow row plantings with high population and no resistance exists, only various degrees of tolerance to the infection. Pathogens can survive up to seven years when Sclerotia are buried in the soil.
When walking your fields, a good tool to take with you, is the pocket field guide published by Ohio State and Purdue universities to help diagnose problems in our fields today.