Soybean Pests

 

Many pests and diseases are rearing their ugly head this year. Fall armyworm, aphids, soybean cyst nematodes (SCN), sudden death syndrome (SDS), and white mold are common problems. Weather and management play a key role in the severity of these pests.

Fall armyworm blow in from the south, most likely on tropical storms. Each female moth lays 10-20 eggs up t 100 eggs which hatch in 5-7 days and live 7-21 days. Eggs have been observed on fence posts, lawns, hayfields, corn, soybeans, and vegetable crops. The eggs hatch and the hungry larvae or caterpillars tend to move in waves, consuming everything in sight, even sometimes their own kind. There are two natural predator wasps that help control fall armyworm. Other options include bacillus thuringiensis (Bt) which is a natural control, neem oil, and pyrethrin insecticides.

Aphids in soybeans are a problem especially during the reproductive stage (R5-R6) with an aphid threshold of 250 per soybean plant. Check 5 plants in 10 locations at least 100 feet from the end rows. Females give birth to live aphids which shed their skin several times. Aphids suck out the plant sap and transmit many diseases. They also produce a sugary honeydew which causes sooty mold (fungus) and is eaten by ants, wasps, and flies. Some spring females produce asexually (no male) and tend to stay on one plant. Other females will mate sexually and produce wings to find another plant to infect. Natural aphid predators include lady bugs, green and brown lacewings, hover flies, midges, big-eyed bugs, damsel bugs, soldier beetles, and parasitic wasp. If numerous predators are found, avoid applying insecticides which greatly reduce predator populations and natural pest control. Several soil health farmers have avoided the cost of applying insecticides because they are achieving natural control. Marigolds, dill, catnip, and cilantro are known to repel aphids. Horticultural soaps and oils can be used along with pyrethroid insecticides.

Sudden Death Syndrome (SDS) and soybean cyst nematode (SCN) damage are often linked together. SDS is a soil borne pathogen that invades the roots and lower stems of soybeans producing a toxin. SDS can devastate soybean fields causing aborted flowers and yellow dying plants. Foliar symptoms include small to pale green leaves early on with small circular spots in the late vegetative stage to early reproductive soybean stage. The area between the leaf veins turn bright yellow then brown as the disease progresses. It is common on sandy soils, top of hills and knolls, but also on plants under stress such as poorly drained or compacted soils.

The USA annual SCN loss is estimated at $1 billion dollars per year. SDS is carried by SCN which feed on the soybean roots. The SCN life cycle is 25-30 days with each adult laying up to 500 eggs. Severe soybean damage occurs when SCN reach 10,000 nematodes per plant, becoming yellow and stunted. Common weed hosts are henbit, purple dead nettle, and mulleins. Farmers usually rotate to non-host crops like corn and wheat to reduce SCN numbers. Planting annual ryegrass or cereal rye as a cover crop has been shown to reduce SCN numbers up to 80% if the cover crop is planted early in the fall when soil temperatures are above 500F. SCN come out of their cysts when soil temperatures are above 500F, and for a short period of time in the fall, they can not recognize a host plant from a non-host plant. If these grasses are planted early, the SCN are tricked into coming out of their protective cyst and do not survive the winter.

White mold or sclerotina (fungus) is a common soybean disease. Corn or wheat are non-hosts to SCN. White mold likes wet humid conditions with low aeration and commonly occurs in high yielding soybeans. There are over 400 hosts to white mold including alfalfa, peas, sunflowers. Weed hosts include henbit, dandelion, curly dock, jimsonweed, field pennycress, and prickly lettuce. White mold germinates from sclerotina which are black structures that look like small rat or mice droppings. The white fungus grows on or inside the stem turning to black sclerotina which is often spread to other fields in combines since it is the size of a soybean seed. Symptoms include wilting plants with grayish green leaves turning tan to brown. Long crop rotations, increasing row spacing, and selecting soybeans with partial white mold resistance can help control white mold. Improving soil health helps reduce or prevent many soil pests and diseases.