Soybean Pests and Diseases: What Goes Wrong and When to Act

Three waves, at three different points in the season

Almost everything that damages a soybean crop arrives in one of three distinct phases, and knowing which phase you are in tells you what to look for. The seedling phase, in the first three weeks, is about birds and slugs eating the plants outright. The pod-fill phase, in high summer, is about sucking insects and caterpillars taking the beans out of the pods. The drying phase, at the very end, is about mold and about beans left too long on the plant.

Because a dry-bean crop stands in the ground for four or five months, it passes through all three, which is longer exposure than a green-picked crop gets. The habit worth building is a slow walk down the rows once a week, looking at the undersides of leaves early on and opening two or three pods at pod fill. Nearly every problem below is easy to deal with in the week it appears and expensive a fortnight later.

Seedlings: birds, mice and slugs

A germinating soybean lifts the two halves of the bean above the soil on its stem, and to a pigeon that is a meal sitting in plain view. Mice work the other end, digging sown seed out of the drill before it germinates at all. Between them they account for more failed soybean sowings than every disease combined, and both are solved by the same thing: cover the row with fine mesh or row cover from the day you sow until the plants have three or four true leaves.

Slugs cause damage that looks similar but is not. Where birds strip the leaves and leave a stem, slugs cut the stem through at soil level and leave the top lying beside it. Cover does not help. Go out after dark with a torch for three consecutive nights and clear hiding places along the edges of the bed.

Damping off takes seedlings in cold, wet soil, collapsing them at the base with a dark, pinched stem. There is no treatment once it starts, and the prevention is not sowing into cold ground in the first place.

1. Cover from the day of sowing Row cover or fine mesh pegged along both edges. Fitting it after you see damage is too late, because a flock can clear a row in a single morning.
2. Identify the damage before responding Leaves stripped and stem left standing means birds. Stem cut through with the top lying beside it means slugs. The two need opposite responses.
3. Sow two seeds per station Gives you a spare at every position. Losing scattered individuals to slugs then costs you nothing rather than leaving gaps.
4. Re-sow gaps within ten days Any later and the replacements will still be green when you cut the block for drying, and you will end up abandoning them.

Leave the cover slack rather than taut. A tight cover gets pushed off by the growing plants within a week, usually without you noticing until the row has been eaten.

Pod fill: stink bugs, borers and empty pods

Shield-shaped bugs, green or brown and about half an inch (12 mm) long, feed by pushing a stylet through the pod wall and sucking the developing bean. The pod stays on the plant and looks normal from outside, but the bean inside is shriveled, dimpled or absent entirely. Finding flat pods on an otherwise healthy plant in late summer is the classic sign, and it is worth opening a dozen pods from around the block to see how widespread it is.

They are large, slow and easy to remove by hand in the cool of the morning. Drop them into soapy water; twice a week through pod fill is genuinely effective at garden scale. Clearing weedy margins over winter removes where they shelter and reduces the following year's numbers.

Pod borers are caterpillars that chew a small round hole in the pod wall, climb in and eat the beans from inside. Look for the neat round entry hole rather than for the caterpillar, and remove and destroy affected pods as you find them. A heavy infestation is worth treating with a bacterial caterpillar control applied in the evening; scattered damage is not worth spraying for.

Aphids and spider mites build up on the leaves in hot, dry weather, the first leaving them sticky and the second finely stippled. Neither usually reaches a level that costs you beans in a garden block, and a strong jet of water plus the ladybirds and hoverflies that follow them will normally sort both out.

1. Open pods, do not just look at them Check a dozen pods from different parts of the block each week during pod fill. Stink bug damage is invisible from the outside of the pod.
2. Hand-pick bugs in the early morning They are cold and slow before the sun is on the block. A container of soapy water and ten minutes twice a week keeps numbers down.
3. Pick off bored pods and destroy them Look for the neat round entry hole. Do not compost them, because the caterpillar simply finishes developing in the heap.
4. Clear the margins in autumn Weedy edges and long grass are where shield bugs overwinter. Clearing them is the one measure that reduces next year's pressure.

Leaf and root disease

Soybean leaf diseases mostly show as spots that spread and merge, causing leaves to yellow and fall early. Losing the lower leaves in late summer is normal ripening; losing leaves from midsummer onwards, while the pods are still filling, is not, and it costs you a poorly filled crop.

Every one of these spreads faster in still, humid air among wet foliage. The controls that work are spacing wide enough for air to move, never working among the plants when they are wet, watering the soil rather than the leaves, and rotating so soybeans do not return to the same ground for three years. There is nothing you spray on a garden crop that substitutes for those four things.

A plant that is pale and stunted with yellow leaves and no obvious spots is usually a nodulation problem rather than a disease. Dig one up and look at the roots: nodules present and pink inside means the plant is fixing nitrogen and something else is wrong; no nodules, or nodules white or green inside, means a seed inoculant at sowing is the answer for next season.

Where you find mottled, puckered or distorted leaves on scattered plants, treat it as a virus. Remove those plants and never save seed from them, because several soybean viruses are carried in the seed itself.

The drying-down hazards, and a record that pays for itself

The last month has its own risks and they are the ones that most often ruin an otherwise good crop. A wet autumn on a nearly ready block causes pod mold, gray or pink on the pod, and can make the beans sprout inside the pod on the plant. Both make those beans unstorable. If a long wet spell is forecast and the pods are tan and rattling, cut the block early and finish it under cover.

The opposite risk is leaving the crop standing too long in hot dry weather, at which point the pods split and fling the beans onto the soil. That is not a pest, but it destroys more of a home crop than any insect does. Once the first split pods appear anywhere, harvest within a day.

Bean weevils come in with the crop rather than finding it afterwards. They lay on the pods in the field and the larvae develop inside the beans, emerging in the jar through neat round holes. Sorting out holed or blemished beans before storage, and freezing the sealed jars for a week, deals with both the visible and the hidden ones.

Keep a short note each year of what appeared, roughly when, and how bad it was. Pest pressure in a given garden is remarkably consistent from year to year, and after two seasons those notes let you put the cover on, start opening pods, or clear the margins a week before the trouble arrives rather than a week after.

The single most useful line in that record is the date you first saw each problem. Knowing that shield bugs turned up in the same week two years running turns a reaction into a plan.