Black Soybean Pests and Diseases: Late-Season Problems and What to Do

A longer season means longer exposure

A black soybean stands in the ground for five to six months, a month or two longer than an ordinary soybean, and it does its most vulnerable work at the end. Pod fill lands in early autumn, when shield bug populations have built up all summer, when caterpillars are at their peak, and when the weather turns wet. That timing, more than any particular pest, is what makes the crop's problems distinctive.

Everything divides into three phases. Seedlings, in the first three weeks, are eaten outright by birds, mice and slugs. Mid-season, the leaves carry aphids, mites and beetles, which rarely matter much. Late season, from flowering to cutting, sucking insects take the beans out of the pods and wet weather molds them, and this is where nearly all the real losses happen.

The habit that pays is a weekly walk down the rows, looking at leaf undersides early in the season and opening pods from flowering onwards. Almost everything below is cheap to handle in the week it appears and expensive a fortnight later.

Seedlings: the fortnight when the crop is most likely to fail

The large seed leaves of a black soybean carry half a big bean each above the soil, and every seed-eating bird in the garden recognizes them. Pigeons and jays will clear a row in a morning. Mice work the other end and dig the sown seed out of the drill before it germinates. Between them they cause more failed sowings than every disease put together.

One measure fixes both: cover the row with fine mesh or row cover from the day you sow, pegged along both edges and left slack enough for the seedlings to lift it. Take it off once the plants have three or four true leaves and the seed leaves have yellowed and dropped. Fitting cover after you see damage is always too late.

Slugs look similar and are not. Birds strip the leaves and leave a bare stem standing. Slugs cut the stem clean through at soil level and leave the top lying beside it. Cover does not help; three consecutive nights out after dark with a torch does, along with clearing the shelter along the bed edges.

Damping off collapses seedlings at the base in cold, wet soil, with a dark, pinched stem. There is no treatment once it starts. The prevention is not sowing into cold ground, which on a crop that gains nothing from an early start is an easy discipline to keep.

1. Cover the row on the day you sow Row cover or fine mesh pegged along both edges, left slack. A tight cover gets pushed off within a week and you will not notice until the row has gone.
2. Read the damage before you respond Leaves stripped with the stem left standing means birds. Stem cut through with the top beside it means slugs. The two need opposite answers.
3. Sow two seeds at every station A spare at each position means scattered losses cost you nothing, rather than leaving gaps a long-season crop gives you no time to fill.
4. Re-sow any gaps within ten days Later than that and the replacements are still green when you cut the block, and you will end up abandoning them.

Pod fill in autumn: shield bugs and borers

Shield-shaped bugs, green or brown and about half an inch (12 mm) long, push a stylet through the pod wall and suck the developing bean. The pod stays on the plant and looks perfectly normal from outside; the bean inside is shriveled, dimpled or gone entirely. This is the single largest cause of loss in a black soybean crop, because the pods are filling exactly when bug numbers peak.

You will not see it by looking at the plant. Open a dozen pods from different parts of the block every week from the moment pods start to swell. A block that looks superb and turns out to be half empty at threshing has almost always been worked by shield bugs for a month unnoticed.

They are large, slow and easy to pick off in the cool of the morning. Drop them into soapy water; ten minutes twice a week through pod fill genuinely holds them down at garden scale. Clearing weedy margins and long grass in autumn removes where they overwinter and cuts the following year's numbers, which is the only measure that reduces pressure rather than managing it.

Pod borers are caterpillars that chew a neat round hole in the pod wall, climb in and eat the beans from inside. Look for the hole, not the caterpillar. Pick affected pods off and destroy them rather than composting them. Heavy infestations are worth a bacterial caterpillar control applied in the evening; scattered damage is not worth spraying for.

1. Open pods weekly, do not just look A dozen pods from different parts of the block. Shield bug damage is completely invisible from the outside of an intact pod.
2. Hand-pick bugs in the early morning They are cold and slow before the sun reaches the block. A tub of soapy water and ten minutes twice a week keeps numbers manageable.
3. Remove and destroy bored pods 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 late autumn Weedy edges and long grass are where shield bugs overwinter. Clearing them is the one thing that lowers next season's pressure.

Aphids and spider mites build up on the leaves in hot dry weather, leaving them sticky or finely stippled. Neither usually costs you beans in a garden block. A strong jet of water and the predators that follow them will normally settle both.

Leaf disease, roots and the humid canopy problem

Leaf diseases show as spots that spread and merge, yellowing leaves and dropping them early. Losing the lowest leaves as the crop ripens is normal. Losing leaves from midsummer onwards while the pods are still filling is not, and on a crop that fills its pods this late it directly costs you weight in the jar.

The reason spacing matters so much on this crop is that a large, densely branched plant makes a still, humid canopy, and every one of these diseases moves faster in still humid air among wet foliage. Wide spacing, watering the soil rather than the leaves, never working among wet plants, and a three-year rotation away from all legumes are the four things that work. Nothing you spray on a garden crop substitutes for them.

A plant that is pale and stunted with yellow leaves but no spots is usually a nodulation failure rather than a disease. Dig one up: 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 use a seed inoculant next season.

Mottled, puckered or distorted leaves on scattered plants should be treated as a virus. Pull those plants out and never save seed from them, because several soybean viruses travel in the seed itself.

The wet-autumn hazards, and a record that pays for itself

The last six weeks carry risks that the standard soybean often escapes simply by finishing earlier. A wet autumn on a nearly ready block causes pod mold, gray or pink on the pod wall, and can make the beans sprout inside the pod while still on the plant. Both make those beans unstorable and both spread through a batch. If a long wet spell is forecast and the pods are already tan and rattling, cut the block early and finish the drying under cover.

The opposite risk is leaving it standing through hot dry weather until the pods split and throw the beans onto the soil. That is not a pest but it destroys more of a home crop than any insect. Once the first split pods appear anywhere in the block, cut within a day.

Bean weevils arrive with the crop rather than finding it later. They lay on the pods in the field, the larvae develop inside the beans, and the adults emerge in the jar through neat round holes. Sorting out holed and blemished beans before storage, and freezing the sealed jars for a week before shelving them, handles both the visible and the hidden ones.

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

The single most useful entry is the date you first saw each problem. Two seasons showing shield bugs in the same week turns a reaction into a plan, and on a crop that fills its pods this late a week of warning is worth a great deal.