Cowpea Pests and Diseases: Aphids, Pod Bugs and Leaf Spots

Aphids on the growing tips: the one to watch

If a cowpea crop is spoiled by a pest, this is nearly always it. The aphid that specializes in cowpeas is small, shiny and near-black with pale legs, and it does not arrive one at a time. A few winged females land on the soft growing tips, and within a week the top three inches (8 cm) of every runner can be sleeved in a solid black mass, packed so tightly that the stem is invisible.

The direct damage is bad enough. Sap loss stunts the tips, new leaves come out crinkled and undersized, the flower buds those tips were about to carry never open, and the honeydew the colony excretes coats the leaves below and grows sooty mold that blocks the light. But the greater harm is that these aphids carry cowpea viruses, and a colony that establishes early can infect a row before it flowers.

The whole management question is timing. Inspect the growing tips twice a week from the moment the plants begin running. At the smudge stage, a colony is dealt with by wiping it off between finger and thumb or knocking it off with a hard jet of water, repeated the next day and the day after. Once the tips are sleeved, ladybird and hoverfly larvae will usually arrive and clean up in a fortnight, but you have already lost that flush of flowers. Rich, over-fed, nitrogen-soft plants attract them far more heavily than lean ones, which is one more reason to keep the fertilizer off.

Pests on the pods

Once pods start forming, attention shifts from the tips to the pods themselves. Pod-sucking bugs and stink bugs push their mouthparts through the pod wall and drain individual developing seeds. The pod stays on the plant but goes dimpled and uneven, and when you shell it some seeds are plump and others are flat brown scraps. These bugs are large, slow in the cool of the morning and easy to pick off by hand into a jar of water, which at garden scale is the whole control program.

Pod borer caterpillars chew a neat round hole in the pod wall and eat the seed from inside, leaving dark pellets of frass caught in the foliage below. Because they spend almost their whole life inside the pod, anything sprayed after they enter is wasted. Hand-picking from the moment the first pods inflate works well on a garden row, and a bacterial caterpillar treatment on the foliage works only if it goes on before the larvae bore in.

Cowpea weevil is a field pest that becomes a storage disaster. The adults lay on maturing pods late in the season, and the eggs travel indoors in seed that looks perfectly clean. Nothing you do in the garden prevents this reliably; the answer is the freezer treatment of every dried batch before it goes into the store cupboard.

Diseases: wet leaves and wet soil

Cowpeas are far less disease-prone than most garden beans, and nearly everything that does trouble them traces back to moisture. The leaf diseases spread by splash and by wet foliage brushing against wet foliage, which is why they run down a row in streaks. Watering at the base rather than overhead, generous spacing so the canopy dries quickly, and a rule against working among the plants while they are wet remove most of the risk before the season starts.

The common leaf problems are a brown or gray leaf spot with a darker margin, a rust that appears as rusty orange pustules on the undersides late in the season, powdery mildew as a white bloom in hot dry spells with cool nights, and bacterial blight as angular water-soaked patches that turn brown and can girdle stems. The first three are usually late enough that they cost you little. Bacterial blight is seed-borne, so never save seed from a crop that showed it.

In the soil, wet ground brings root and stem rots that kill plants one at a time from the base, and dry ground under heat stress brings charcoal rot, which shows as a stem that dries out and turns ashy gray just above soil level. Root-knot nematodes cause stunted plants with knotted, galled roots and are worth checking for whenever a patch of the row is inexplicably poor while the rest thrives. All three are managed by rotation and by getting the drainage and the watering right, not by treatment.

Knowing when to stop nursing a row

There is a point where continuing to fight for a cowpea row costs more than it returns. If virus symptoms are showing across most of the plants while the pods are still flat, the crop will not fill properly and the sensible move is to pull it, bin the debris and put the bed to something else. Viruses do not recover and infected plants become a reservoir for the rest of the garden.

The judgement changes once the pods have inflated. At that stage the seed inside is often sound even when the foliage is failing, and pulling the plants and drying them under cover salvages most of the harvest. Diseased foliage on a plant whose pods are full size is not worth treating; it is worth harvesting around.

Be strict about seed saving. Bacterial blight and several of the viruses travel in seed, and saving from an affected crop is how a clean garden gets a permanent problem. If a row showed either, eat the crop and buy fresh seed.

Bin diseased cowpea debris rather than composting it. Leaf spot and blight organisms survive on residue in the soil for two or three years, and a home heap rarely gets hot enough throughout to kill them.

Rotation, hygiene and what to record

Three practices carry most of the prevention: a three year gap before cowpeas return to the same ground, clean seed from a healthy crop or a fresh packet, and never touching the plants when they are wet. Add generous spacing and no nitrogen, and you have removed most of the risk before anything germinates.

The record that pays for itself is the date you first saw aphids on the tips, and where in the row. Colonies almost always start in the same place each year, usually the most sheltered and least breezy corner of the bed, and knowing that lets you inspect the right ten feet twice a week rather than the whole row. Note whether the bed had compost or fertilizer added, because a soft crop and a heavy aphid year go together so consistently that the connection is worth having in writing.