Gadenin日本語Get the appBlack bean aphid: the one pest that matters
Black bean aphid arrives as winged females in late spring and early summer and heads straight for the softest tissue on the plant, which is the growing tip. Within a fortnight a few individuals become a black crust wrapping the top four inches (10 cm) of stem, and from there they spread down onto the flower trusses and the developing pods. Infested tips stop extending, flowers abort, and pods come out distorted and sticky with honeydew, which then grows sooty mold.
The defense is structural, not chemical. Sow in autumn so the plant is flowering and setting pods before the aphids fly, and pinch out the growing tip as soon as the lowest pods have set. Between those two things the crop simply passes through the vulnerable window before the pest is there in numbers.
Check the tips every few days from the moment the first flowers open. Aphid colonies are visible from a yard away once they are established but almost invisible in the first week, and the first week is when removing a tip solves the problem entirely.
- Inspect the growing points twice a week Look at the underside of the top leaves and the stem immediately below the tip. Ants running up and down the stem are a reliable sign that aphids are already there, because ants farm them for honeydew.
- Pinch out infested tips and remove them Snap off the top four to six inches (10 to 15 cm) and carry it off the plot in a bag. Dropping it under the row simply relocates the colony.
- Wash off what is left with plain water A hard jet of water knocks aphids off the plant and most do not climb back. Repeat every two or three days while numbers are low, in the morning so the foliage dries.
- Leave the predators alone Ladybird larvae, hoverfly larvae and lacewings clear a colony in about a week. Any broad-spectrum spray kills them faster than it kills aphids and the colony rebounds worse.
- Cut the crop when it is done A finished plant left standing is an aphid nursery next to whatever you plant after it. Cut the tops at ground level as soon as picking ends.
Chocolate spot: the disease of wet, crowded rows
Chocolate spot is a fungal disease that shows as small reddish-brown spots scattered over the leaves. In its mild form the spots stay small and separate, the plant carries on and you lose nothing. In its aggressive form, which follows prolonged wet weather on a crowded, poorly drained row, the spots run together into large dark blotches, the leaves collapse, stems go black and the plant dies before the pods fill.
Everything that prevents it is about air and soil. Keep the double rows 24 to 30 inches (60 to 75 cm) apart, stagger the seed so no plant sits directly behind another, keep the ground weed free, and do not overfeed with nitrogen, which produces the dense soft foliage the fungus prefers. Acidic and potassium-poor soils make it much worse, so a dressing of lime on acid ground and wood ash or a potassium feed is genuine prevention rather than a general tonic.
Once the aggressive form has taken hold there is no cure. Remove the worst-affected plants to slow the spread, pick whatever pods have filled, and clear the debris off the plot rather than composting it. Then do not grow beans on that ground for three years.
| Problem | What you see | Cause | Action |
|---|---|---|---|
| Black bean aphid | Black clusters on the tip, sticky leaves, ants on the stems | Winged aphids arriving in late spring | Pinch out the tip, wash off the rest, protect predators |
| Chocolate spot | Reddish-brown leaf spots merging into black blotches | Wet weather, crowding, acid or potassium-poor soil | Space wider, lime acid soil, remove the worst plants |
| Bean rust | Small raised orange-brown pustules on leaves and stems | Warm humid weather, late in the crop's life | Usually too late to matter. Pick and clear the crop |
| Pea and bean weevil | Neat U-shaped notches cut into leaf margins | Adult weevils feeding at night on young plants | Cosmetic on established plants. Row cover young ones |
| Root rot and foot rot | Yellowing, wilting, blackened stem base, plant lifts easily | Waterlogged soil, beans grown too often in one spot | Improve drainage, rotate on three years, do not replant |
| Pigeon damage | Leaves torn to ragged stubs, whole young plants stripped | Birds feeding hardest in cold weather | Net the row until plants are established |
Rust, weevil notches and things not worth acting on
Bean rust turns up as small raised orange-brown pustules on the leaves and stems, usually late in the crop when the pods are already filling. At that stage it takes very little off the yield, and the honest response is to finish picking, cut the plants down and clear the residue. Rust spores overwinter on debris, so leaving a rusty row standing is what carries it into next year.
Pea and bean weevil cuts neat U-shaped notches around the edges of the leaves. On a young plant just after emergence heavy notching can check growth, and row cover over the row for a fortnight solves it. On an established plant the notching looks alarming and costs essentially nothing. Ignore it.
Some spotting on the lowest leaves late in the season is normal senescence, not disease. The test is direction: leaves yellowing and dying from the bottom up while the top is green and podding is the plant finishing its job. Spots appearing on healthy mid-height leaves and spreading upward is chocolate spot, and that needs your attention.
Rotation, hygiene and what to record
Both of the serious problems on this crop build up in soil and debris, so rotation is the cheapest control you have. Keep broad beans, peas and other legumes off the same ground for three years. Cut the finished tops at ground level and remove them, but leave the roots in the soil, because the nitrogen-fixing nodules are worth having and the roots themselves do not carry the diseases in the way foliage does.
Record the date you first see aphids on the tips and the date you first see chocolate spot, along with which row and which sowing. After two seasons you will know whether the aphids reliably arrive before or after your autumn row sets pods, and whether chocolate spot follows a particular wet spell or a particular corner of the plot. Both answers change what you do next year, and neither is guessable from memory.
If a row has been badly hit by chocolate spot or foot rot, note the bed clearly. That ground should not carry beans again for three years, and the note is the only thing that will still be true next spring when the bed looks perfectly healthy.