Green Bean Problems: Aphids, Rust, Mosaic Virus and Bean Beetles

Start with where the damage is

On beans, the location of the trouble narrows it down faster than the appearance. Damage concentrated on the soft growing tips means sap-sucking insects. Spots and pustules on the older lower leaves mean fungus or bacteria, spread upward by rain splash. Mottling and distortion across the whole plant with no visible organism means virus. Holes and lace-like leaves mean chewing beetles or slugs.

Check the undersides of the lower leaves every time you pick. It costs a few seconds while you are already crouching, and nearly every serious bean problem shows itself down there several days before you would notice it from standing height.

Black aphids on the growing tips

Dense colonies of black or dark green aphids on the soft tips and flower stalks are the most common bean pest. They distort the tips, coat everything in sticky honeydew that then grows black sooty mold, and worst of all they carry mosaic virus from plant to plant. That last point is why you deal with them early rather than tolerating a few.

While a colony is confined to the tips, pinching out the affected inch or two of shoot and dropping it in a bucket removes almost all of them in one go. A jet of water knocks off most of the rest. Ladybird and hoverfly larvae will clear a light infestation within a fortnight if you have not sprayed them out of existence, so hold off on broad insecticides.

1. Pinch out infested tips Take the top 2 inches (5 cm) of any shoot carrying a colony and bin it. On a pole bean that has reached the top of its support you were going to pinch the tip anyway.
2. Wash the rest off A firm jet of water aimed up under the leaves, on two or three consecutive mornings, breaks the colony up. Dislodged aphids rarely climb back.
3. Check for ants Ants farm aphids and drive off the predators. A line of ants running up a cane means the colony will keep coming back until you interrupt them at the base.
4. Spray only as a last resort If a colony has spread down the whole plant, use insecticidal soap approved for edible crops in the evening, covering the leaf undersides. Observe the interval before picking on the label.

Bean rust and gray mold

Rust appears as small rusty orange pustules on the undersides of the leaves, each often ringed with a pale halo, and it thrives in warm, humid weather on crowded plants. Left alone it turns the leaves yellow and drops them from the bottom of the plant upward, and a pole bean can be stripped bare by late summer. Gray mold shows instead as a fuzzy gray growth on dying flowers and on pods touching wet soil.

Both are managed the same way: air and dryness. Pull off affected leaves and take them off the plot, thin out crowded growth so air moves through, keep water off the foliage, and never handle the plants while they are wet. A sulfur or copper fungicide approved for beans where you live slows rust if you start at the first pustules, but it will not rescue a plant already half defoliated.

1. Remove and bag affected leaves Take rusted leaves off at the base and carry them away in a bag. Dropping them on the bed puts the spores back within splashing range.
2. Open the plant up Thin congested growth in the middle of a wigwam and strip yellowing leaves from the bottom 12 inches (30 cm) of the stems so air can move.
3. Clear the ground in autumn Rust and blight overwinter on bean debris. Pull every stem and leaf off the plot at the end of the season rather than digging it in.

Do not compost diseased bean haulm in a cool heap. Home compost rarely gets hot enough to kill rust spores, and you will spread it back over the garden next spring.

Mosaic virus: pull the plant

Mosaic virus shows as a patchy yellow and green mottle across the leaves, often with puckering or downward curling, and the plant stays stunted and sets few pods. There is no treatment. Every day an infected plant stays in the row is another day aphids can carry the virus to its neighbors, so lift it, roots and all, and dispose of it away from the bed.

Prevention is aphid control, clean seed, and not smoking or handling tobacco near the plants. Wash your hands before working the row after removing an infected plant, and if you save your own seed, never save it from a plant that showed mottling.

1. Confirm it is not a nutrient problem Nutrient deficiency yellows evenly, usually between the veins and usually on the older leaves. Mosaic is patchy and irregular and the leaf itself is distorted.
2. Lift the whole plant Pull it with its roots on a dry day and bag it. Leaving the stump in the ground leaves a virus reservoir for aphids to feed on.
3. Deal with the aphids next The virus arrived on an insect. Removing the plant without reducing the aphid population just delays the next case.

Beetles, slugs and the seedling stage

Bean leaf beetles and bean weevils chew round holes in leaves and reduce them to lace, and their larvae can damage roots and pods. While numbers are low, hand picking into a jar of soapy water first thing in the morning, when the beetles are sluggish, genuinely works. A floating row cover over young plants keeps them off entirely until flowering, when it has to come off for pollinators.

The riskiest fortnight of a bean's life, though, is between emergence and the first true leaves. Slugs and snails can take a whole row in one damp night, and birds pull the seedlings out for the seed still attached. Cover the row, go out after dark with a torch for a few nights after emergence, and the crop gets past it.

Write down what appeared and when, and where in the plot it started. Bean pests and diseases come back to the same ground and in the same week each year, and a note of the first sighting date is what lets you get the row cover on before it happens rather than after.