Runner Bean

Slugs and snails are nocturnal mollusks that rasp ragged holes in soft leaves, shear off seedlings at ground level, and chew into ripe fruit that touches the soil. They feed at night and in the early morning and spend the day hidden under boards, pots, dense mulch, leaf litter, and low foliage, becoming active in exactly the cool, damp, shaded conditions that suit young plants. Damage is heaviest in wet spells and on tender spring growth: a single wet week can erase a row of seedlings while an established, tough-leaved plant of the same species is barely marked. They lay clusters of small pearly eggs in damp soil and under debris, so a garden with plenty of cover carries a resident population year after year rather than being invaded from outside.

Watch for: Irregular ragged holes chewed out of the middle of leaves and along the edges - not the clean-edged cuts a caterpillar leaves. Seedlings that vanish overnight, or that are cut off at soil level with nothing left behind. Silvery dried slime trails across leaves, soil, pots, and paving in the morning: this is the giveaway that tells you it was mollusks rather than an insect. Chewed pits in ripe fruit, especially where it rests on the ground. Damage that shows up on the lowest, dampest leaves first and gets worse after rain or evening watering. Turn over a board, an upturned pot, or a clump of dense mulch during the day and look for the animals themselves sheltering in the cool underneath.

Prevent: Take away the daytime shelter first: lift boards and upturned pots, clear piles of debris, keep dense mulch pulled back from crowns and stems, and give crowded plantings enough airflow to dry out. Water in the morning rather than the evening so the surface is dry by the time they come out. Copper tape around containers and raised beds gives them a shock they will not cross, and it is the one barrier that reliably works. Get plants past the vulnerable stage quickly - start seedlings in pots and set them out at a decent size rather than direct-sowing into a cold wet bed, since most plants stop being worth defending once the leaves toughen. Leave the ground beetles, frogs, toads, and ground-feeding birds alone, because they eat mollusks all night for free and broad-spectrum slug pellets kill them too. Where thick-leaved or resistant varieties exist for what you are growing, they take far less damage.

Treat: Hand-picking works and costs nothing: go out an hour after dark with a headlamp, drop what you find into soapy water, and repeat nightly for a week or two - persistence is what wins, not any single product. Shallow beer traps sunk to soil level catch them too, but need emptying and refilling daily to be worth anything. Iron-phosphate bait is the standard control and is far lower-risk around pets and wildlife than the old metaldehyde products, but it is not harmless: many formulations include EDTA, and a dog that eats a quantity of pellets can develop iron toxicosis. Scatter it thinly per the label, never in piles, and keep dogs off a baited bed. Clear fallen foliage and debris in autumn, where the eggs are laid. In a wet climate this is a population you manage each season rather than one you eradicate - and on most established plants light grazing is cosmetic and needs nothing at all.

Runner beans are sown into cool damp spring soil, which is peak slug weather, and a seedling chewed at the growing point is gone for good. This is the single best reason to start plants in deep pots and set them out already climbing.

The defining runner bean problem, and the reason so many American gardeners give up on the crop: a vine covered in flowers that sets almost no pods. Runner beans are a highland Mexican species pollinated by bumblebees and hummingbirds, not self-pollinating inside the closed flower like snap and pole beans, so three separate things can each stop the crop. Sustained heat above about 85°F - and warm nights in particular - aborts the flowers outright. Dry roots during bloom does the same. And a shortage of large bees, whether from cold wet weather that keeps them grounded or from insecticide use, leaves the flowers unpollinated no matter how good the plant looks.

Watch for: Weeks of heavy bloom with hardly any small pods forming behind the flowers. Flowers browning and dropping off the truss cleanly, leaving bare stalks. Pod set stopping during a heat wave and starting again a week after it breaks - the clearest confirmation that heat is the cause. Short, curled, or lumpy pods with only two or three seeds developing, which points to partial pollination rather than none. Very little bumblebee traffic on the flowers, or holes chewed through the base of the flower tube by bees robbing nectar without entering the front.

Prevent: Cool the roots and keep them evenly moist - a deep mulch and base watering through the whole bloom period do more than anything else. Site the plants where they get afternoon shade in any zone where summers run hot, and sow early enough that the first flush of bloom lands before peak heat. Plant strong bumblebee forage nearby - borage, sunflowers, and flowering herbs - and never spray insecticide onto or near open flowers, since the pollinators are the crop. Where heat is the recurring problem, grow a self-pollinating stringless variety such as Moonlight or Firestorm, which carry common-bean genetics and set pods without insects and in warmer weather.

Treat: There is nothing to spray and nothing to fix on a plant that is already blooming into a heat wave - the vine is healthy and will resume setting when the weather breaks, so keep it watered, mulched, and picked. Strip off any leathery over-mature pods, which independently signal the plant to stop. If cold wet weather has grounded the bees, you can hand-pollinate by opening the keel of a flower with a fingertip, though at trellis scale that is a curiosity rather than a solution. If it happens every year in your garden, the honest fix is a variety change rather than a technique change.

Worth reading before planting rather than after: this is not a disease and not a soil problem, and it is the single most common reason a runner bean disappoints in the United States.

Aphids are small soft-bodied sap-suckers - green, black, gray, pink, or waxy white, and mostly wingless - that gather in dense colonies on the tenderest new growth: shoot tips, buds, and the undersides of young leaves. They breed extremely fast in mild weather, so a few can become hundreds inside a week. Their feeding curls and distorts soft growth, and the sticky honeydew they excrete coats everything below and turns black with sooty mold. Ants often farm and defend the colonies for that honeydew. A few aphids are normal on almost any plant and a healthy plant shrugs them off; only dense, persistent colonies do real damage, and some aphids also spread plant viruses as they feed.

Watch for: Dense clusters of tiny insects packed onto shoot tips, flower buds, and the undersides of young leaves. Curled, cupped, or distorted new growth that never opens properly. A sticky shine on the leaves - and on whatever sits under the plant - that later goes black with sooty mold. Tiny white cast skins left behind where a colony has been feeding. Ants running steadily up and down the stems, which is often the first thing you notice. Mottled, streaked, or misshapen leaves that do not grow out can mean a virus the aphids brought with them rather than the feeding itself.

Prevent: Go easy on nitrogen: the soft, lush growth that heavy feeding produces is exactly what aphids breed on. Check new growth once a week through spring and early summer so you catch colonies while they are still small, and rub the first few out with your fingers. Leave flowering plants nearby and skip broad-spectrum sprays - ladybugs, lacewings, hoverflies, and parasitic wasps do most of this work for free, and they only stay if there is something for them to eat. Keep ants off the plant where you can, since they protect aphids from those predators. Water steadily so plants are not stressed, and check any new plant over before you bring it home or move it indoors, since aphids usually arrive on something else.

Treat: Knock the colonies off with a hard jet of water and repeat every few days - on most plants that alone ends it. Pinch out or prune off the worst-infested tips rather than treating the whole plant. If colonies keep rebuilding, spray insecticidal soap or horticultural oil directly onto the insects, covering leaf undersides, and repeat weekly until they are gone; treat early or late in the day, and never spray a wilted or heat-stressed plant. Avoid broad-spectrum insecticides - they kill the ladybugs, lacewings, and parasitic wasps that were about to solve the problem for you, and aphid numbers usually come back worse afterward. Anything you spray must be labeled for the plant you are treating; follow that label and observe its pre-harvest interval. On anything you eat, wash leaves, heads, and buds well before use, since aphids hide down in crowns, folds, and tight growth.

The black bean aphid is the one that finds runner beans, massing black on the shoot tips and flower trusses. Pinch out the infested tips and hose the rest off - and be careful what you spray, because this crop depends on bumblebees to set pods at all.

A seed- and splash-borne fungus (Colletotrichum lindemuthianum) causing dark, sunken lesions on pods, stems, and leaf veins - and because it is seed-borne, it carries straight into next year's crop through saved seed, making it especially serious for dry-bean growers.

Watch for: Sunken, dark-bordered spots on pods (pink ooze in humidity), reddish-brown streaks on stems and leaf veins, seed with dark blotches.

Prevent: Start with clean, disease-free seed; never save seed from infected plants; never work wet plants; rotate 2-3 years; clear debris.

Treat: Remove and destroy infected plants; copper can protect healthy foliage in a wet season. Don't save seed from infected plants.

Worth knowing about here because runner beans are grown in exactly the cool damp summers this fungus likes, and because gardeners often save the big attractive seeds - discard any that are blotched.

A coppery, black-spotted beetle that looks like an oversized ladybug and is in fact a member of the same family - but this one eats plants. The spiny yellow larvae do most of the damage, feeding on the undersides of bean-family leaves and skeletonizing them into lace, which cuts the photosynthesis that fills the pods. Both adults and larvae feed, and over a long season the generations stack up.

Watch for: Fuzzy spiny yellow larvae and neat clusters of yellow eggs on the leaf undersides, lacy skeletonized leaves with only the veins left, and coppery black-spotted adults on the foliage. The larvae are usually well established underneath before the adults become obvious, so turn the leaves over.

Prevent: Float row cover over young plants, scout leaf undersides weekly from midsummer and crush the egg clusters before they hatch, grow early varieties that finish before peak beetle season where you can, clean up bean-family debris in fall, and rotate the bean bed.

Treat: Hand-crush eggs and larvae on the leaf undersides - done every day or two while the pods are filling, this alone controls a home planting. Insecticidal soap or neem knocks back a heavy larval outbreak, and the parasitic wasp Pediobius is available for large plantings. Do not spray anything onto open flowers.

A long-season crop gives the beetle time to build up, so scout weekly from midsummer. Do not spray anything onto open flowers here - the bumblebees are what set the pods.

A fungal disease (Uromyces) of bean-family plants, producing raised rusty orange-brown pustules on the leaves and sometimes the stems, mostly on the undersides and ringed with yellow. Heavy infection browns and drops the foliage, and losing leaf area during pod fill directly shrinks the crop. It is worst in warm, humid weather and wherever the leaves stay wet.

Watch for: Small raised rusty-orange pustules ringed with yellow, mostly on the leaf undersides and usually starting low in the plant where humidity is highest, followed by yellowing, browning and early leaf drop once it takes hold.

Prevent: Grow resistant or tolerant varieties where they are offered, space and train for airflow, water at the base so the leaves stay dry, and rotate beans on a two to three year cycle. Remove crop debris after harvest rather than leaving it to carry the fungus over.

Treat: Strip the worst-affected leaves early, starting with the lower ones. Sulfur slows the spread in a wet season; copper is not effective against rust. A late infection often still allows a full harvest, and fungicides are rarely needed in a home garden.

A dense runner bean canopy holds moisture, so rust often starts deep inside the vines where you cannot see it. Thin the interior growth if the plant is a solid wall of leaves by August.