Gadenin日本語Get the appBlackfly on the growing tips
Black bean aphid arrives on the soft growing tips in early summer, usually in a single day, and builds into a black crust around the top six inches of stem within a week. Heavily infested tips curl, stop extending, and the honeydew they drop coats the leaves below and turns them sooty black. It rarely kills a runner bean but it can cost weeks of growth at exactly the wrong moment.
The best control is the one you do before it appears: pinch out the growing tips as soon as the plants reach the top of the supports. The soft tip is the only part the aphid really wants, and removing it makes the plant much less attractive. Where it has already arrived, a hard jet of water from a hose knocks most of the colony off, repeated on two or three consecutive days.
Resist reaching for an insecticide. Runner beans depend on bumblebees to set pods at all, and anything that reduces bee visits costs you far more crop than the aphids do. Ladybird and hoverfly larvae clear up blackfly within a fortnight if you leave them alone, and a garden that is sprayed has neither.
| Problem | What you see first | Action |
|---|---|---|
| Black bean aphid | Black crust on the top few inches of stem | Pinch out tips, jet with water, leave predators to finish |
| Slugs and snails | Seedlings sheared off at ground level overnight | Protect only until plants are a foot tall, then ignore |
| Halo blight | Small water-soaked spots with a pale yellow ring | Remove worst leaves, stop overhead watering, never save seed |
| Red spider mite | Fine pale speckling on upper leaf, webbing beneath | Water heavily and raise humidity. It is a symptom of drought |
| Bean seed fly | Sown seed fails, small maggots in the remains | Sow into warm soil so emergence is fast. Resow |
| Birds on the flowers | Flowers stripped or shredded, no slits at the base | Net the flowering zone or thread black cotton across it |
Halo blight and the wet canopy
Halo blight is a bacterial disease and shows as small water-soaked spots on the leaves, each surrounded by a distinct pale yellow ring that gives it its name. In a wet season it spreads fast through a dense canopy, moving on splashing water, and severe cases stunt the plants and mark the pods with greasy patches. There is no cure once it is in a plant.
Everything about controlling it is about keeping the leaves dry and the air moving. Water at the base and never over the top. Space the plants properly rather than crowding them. Do not work along a wet row, because your sleeves carry the bacteria from plant to plant more efficiently than the rain does. Pick off and bin the worst affected leaves as soon as you see them, and take the whole plant out if it is badly hit while still small.
Halo blight travels on seed, so never save seed from an affected row, and do not compost the debris. At the end of the season pull the top growth and bin or burn it rather than adding it to the heap, and do not plant beans on the same ground for three years.
Gray mold is the other wet-weather problem, appearing as fuzzy gray growth on dying flowers and then rotting the young pods behind them. It follows the same rules: remove the dead flowers and any rotting pods, keep the canopy open, and it will not spread far.
- Water the ground, not the plant Aim the can or hose at the soil at the base. Overhead watering in the evening is the most reliable way to spread bacterial disease through a bean row.
- Stay out of a wet row Pick and tie in when the foliage is dry. If you must go in after rain, do the healthy end of the row first and the suspect end last.
- Clear dead flowers and rotting pods Go over the row every week or two and remove anything that is browning or slimy. Gray mold works forward from dead tissue into live pods.
- Bin the debris, do not compost it Bean tops from a diseased row go to the bin or the fire. Home compost piles rarely get hot enough to kill halo blight bacteria.
Yellowing leaves at the very bottom of a tall plant in late summer are normal ageing, not disease. Look for the yellow halo around a defined spot before you decide it is blight.
Slugs early, mites late
Slugs and snails are only a problem in the first three weeks. A runner bean seedling shorn off at ground level overnight does not regrow, which is why sowing a few spares at the end of the row is worth doing. Protect the row until the plants are a foot tall with whatever method you use, then stop, because from that point the stems are too tough to interest them and the effort is wasted.
Red spider mite is the opposite: a late, hot, dry weather problem, and it is best understood as a symptom rather than a cause. Fine pale speckling appears on the upper leaf surface, and if you turn a leaf over in strong light you will see the mites themselves and, later, fine webbing in the leaf axils. Plants under proper irrigation almost never get it badly.
The treatment is water. Soak the root zone heavily, hose the undersides of the leaves in the morning for several days running, and the population collapses because the mites need dry conditions to breed. If a container-grown plant is heavily infested and already dying back, cut your losses and take it out rather than letting it seed mites into the rest of the plot.
Note what appeared and when
Record the date you first saw each problem and where in the row it started. Blackfly and halo blight both arrive at roughly the same point in the season each year at a given site, and knowing that you first saw blackfly in the same week two years running lets you pinch the tips out ahead of it rather than after.
Record which end of the row was worst too. A blight that always starts at the same end is telling you something about shade, shelter or drainage at that spot, and the fix is often to change the layout rather than to treat the plants.