Gadenin日本語Get the appLeaf spot and rust: the two that defoliate
Leaf spot is the disease you will meet every year. It shows as small round brown spots on the lower leaves from midsummer, often with a yellow halo, and it works upward through the plant. Badly affected leaflets drop, and a plant that loses its lower canopy in the last month before harvest stops filling pods and also becomes far harder to lift, because the weakened pegs break and leave the crop in the ground. Rust appears as orange powdery pustules on the underside of leaves and behaves similarly, moving fast in warm humid spells.
Both are splash-spread and both need leaf wetness to infect. Water at the base in the morning rather than overhead in the evening, space the rows widely enough that air moves through the canopy, and remove and destroy the crop residue at the end of the season rather than digging it in. Once a quarter of the lower leaves are spotted, act on the assumption that the crop is on a clock: plan to test dig earlier and lift as soon as the majority of pods read mature.
| Symptom | Likely cause | What to do |
|---|---|---|
| Round brown spots with yellow halos, lowest leaves first | Leaf spot | Water at the base, thin the canopy, clear residue in autumn |
| Orange powdery pustules under the leaves | Rust | Improve airflow, remove worst plants, harvest promptly |
| White fungal threads at the crown, wilting branches | Southern blight or white mold | Pull and destroy affected plants, do not compost them |
| Stunted plants, knotted swellings on the roots | Root-knot nematode | Rotate away for three years and grow a non-host crop |
| Silvery flecking and distorted young leaves | Thrips | Usually tolerable; keep plants watered so they grow through it |
| Holes in the mound, pods gone, plants undisturbed | Voles, mice or squirrels | Trap or fence early; they will clear a row in days |
Crown rots and why buried foliage is dangerous
Southern blight, sometimes called white mold, is the most damaging peanut disease in hot humid gardens. It appears as a fan of white fungal threads at soil level around the crown, followed by wilting of one branch and then the whole plant, and small round tan bodies the size of a mustard seed among the threads. It lives in the soil and on undecayed plant material, which is exactly why you spread the branches out over the mound when hilling rather than burying leaves under soil.
There is no rescue for an infected plant. Pull it, along with the soil immediately around the crown, and put it in the rubbish rather than the compost pile; the small resting bodies survive composting easily. Prevent it by rotating away from peanuts, beans and other legumes for three years, by keeping the crown free of buried organic matter, and by avoiding heavy watering in hot still weather. Where the disease has appeared once, remember the spot and grow something unrelated there for several seasons.
Any peanut plant you pull for disease should be recorded by position. Southern blight and nematodes both build up in specific patches of a bed, and a note of where it happened is worth more than any treatment.
Empty pods, and the ones that are not a disease
A great deal of peanut disappointment is not caused by any pest. Pods that look normal and are empty or half-filled when cracked are usually short of calcium in the top few inches of soil, because the pod absorbs calcium directly rather than through the plant. The fix is a gypsum dressing worked into the surface around the plants as flowering starts, and it is preventive only; nothing added after the pods have formed will fill them. The large-seeded types suffer worst.
The other non-disease failure is pegs that never entered the ground. If you lift a big healthy plant and find very few pods, look at what the surface was like in midsummer. A baked crust, a layer of mulch, compacted soil where you knelt or a bed that dried out during flowering all produce the same result: plenty of flowers, almost no pods. That diagnosis is only available if you noticed the plant flowering well, which is one more reason to record the date of first flower.
- Crack a sample before blaming pests Open ten pods from a poor plant. Empty shells point to calcium; missing pods altogether point to the soil surface during pegging.
- Check for pegs that stopped short Look for dried pegs bent sideways above the soil. That is the signature of a crust or mulch barrier, not of any pest.
- Rule out the diggers Loose soil, small tunnels and scattered empty shells at the surface mean rodents took the crop. The plant tops often look untouched.
Insects and animals worth acting on
Above ground, peanut carries the usual summer legume insects: aphids on the growing tips, thrips causing silvery distortion on young leaves, and caterpillars chewing the foliage in late summer. A vigorous plant grows through all of them and spraying is rarely justified; the exception is a heavy aphid colony early in the season, which is worth washing off because aphids spread virus. Check the undersides of leaves when you are already in the bed for another reason, rather than making a special inspection.
Below ground is where the real losses happen. Wireworms bore into developing pods, and voles, mice and squirrels will empty an entire row in the fortnight before harvest, often working from one end and leaving the tops standing so that nothing looks wrong. If you see shells at the surface or fresh runs through the mound, harvest at once even if the test dig says another week, and next season set traps or a buried wire barrier before flowering rather than after.