Gadenin日本語Get the appPecan scab: the one that matters in humid climates
Scab is a fungal disease and it is the limiting factor for pecans anywhere with warm, wet, humid summers. It shows first as small olive-black spots on the undersides of young leaves and on leaf stalks, then on the shucks as sunken black lesions. Badly scabbed shucks stick shut, never split, and the nuts inside stay small and empty. A severe year can take the entire crop off a susceptible tree.
The fungus overwinters on shucks, leaf stalks and infected shoots left in the tree and on the ground, and rain splashes the spores onto the new growth each spring. Every wetting period during the growing season is a new infection cycle, which is why the disease explodes in a wet summer and barely appears in a dry one.
For a home tree there is very little chemical help. Effective control means repeated fungicide sprays from bud break through shuck expansion, applied to the top of a 70-foot (21 m) crown, which is not achievable from the ground. What you can do is reduce the spore load and improve drying: rake up and remove fallen shucks and leaves in winter, remove low limbs and crowding trees so air moves through, and never plant a pecan in a still, damp hollow. In a genuinely humid region, accept that a susceptible tree will crop well only in dry summers.
| Symptom | Likely cause | When it appears | What to do |
|---|---|---|---|
| Olive-black spots under young leaves | Scab | Spring, after wet spells | Sanitation, airflow; remove fallen debris |
| Sunken black lesions on shucks | Scab | Mid to late summer | Expect small nuts; rake shucks in winter |
| Sticky leaves and black sooty film | Aphids | Midsummer onward | Tolerate; predators usually catch up |
| Small round hole in the shell | Nut weevil | At harvest | Destroy affected nuts, do not compost |
| Nuts webbed together at the tips | Casebearer larvae | Early summer | Prune out reachable clusters |
| Small crinkled leaves, short shoots | Zinc deficiency | Spring flush | Foliar zinc on expanding leaves |
| Nuts drop small and green | Natural drop or poor pollination | Early summer | Check partner trees at bloom next year |
Insects worth knowing
Pecan nut casebearer larvae bore into small nutlets in early summer and web several nuts of a cluster together, hollowing them out. Damage is concentrated and obvious if you look at the cluster tips in early summer. On a small tree, cutting out the webbed clusters you can reach removes the generation feeding in them.
The pecan weevil arrives late, from the soil, as the kernels harden. Adults feed on nuts and lay eggs inside; the grub eats the kernel and chews its way out through a neat round hole about a sixteenth of an inch (1.5 mm) across, then drops to the soil to overwinter. Any nut with that hole should be destroyed rather than composted, and floating nuts off in water at harvest removes most of them from the batch.
Aphids feed on leaf undersides through summer, and their honeydew grows black sooty mold over the leaves. Heavy infestations cause early leaf drop, which cuts into next year's crop. Predators normally catch up on their own, and broad-spectrum insecticide sprays on a large tree usually make aphids worse by killing those predators. Leave them alone unless leaves are dropping in midsummer.
Nut drop, and telling the harmless kind from the rest
Every pecan drops a great many small nuts in early summer, and most of it is normal. The tree sets far more female flowers than it can fill and sheds the surplus in two waves: unpollinated flowers first, then a second drop of pea-sized nutlets as the tree balances its load against water and reserves.
The drop is telling you something only when it is total. If practically every nutlet goes and the tree is otherwise healthy, look at pollination: you probably lack a partner in the opposite timing group, or the weather during the shedding window was persistently wet. If the drop happens later, when nuts are already sizing, suspect drought or an enormous crop the tree cannot carry.
Late frost is a separate case and unmistakable at the time. A hard freeze after bud break blackens the new shoots and flower clusters within a day or two, and the crop for that year is gone regardless of anything else. Note the date it happened; a site that frosts late repeatedly is telling you something permanent about where the trees are planted.
Keep a short note each season of what you saw and when: the first scab spots, the week nutlets dropped, whether summer was wet or dry. Pecan problems repeat on a weather cycle, and three years of notes let you predict a bad scab year from the spring rather than discover it at harvest.
What actually reduces trouble on a home tree
Nearly everything that works on a garden pecan is cultural rather than sprayed, because the tree is simply too tall to treat. The measures below cost nothing and compound over years.
Rake and remove fallen shucks, leaves and prunings every winter; they carry both scab spores and overwintering insects. Keep grass and weeds off the root zone and mulch instead, so the tree is not competing for water during kernel fill. Water deeply through late summer in any dry spell, because a drought-stressed tree gives poor kernels and is more vulnerable to everything else. Thin nuts in early summer of a heavy year so the tree keeps reserves and does not swing into a barren season.
And give the crown room. Crowded pecans stay wet after rain, feed scab, shade their own inner branches and stop bearing on them. Removing a neighboring tree does more for the health of a mature pecan than any spray a home grower can apply.