Apple Pests and Diseases: Codling Moth, Scab and Canker

The year has an order to it

Apple troubles arrive in a fixed sequence, and knowing the order turns pest control from a reaction into a routine. Dormant winter is when you deal with canker and clear up the sources. Bud burst to bloom brings scab infection periods and aphids. Petal fall to midsummer is codling moth and sawfly. Late summer is when scab, mildew and bitter pit all become visible on the fruit, long after the point where anything could have been done.

Almost every serious apple problem is easier to prevent in the previous season than to treat in this one. Scab spores overwinter in the fallen leaves under your own tree, codling moth larvae in bark crevices on the trunk, and canker spreads from wood you left in the tree, so autumn and winter cleanup is the most effective spray-free control there is. Walk the tree once a week from bud burst, looking at new leaf undersides, developing fruitlets and the bark of the main limbs.

Scab: a wet-leaf disease, controlled before you can see it

Scab shows as olive-green blotches on leaves that turn brown and drop early, and as rough, cracked brown patches on the fruit. The apples are still edible, but they crack, they will not store, and a badly infected tree loses so many leaves by late summer that it cannot build next year's fruit buds.

The fungus needs leaf wetness to infect, and the warmer it is the shorter the wet period needed, which is why a mild wet spring produces a bad scab year. Infection happens weeks before you see a mark, so by the time blotches appear that round is over.

The controls interrupt the cycle rather than curing the symptom. Rake up and remove every fallen leaf, because next spring's spores overwinter in them. Prune to an open framework so leaves dry quickly after rain, never water over the canopy, and remove infected shoots and mummified fruit left hanging in the tree. Where scab is a yearly problem, choosing scab-resistant stock for a new tree beats any spray program.

Powdery mildew is the other common leaf fungus and behaves in the opposite way, thriving in warm dry weather. It coats shoot tips in white felt; cut those tips out as you see them, and again in winter, because the fungus overwinters inside the buds.

Codling moth: the grub in the core

A tunnel running to the core, packed with brown crumbly frass, and a small hole at the side or at the calyx end, is codling moth. The adult is a small gray-brown moth that flies at dusk in warm, still weather from around bloom onwards. She lays eggs on leaves and fruitlets, and the newly hatched caterpillar has a short window in which it must find a fruit and bore in. Once it is inside, nothing reaches it.

That window is the whole of codling moth control: everything useful happens between egg laying and the grub entering the fruit, roughly two to three weeks after the moths start flying. Pheromone traps are how you find it without guessing. Hang one trap per tree at head height shortly before bloom and check it twice a week; the first sustained catch tells you the flight has begun. In warmer areas there is a second generation in late summer, so keep the trap up all season.

1. Trap to time it, not to control it One pheromone trap per tree, hung before bloom. A run of catches on consecutive checks marks the start of the flight. Note the date; the entry period follows one to three weeks later depending on warmth.
2. Band the trunk in midsummer Wrap a four-inch (10 cm) band of corrugated cardboard around the trunk. Mature grubs leaving fruit crawl down and pupate under it. Remove and burn the band in autumn, and replace it if you want a second catch.
3. Bag or barrier individual fruit On a small tree, slipping a paper or fine mesh bag over each thinned fruitlet after the early drop is close to complete control, and needs no chemicals at all. It is laborious, but on a dwarf tree it is an hour's work.
4. Pick up every drop, every week Fallen apples usually contain a live grub that will crawl out and overwinter under the tree. Collecting drops weekly through summer cuts next year's population more than anything else you do.
5. Scrape loose bark in winter Larvae overwinter in cocoons in bark crevices and under loose flakes. A stiff brush over the trunk and main limbs in the dormant season removes a large share of them.

Do not confuse codling moth with sawfly. Sawfly damage appears much earlier, just after petal fall, leaves a ribbon-like scar winding across the skin and a wet, dark, foul-smelling tunnel. Sawfly-damaged fruitlets drop early, and removing them promptly is the control.

Canker, aphids and the disorders that are not pests at all

Canker appears as a sunken, cracked, elliptical patch of dead bark, often centered on a pruning cut, a rubbed branch or a leaf scar. The wood beyond it dies back. There is no treatment for the affected tissue: cut the branch out well below the canker into clean wood, or on a limb too large to lose, pare the dead bark back to a healthy margin. Prune in dry weather, avoid leaving stubs, and remove rubbing branches, because those are the wounds canker enters through.

Aphids curl the new leaves and, in the case of the woolly kind, sit in white cottony masses in bark cracks and around old pruning wounds. Light infestations are dealt with by predators if you have not sprayed them out; heavy ones on shoot tips can be rubbed off or washed off. Persistent woolly aphid is often a sign of a tree carrying old, unhealed wounds.

Two common complaints are not pests. Bitter pit shows as small sunken brown spots under the skin and brown flecks in the flesh, and it is a calcium distribution problem driven by irregular watering, heavy nitrogen and overlarge fruit on a lightly cropping tree. Even watering, restrained feeding and proper thinning fix it. The other is a heavy natural fruit drop in early summer, which is the tree shedding what it cannot carry and is entirely normal.

Knowing when to cut your losses

Some situations are not worth nursing. A limb with canker girdling more than about half its circumference will die anyway, and leaving it keeps a spore source in the middle of the tree, so take it off. The same judgement applies to fruit: once a codling grub is inside an apple there is no saving it, so pick it off and remove it from the site rather than dropping it under the tree.

For a badly neglected tree with several problems at once, work in the same order every time: remove dead and cankered wood, clear everything off the ground, open the canopy over two or three winters, and only then judge what remains. Most of the pest pressure disappears with the hygiene.

Keep a note of the date of the first codling moth catch, the date you first saw scab lesions, and which trees and which sides were affected. Those dates repeat within about a week from year to year in the same garden, and problems cluster: a tree at the shaded end of a row that scabs every year is telling you something about airflow that no spray will change.