Canker in cider orchards

The fungus produces spores in wet weather and they are dispersed by rain splash. They cannot penetrate intact bark, so infection depends on an opening. The largest single opportunity of the year is leaf fall, when every abscised leaf leaves a fresh scar that remains susceptible for a period; rain during leaf fall therefore does a great deal of the year’s infecting.

Other openings matter too. Pruning cuts made in wet weather are entry points, which is the practical argument for pruning in dry conditions rather than simply in winter. Picking wounds, frost cracks, scab lesions, woolly aphid damage and the stubs left by careless pruning all serve. Nursery trees can arrive already infected, with symptoms appearing only after planting, and this is a well-recognised route by which a new orchard starts life with the disease already in it.

Once established, the fungus grows slowly in the bark and cambium, producing a sunken, cracked lesion with concentric ridges of callus where the tree has tried and failed to wall it off. In damp weather the canker produces creamy spore masses, and in autumn small red fruiting bodies, both of which supply the inoculum for the next round of infection on the same tree.

A canker that encircles a shoot or branch girdles it, cutting off the transport of water and photosynthate, and everything distal to it dies. On a young tree a canker on the main stem is fatal. On a mature tree the process is one of attrition: spur by spur, limb by limb, over years and decades, with each dead limb reducing the crop and each dead limb also being a source of further spores.

This is why so many veteran cider orchard trees have the characteristic look they do — a partly hollow trunk carrying a reduced, gappy crown with dead limbs among the living. It is also why old orchards decline in yield rather than failing suddenly, and why an orchard that has gone unmanaged for twenty years typically has substantial canker in it.

Cultivar susceptibility varies widely and is well documented in the pomological literature. Kingston Black, for instance, is repeatedly described as canker-prone, and its reputation as a difficult variety to grow rests substantially on that. Choosing cultivars for a wet site without regard to their canker record is a decision the grower will meet again in fifteen years.

There is no cure. Management is a matter of reducing inoculum and reducing entry. Cutting out cankered wood well below the lesion, in dry weather, and removing it from the orchard rather than leaving it in the alley, is the core operation. Large cuts are painted or protected in some systems; the practice is not universal and its value is debated.

Preventive treatment concentrates on the leaf-fall window and on pruning wounds, and in commercial orchards involves autumn fungicide applications timed to leaf fall. In an unsprayed traditional orchard none of that is available, and management consists of cutting out what can be reached and accepting the rest.

The wider levers are cultural. Drainage matters, because canker is worse on wet sites and on trees under stress. Nutrition matters, because both deficiency and excess vigour increase susceptibility. Cultivar choice matters most of all in a new planting. And in a veteran orchard being kept for habitat rather than yield, a certain amount of canker is part of the process that produces the dead wood the habitat depends on, which is an awkward but honest thing to say about a disease.