Bacterial canker of stone fruit

Symptoms

Sunken, necrotic wounds (cankers) form on the trunk and branches, from which – especially in spring – amber gum oozes (gummosis). The bark over the wounds dies and cracks, and infected buds and short shoots fail to break or die in spring, so whole sections of the crown dry out. Small, brown, watery spots may appear on the leaves and fall out of the blade ('shot-hole'). The disease affects stone-fruit trees – sour cherry, sweet cherry, plum, peach and apricot – and when it girdles a branch it kills it. The cause is two bacterial pathovars — Pseudomonas syringae pv. syringae and pv. morsprunorum — with similar symptoms that cannot be told apart with the naked eye. A characteristic early symptom is the dying of buds and flowers in spring (so-called blossom blast): buds turn brown and fail to open, and individual branches suddenly wilt and dry out, holding their browned leaves for a long time. The gum oozing from the cankers is glassy to amber and appears most abundantly in spring and early summer, after the sap starts to flow; in high summer the ooze usually stops. When the bark is cut at the edge of a canker, the tissue beneath is often watery, rusty-brown, with a sour, fermenting smell (typical of pv. syringae). The leaf spots are at first small, angular, watery and surrounded by a lighter halo; their dead centres crumble away, producing a 'shot-hole' effect. The symptoms are easily confused with frost damage to the bark and with fungal shoot dieback (brown rot) and fungal shot-hole of the leaves (Wilsonomyces carpophilus) — the key then is the season of peak activity and the presence of fungal sporulation. The disease intensifies in cool, wet weather in spring and autumn; in summer, in warmth and drought, the bacterium is least active.

Treatment

There is no effective chemical control after infection – the basis is cutting out infected shoots and branches with a margin of healthy wood. Pruning is done in dry weather in summer, when bacterial activity is lowest and wounds heal quickly, not in winter or early spring. Tools are disinfected between cuts and large wounds are protected; heavily infected, girdled trees are removed. In practice pruning is carried out in high summer (July–August), in dry weather and with a rain-free forecast, removing the branch at least 15–30 cm below the visible edge of the canker, down to healthy, light-coloured wood. Tools are disinfected before each successive cut (for example with 70 percent alcohol or a hypochlorite solution), and the removed material is taken out of the orchard and destroyed — dried cankers remain a source of bacteria for the following season. Larger wounds are worth protecting with a wound-sealing product, guarding them against drying out and being recolonised. Copper products act only to limit and prevent — they do not 'cure' an existing canker, and some strains have developed tolerance to copper, so they should not be treated as a remedy. Trees with the trunk or leader girdled by a canker around the whole circumference cannot be saved and must be removed so they do not infect neighbouring trees.

Prevention

The key is limiting the wounds through which the bacterium enters – avoiding pruning in wet and frosty periods and protecting trunks from frost and sun damage (whitewashing). Plant healthy material on airy, non-flooded sites, choose less susceptible varieties and rootstocks and keep the trees in good condition with balanced fertilisation. Copper treatments in autumn (at leaf fall) can reduce the source of infection. The bacterium overwinters in cankers, in buds and on the surface of the tree, and spreads with raindrops and on tools; the autumn scars left by fallen leaves and micro-injuries are the main gateways for entry — hence the importance of a copper spray during leaf fall, and in regions of heavy infection also at bud swell. Whitewashing trunks with lime from autumn onward reduces bark cracking from frost and winter sun, which opens the way to infection. It is worth planting trees on less susceptible rootstocks and choosing cultivars with higher tolerance, from certified, healthy nursery material. Excessive, late nitrogen fertilisation is avoided — lush, unlignified shoots are more susceptible — and good drainage and airy spacing are provided so that the crown dries quickly. Formative and sanitary pruning is planned for the dry summer period, never on cool, wet days in autumn and early spring.

How to tell it apart from look-alikes

Control — organic and chemical methods

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