Bacterial canker on kiwi — symptoms and treatment

Severity: high

What is Bacterial canker

Bacterial canker of kiwi is caused by Pseudomonas syringae pv. actinidiae (Psa). It has devastated kiwi orchards worldwide, forcing growers to pull out and replant with different varieties. It is rated high severity for good reason: Psa is not a leaf-spot nuisance but a systemic bacterium that gets into the vascular system of the vine, and once it reaches the trunk and main leaders, the vine is lost. It attacks leaves, canes, trunks and flower buds.

Symptoms

The first thing most growers see is on the leaves: small angular spots, reddish-brown and often ringed by a bright yellow halo, frequently limited by the veins. The diagnostic symptom, though, is on the wood. Infected canes exude ooze — milky white at first, turning reddish as it ages — seeping from cracks, buds and pruning wounds. Behind the ooze the cane dies back, the wood beneath the bark stains, and if the bacterium reaches the trunk the whole vine collapses.

Key signs:

• Early: reddish-brown angular leaf spots with yellow halos
• Early: wilting or browning of flower buds before opening
• Advanced: milky white to reddish ooze on canes, especially in spring
• Advanced: cane dieback, staining under the bark, vine death

Causes and conditions

Psa spreads in rain and wind-driven rain, in irrigation water, on pruning tools, on hands and clothing, and in infected planting material — which is how it crossed continents. It enters through wounds and natural openings: pruning cuts, leaf scars at leaf fall, frost cracks, hail damage and wind rub. The bacterium favours cool, wet conditions, so autumn rains, mild wet winters and cool wet springs are its seasons; it is most aggressive when a wet spell coincides with fresh wounds. Stressed vines succumb faster.

Treatment

There is no cure. Once Psa is systemic, management means removing infected wood and protecting what is left.

Pruning and sanitation — cultural

Throughout season. Remove and destroy infected canes. Use resistant varieties (e.g., Actinidia chinensis 'Zesy002'). Sterilize pruning tools. Cut well back into clean, unstained wood, prune in dry weather, and disinfect between every cut on infected material — contaminated secateurs will inoculate every vine down the row.

Copper hydroxide — chemical

Autumn and spring. Apply copper sprays in autumn at leaf fall and spring at bud break. Repeat during prolonged wet weather. Those timings target the moments the vine is most exposed: fresh leaf scars in autumn, soft new tissue in spring. Copper is protective only — it kills bacteria on the surface and does nothing once they are inside the vine. Pre-harvest interval: 21 days. Pesticide registrations vary by country — check local approval before use.

Prevention

• Plant resistant varieties and certified, Psa-free material — the foundation, not the fallback.
• Sterilize pruning tools between vines, every time.
• Prune in dry weather, never during or just before a wet spell.
• Shelter the orchard from wind to reduce wind-driven rain and wind-rub wounds.
• Remove and destroy infected canes and prunings — never leave them on the orchard floor.
• Avoid overhead irrigation and keep vines free of waterlogging and other stress.

Frequently asked questions

Is it contagious to my other kiwi vines? Very. Psa moves in rain, wind and irrigation water, and fastest of all on unsterilised pruning tools. Work through clean vines first and infected ones last.

Can I save an infected vine? If the infection is confined to canes, cut them out well below the visible damage and keep watching. Once ooze and staining reach the trunk or main leaders the vine will not recover — remove it and replant with a resistant variety.

Can I eat the fruit after spraying? Yes, after the pre-harvest interval — 21 days for copper hydroxide. The disease affects the vine and the crop load rather than making the fruit unsafe.