Bacterial blight on hazelnut — symptoms and treatment

Severity: medium

What is Bacterial blight

Bacterial blight of hazelnut is caused by Xanthomonas arboricola pv. corylina, a bacterium that infects leaves, shoots, branches and the nuts themselves. It is spread by rain and wind. Unlike a fungus, it cannot force its way into healthy tissue — it needs an opening, which it finds in natural pores, leaf scars and any wound. Severity is rated medium: mature trees usually grow through it, but young trees and nursery stock can be killed by girdling cankers, and heavy infection costs both shoots and saleable nuts.

Symptoms

The most recognisable sign is on the foliage: small dark spots scattered across the leaf, each ringed by a yellow halo where the surrounding tissue has been bleached. From the leaves the infection moves into the shoots, which wilt and die back from the tip, sometimes with the dead leaves still clinging. On young branches, sunken cankers develop and can girdle the wood, killing everything above them. The nuts are not spared — spots appear on the husks and shells, downgrading the crop.

Key signs:

• Early: dark spots on leaves surrounded by yellow halos
• Early: spots on developing nuts
• Advanced: shoot dieback, working back from the tip
• Advanced: cankers on young branches, sometimes girdling them

Bacterial blight can be mistaken for Eastern filbert blight, but that disease produces rows of black stromata on the bark of its cankers. Black bumps mean the other, more serious problem.

Causes and conditions

The bacterium survives from year to year in cankers on branches, in buds and in infected leaf litter. When it rains, bacteria ooze out and splash onto new growth; wind-driven rain carries them further through the orchard. Infection follows wet weather, and it is worst when a wet spell coincides with soft, actively expanding tissue in spring. Wounds are the open door — hail, frost damage, wind rub, pruning cuts and the fresh leaf scars left at leaf fall all give the bacterium entry. Nursery trees and young orchards are the most vulnerable: all young wood, and handled more.

Treatment

There is no spray that cures an infected branch. Management is about protecting new growth during wet weather and cutting out the reservoir of bacteria the orchard carries.

Copper hydroxide — chemical

Bud break. Apply copper at bud break and during wet weather in spring. Copper acts on the plant surface, killing bacteria before they get in, so timing matters more than dose: it must already be on the tissue when the rain arrives. Pre-harvest interval: 21 days. Pesticide registrations vary by country — check local approval before use.

Alongside spraying, prune out cankered and dying branches back into healthy wood and take them out of the orchard. Prune in dry weather, and disinfect tools between trees so the blades are not carrying the bacterium from one to the next.

Prevention

• Plant certified, disease-free nursery stock — infected young trees bring the bacterium in with them.
• Prune only in dry weather, and disinfect pruning tools between trees.
• Remove and destroy cankered branches and fallen infected leaves.
• Protect young trees from wind and hail damage; every wound is an entry point.
• Avoid overhead irrigation, which mimics the rain the bacterium depends on.

Frequently asked questions

Is it contagious to my other trees? Yes. Rain splash and wind-driven rain move the bacterium from cankers to healthy trees, and contaminated pruning tools move it much faster than the weather does.

Can I eat the nuts after spraying? Yes, once the pre-harvest interval has passed — 21 days for copper hydroxide. Spotted husks and shells are a market problem, not a food-safety one.

When should I treat? At bud break, and again during wet weather in spring. Copper is preventive: applied after the spots and dieback appear, it does nothing for tissue already infected.