Walnut blight — symptoms and treatment

Severity: high

What is Walnut blight

Walnut blight is caused by the bacterium Xanthomonas arboricola pv. juglandis. It is the most important disease of walnuts. Unlike the leaf diseases of walnut, it goes straight for the crop: catkins, young nuts and shoots are all attacked, and infected nuts turn black and shrivel rather than filling. Because it is bacterial, no fungicide will cure it and no spray rescues a nut once the bacteria are inside — the whole strategy is to protect the developing crop before infection happens.

Symptoms

The disease announces itself with black, water-soaked spots. That greasy, translucent look, as though the tissue had been soaked through, is the bacterial signature and separates blight from the dry brown spotting of walnut anthracnose. The spots appear on the catkins and on young nuts, and infected nuts turn black and shrivel, often dropping rather than filling out. Nuts struck later may hang on but carry a blackened, sunken lesion, and the damage can reach through the husk to spoil the kernel. Leaf spots appear as well, and shoot dieback follows where the bacteria establish in young wood, killing back the current season's growth. The catkins matter more than their size suggests: they are an early infection court, and blighted catkins seed bacteria into the tree just as the nuts are setting.

• Early: black water-soaked spots on catkins; leaf spots
• Progressing: water-soaked spots on young nuts; nuts turn black
• Advanced: nuts shrivel and drop; shoot dieback

Causes and conditions

The bacteria overwinter in infected buds, blighted twigs and old catkins, and emerge in spring to be spread through the tree by rain splash. Water is the whole story. Bacteria need free water to move and a wound or natural opening to enter, so the disease tracks rainfall almost exactly: rainy springs bring severe outbreaks, dry springs suppress it. The vulnerable window opens at catkin emergence and stays open through bloom and nut set, when tissue is young and easily invaded, and pollen from infected catkins can carry bacteria to receptive flowers. Wind-driven rain both spreads bacteria and forces them into tissue, which is why a stormy spring during bloom is the worst case.

Treatment

Copper hydroxide — chemical

Apply copper sprays at catkin emergence, full bloom, and post-bloom. Critical in rainy springs. Those three timings are not arbitrary — they bracket the period when catkins, flowers and newly set nuts are exposed, and copper works by sitting on the tissue surface as a protectant barrier before the bacteria arrive. A spray applied to already-blackened nuts does nothing for them; it only protects what is still clean. Coverage of the developing nuts and catkins is what determines whether the programme works. Pre-harvest interval: 21 days. Pesticide registrations vary by country — check local approval before use.

Prevention

• Prune out blighted shoots and dead twigs, and remove old catkins and mummified nuts, to cut the overwintering reservoir
• Prune for an open canopy that dries quickly after rain
• Avoid overhead irrigation, which recreates the rain splash the bacteria depend on
• Plant later-leafing cultivars where available, so the vulnerable stage misses the wettest weather
• Disinfect pruning tools between trees and prune in dry conditions

Frequently asked questions

Can I still eat the nuts? Nuts with black, shrivelled lesions are not worth eating — the damage often reaches the kernel. Clean nuts from the same tree are fine.

When should I treat? At catkin emergence, full bloom and post-bloom. Sprays must go on before infection, and they matter most in a rainy spring.

Will a fungicide work? No. This is a bacterial disease, so fungicides aimed at fungi do not touch it. Copper is the material used, and it is protective rather than curative.