Walnut Anthracnose Disease

One of the most important fungal diseases (Gnomonia leptostyla) becomes widespread due to increased humidity and leads to reduced tree growth and general weakness over several years. Symptoms of this disease include the appearance of brown spots on the underside of leaves, leaf yellowing, and browning of leaf margins. Under suitable temperature and humidity conditions, the pathogen spreads and, in addition to leaves, attacks petioles and fruits, causing fruit drop, black wrinkled spots, and hollow fruits.

Control and Treatment of Walnut Anthracnose

The use of resistant cultivars, collecting and burning infected plant parts such as fallen leaves and fruits from the ground surface, and carrying out two spraying stages—first in mid-autumn and second before bud swelling—using copper-based compounds such as Bordeaux mixture and copper oxychloride are recommended. Spring spraying at the hazelnut-sized fruit stage, after catkin drop, and post-harvest spraying are also advised. In addition, fertilization with resistance-enhancing fertilizers such as potassium, calcium-boron, and phosphorus-potassium fertilizers plays an important role in disease control.

Walnut Blight Disease (Walnut Kernel Rot)

The causal agent of walnut blight is a bacterium (Xanthomonas arboricola), which is the main cause of walnut kernel blackening and rot. It is one of the most serious leaf and fruit diseases and is very common in temperate regions with rainy springs.

Symptoms of walnut blight appear on leaves, fruits, branches, and buds as small spots on leaves that gradually expand and turn black. When the fruit reaches the size of a pea, symptoms appear as black spots at the blossom end, which gradually enlarge, become sunken, crack, and eventually cause the entire shell and kernel to turn black and shriveled.

The pathogen overwinters on fallen leaves and fruits on the soil surface and spreads to other plants through rain and insects, causing severe damage in walnut orchards.

Control and Treatment of Walnut Blight

The best approach is the use of resistance-enhancing fertilizers such as amino-potassium and phosphorus-potassium during the tree’s spring growth stages, along with two spraying applications using Bordeaux mixture and copper oxychloride. The best time for fungicide application in walnut trees is before bud break in late winter (late March) and again in autumn after leaf fall. An additional spraying after flowering and catkin drop is also recommended.

Walnut Tree Pests

Leopard Moth (Wood-Boring Worm) in Walnut

The leopard moth (Zeuzera pyrina) is one of the most important pests of fruit trees, especially walnut trees. It is a wood-boring pest commonly found in regions with water scarcity and drought. This pest has many hosts, including both fruit-bearing and non-fruit-bearing trees, but it is most frequently observed on walnut, apple, pear, and quince trees.

Biologically, it is a moth with white wings and black spots and goes through several life stages including egg, larva, pupa, and adult insect. The larvae, which are the destructive stage, emerge from eggs from late May to September, enter one-year-old branches, and begin feeding on wood tissue and vascular bundles. Gradually, they move into larger branches and tree trunks. In many regions, one generation of this pest may take up to two years to complete.

Damage Symptoms

Damage to vascular tissues results in symptoms such as dieback of branch tips, semi-dry green shoots, frass near buds, holes on the trunk and thick branches, and the presence of sawdust-like material.

Control Methods

The best way to control this pest is to reduce the likelihood of infestation by ensuring regular irrigation and strengthening trees through organic, nitrogen, and potassium fertilizers.

Pruning infected branches and burning them, inserting wires into entry holes to kill larvae, using poisoned baits such as Zeuzeran paste, placing cotton soaked with insecticide (e.g., chlorpyrifos) inside holes, and injecting insecticide into the holes are among the recommended control methods.

Since most of the pest’s life cycle occurs inside the trunk, separate spraying is not recommended. Spraying should be carried out simultaneously with treatments for other pests such as codling moth using insecticides like chlorpyrifos or phosalone. Installing delta traps containing pheromones to attract male insects for monitoring and control is the best control method. Typically, 5 to 10 traps per hectare should be installed at a height of 1.5 meters.

Walnut Gall Mite

The walnut gall mite (Eriophyes tristriatus) and the felt or blister mite (Eriophyes erineus) are very similar and are often mistaken for one another by farmers and experts. The gall mite damages leaves and fruits, while the felt mite mainly affects walnut leaves.

The first symptoms of gall mite infestation are gall-like swellings on walnut leaves, which turn green with rising temperatures from May and are mostly seen along leaf veins. From June to August, as temperatures rise, gall formation increases and mite populations inside the galls grow rapidly. This pest is mostly observed in the middle canopy of walnut trees older than 15 years and may also cause galls on fruits. The presence of galls reduces photosynthesis, causes yellowing, smaller leaves and fruits, leaf curling, and fruit drop.

Control Methods

Due to the presence of beneficial insects in spring and summer, the best time to use effective miticides is in autumn and winter, when gall mite populations emerge from the galls (from mid-October onward). Botanical miticides such as Bio-mite or low-risk organic miticides such as Nissorun or Kanemite can be applied once or twice at 15-day intervals.

In addition to spraying, collecting and burying infected fallen leaves during winter and applying miticides such as bromopropylate and propargite at a concentration of 1‰ in combination with horticultural oil (winter oil) in late winter are recommended for controlling both gall and felt mites.

Conclusion

In this article, important and destructive walnut pests—including walnut gall mite and walnut felt mite, the leopard moth pest, walnut blight, and walnut anthracnose—as well as the best control and treatment methods for various walnut tree pests and diseases, have been introduced.