Walnut rot disease, also known as blackwater disease, is commonly found in regions such as Henan, Shanxi, Shandong, and Sichuan. It can affect both young and mature walnut trees. Once the trees enter the fruiting stage, improper cultivation practices—such as insufficient fertilization, lack of water, excessive fruit load, and weak tree vitality—can lead to severe damage from this disease. As a result, the tree's health deteriorates, often leading to the death of entire plants.
The symptoms of walnut rot primarily target the bark of the trunk and branches. Depending on the age of the tree and the affected area, the manifestations vary. Initially, lesions appear hidden beneath the bark, commonly referred to as "wet skin." Over time, multiple lesions may merge into large patches, with white mycelium accumulating around them. Black spores may also be visible from the bark. In advanced stages, lesions can grow up to 20–30 cm long, causing the bark to split vertically and release black water, giving the disease its alternative name, "blackwater disease." The affected area appears dark and glossy, like it has been painted with black paint.
When saplings are affected, the lesions typically appear spindle-shaped, dark gray, and water-soaked, with slight swelling. Pressing the area will cause foamy liquid to exude. Small black dots, which are conidia of the pathogen, are scattered across the lesions. Under high humidity, orange gel-like filaments may emerge from these spots, indicating the presence of conidial structures. As the disease progresses, the lesions spread along the trunk and branches, eventually cracking and releasing large amounts of black water. If the lesions encircle the trunk completely, the side branches or even the entire tree may die, especially in young plants.
Shoots are mainly affected on lateral branches of young or two- to three-year-old trees. Infected parts gradually lose their green color, and the cortex and wood quickly dry out. The entire shoot becomes desiccated, with black conidiospores appearing on the lesions.
Microscopic examination reveals that the causal agent is the fungus *Cytospora juglandicola* (family Sphaeropsidales). Under the microscope, the conidia are embedded in cork tissue, multi-chambered, irregularly shaped, dark brown, and have long necks. The spores themselves are colorless and banana-shaped. The pathogen overwinters in infected trees, surviving in the form of mycelium and conidia. When sap flow begins in early spring, spores are dispersed by rain, wind, and insects, entering through wounds and spreading throughout the tree. From April to September, mature conidia are released under high humidity, leading to repeated infections. The disease peaks in spring and autumn, particularly between mid-April and late May.
Walnut trees are generally vulnerable due to poor management practices, poor soil conditions, inadequate drainage, insufficient nutrients, weak tree vigor, or damage from frost or salt. Effective control measures include improving overall orchard management, enhancing tree nutrition, and boosting resistance to cold and disease, which serves as a fundamental strategy for prevention and treatment.
Regular monitoring is essential. As soon as lesions are detected, they should be treated promptly. Disinfectants such as 50% thiophanate-methyl wettable powder diluted 50 times, 50% eutectic wettable powder diluted 50 times, 5–10 degree lime sulfur, or a 1% copper sulfate solution can be used. After disinfection, apply Bordeaux mixture to protect the wound. It’s best to scrape the lesions in a diamond shape, ensuring the scraped area is smooth and clean to promote healing. The removal area should extend at least 1 cm beyond the discolored necrotic tissue. The principle is “scrape early, scrape more, and scrape thoroughly.†Spring is the main period, followed by autumn, but regular inspections and scraping should not be neglected. Collected scrapings should be burned immediately to prevent further spread of the disease.
After harvesting walnuts, combine pruning with cutting away diseased branches, removing infected bark, and burning the debris to reduce the source of infection. During winter, applying white wash to the trunks helps prevent frost and insect damage, which can weaken the tree and increase susceptibility to disease.
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