Walnut rot disease, also known as blackwater disease, is commonly found in regions such as Henan, Shanxi, Shandong, and Sichuan. It affects 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 vigor—can lead to severe damage caused by the disease. This results in reduced productivity and even the death of entire plants.
The symptoms of the disease primarily affect the bark of the trunk and branches. Depending on the age of the tree and the location of the infection, the signs may vary. Initially, lesions appear hidden within the bark, often referred to as "wet skin." Over time, multiple lesions can merge into large patches, with white mycelium forming around them and black spores emerging from the bark. In advanced stages, the lesions can grow up to 20–30 cm long, causing the bark to crack vertically and release black water, giving the disease its name. The affected area appears dark, like it has been painted with black paint.
On saplings, when the trunk or lateral branches are infected, the lesions tend to be spindle-shaped, dark gray, and water-soaked, with slight swelling. Pressing on the area releases a foamy liquid. Black dots are scattered across the lesions, which are conidia of the pathogen. Under high humidity, orange gel-like filaments emerge from these spots, indicating the presence of conidial structures. As the disease progresses, the lesions expand and eventually crack, releasing large amounts of black water. If the lesions encircle the trunk, the tree may die within a week.
Shoots are mainly affected on the side branches of young or 2- to 3-year-old trees. Infected areas gradually lose their green color, and the cortex and wood quickly dry out, leading to the death of the entire shoot. Black conidia are visible on the affected areas.
Microscopic examination reveals that the pathogen is *Cytospora juglandicola*, a fungus belonging to the genus *Sphaeropsidales*. The conidia are buried beneath the cork layer, multi-chambered, irregularly shaped, dark brown, and have a long neck. The spores are colorless and banana-shaped. The pathogen overwinters in infected trees, surviving through mycelium and conidia. When sap begins to flow in early spring, spores are spread by rain, wind, and insects, entering through wounds and spreading further. From April to September, under high humidity, mature conidia are released, leading to repeated infections until they return to dormancy. Spring and autumn are peak periods, especially between mid-April and late May.
Walnut trees are generally vulnerable due to poor management, poor soil conditions, inadequate drainage, insufficient nutrients, weak tree health, or damage from frost or salt. To manage this disease effectively:
- Improve overall orchard management, enhance tree nutrition, and strengthen tree vitality. This is the fundamental step in prevention and treatment.
- Regularly inspect trees and treat lesions promptly. Disinfect using 50% thiophanate-methyl wettable powder diluted 50 times, 50% eutectic wettable powder at 50 times, or 5–10 degree lime sulfur, or a 1% copper sulfate solution. Apply Bordeaux mixture to protect the wound. It's best to scrape the lesion in a diamond shape, ensuring smooth edges for faster healing. Remove diseased tissue beyond 1 cm from the discolored area. Scrape early, thoroughly, and frequently. Focus on spring, followed by autumn, but maintain regular checks throughout the year. Collect and burn all removed parts to prevent reinfection.
- After harvesting, combine pruning with the removal of pests and diseased branches. Clean and burn any infected bark to reduce the source of pathogens.
- In winter, apply white wash to the trunks to prevent frost and insect damage.
By implementing these measures, growers can significantly reduce the risk of walnut rot and maintain healthier, more productive orchards.
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