The Latest Control Methods for Walnut Rot Disease

Walnut rot disease is caused by a fungus, with spring and autumn being the two peak periods of the year, especially from mid-April to late May when the damage is most severe. Generally, walnut trees that are extensively managed, have thin soil layers, poor drainage, insufficient fertilizer and water, weakened tree vigor, or suffer from frost damage and salt damage are susceptible to this disease. Let us take a look at the control methods for walnut rot disease below!

Symptoms of Walnut Rot Disease

Walnut rot disease mainly affects branches and trunks. On the main trunk and lateral branches of young trees, the lesions are initially nearly spindle-shaped, dark gray, water-soaked and swollen. When pressed by hand, a foamy liquid flows out. The diseased bark turns brown and smells of distiller's grains. Later, the diseased bark loses water and sinks, and many small black dots are scattered on the lesions. When humidity is high, orange-red gelatinous substances ooze from the small black dots. In severe cases, the lesions expand, causing the cortex to split longitudinally and discharge black water. In the early stage of infection on the main trunk, symptoms are hidden in the phloem and are not easily seen from the outside. When symptoms become visible, the subcutaneous diseased area has already expanded more than 20 to 30 cm, discharging viscous black water that often smears on the trunk. In the later stage, black water flows out along bark cracks, and after drying it becomes shiny, as if a layer of black lacquer has been painted on.

Occurrence Pattern of Walnut Rot Disease

The pathogen overwinters in the diseased parts as mycelium, stroma, and pycnidia. In the following spring, after the walnut tree sap begins to flow, conidia are produced when suitable conditions for disease development occur. The conidia are spread by wind, rain, or insects, and invade through grafting wounds, injuries, and other openings. After the disease occurs, it gradually expands. Multiple infections can occur during the growing season, with spring and autumn being the two peak periods of the year, especially from mid-April to late May when the damage is most severe. Generally, walnut trees that are extensively managed, have thin soil layers, poor drainage, insufficient fertilizer and water, weakened tree vigor, or suffer from frost damage and salt damage are susceptible to this disease.

Control Methods for Walnut Rot Disease

1. Strengthening tree management and enhancing tree vigor are the basic measures for controlling walnut rot disease. Through deep plowing to improve soil, intertillage and weeding, increased application of organic fertilizer, rational intercropping, timely irrigation and drainage, and rational pruning, the disease resistance of walnut trees can be increased.

2. After harvesting walnuts, in combination with pruning, cut off diseased and pest-infested branches and scrape off diseased bark. At the same time, maintain good sanitation in the walnut orchard, promptly remove diseased branches, dead branches, and diseased branches from the orchard, and burn them together outside the orchard to reduce sources of the pathogen.

3. Whitewash the walnut trunks, especially newly planted young trees, with greater attention to trunk whitewashing in winter and summer to reduce channels for pathogen invasion. The whitewash formula is 12 kg quicklime, 1.5 kg salt, 250 g vegetable oil, 500 g sulfur powder, and 50 kg water.

4. In spring, thoroughly scrape off the lesions until new bark is slightly exposed. The scraping range should be about 0.5 cm wider than the discolored necrotic tissue. Apply 5 to 10 degrees Baume lime sulfur mixture or 1% copper sulfate solution to the scraped wounds, and then burn the scraped bark and other materials together.

5. Spray the canopy before disease onset or in the early stage of disease. You can use 50% carbendazim wettable powder at 800 times dilution, or 75% chlorothalonil at 600 times dilution, or 50% thiophanate-methyl wettable powder at 1000 times dilution. Spray once every 7 days, for 3 consecutive sprays.