Symptom Identification and Control Methods of Walnut Rot Disease

  1. Distribution and occurrence characteristics. Also known as black water disease, it occurs in Henan, Shanxi, Shandong, Sichuan and other places. The pathogen overwinters in the diseased parts as mycelium or stroma and pycnidia. After the sap of the walnut tree begins to flow the following spring, when suitable conditions for disease development occur, conidia are produced; the conidia are spread by wind and rain or insects, and invade through grafting wounds, pruning and saw cuts, wounds and other sites. After the disease occurs, it gradually expands and does not stop until just before overwintering. Multiple infections can occur during the growing season. Spring and autumn are the two peak periods of disease occurrence in a year, especially from mid-April to late May, when the damage is most severe. Generally, walnut trees with extensive management, thin soil, poor drainage, insufficient fertilizer and water, weakened tree vigor, or those suffering from frost damage and saline-alkali damage are susceptible to this disease. Whenever air humidity is high, multiple infections and damage can occur, until infection stops just before overwintering.

  2. Pathogen and damage symptoms. It is a fungal disease caused by Cytospora juglandina. The incidence rate of severely affected walnut trees can reach more than 80%. The large branches of diseased trees gradually die, and in severe cases the entire tree dies.

  It mainly damages the bark of branches and trunks. Because of differences in tree age and infected parts, the disease symptoms also differ. After the main trunk of a large tree is infected, the lesions are initially hidden within the cortex, commonly known as "wet sac bark." Sometimes multiple lesions join together into large patches, with large amounts of white mycelium gathering around them and black mucus overflowing from within the cortex layer. In the later stage of disease development, lesions can expand to 20-30 cm in length. The bark splits longitudinally, and black water flows out along the cracks in the bark and becomes shiny after drying. After the main trunk and lateral branches of young trees are damaged, the lesions are initially nearly spindle-shaped, dark gray, water-soaked, and slightly swollen. Pressing the diseased part with a finger causes liquid with bubbles to flow out, with a smell of distiller's grains. Many small black dots, which are the pathogen's pycnidia, are scattered on the lesions. When air humidity is high, orange-red gelatinous filamentous cirrhi of conidia emerge from the small black dots. The lesions develop longitudinally and transversely along the trunk; in the later stage, the cortex of the lesions splits longitudinally, and large amounts of black water flow out. When the lesion encircles the trunk for one full circumference, it causes the lateral branches of young trees or the entire plant to die. Damage to branches mainly occurs on vegetative branches or 2-3 year old lateral branches. The infected parts gradually lose their green color, the cortex separates from the wood and rapidly loses water, causing the entire branch to dry up, with pycnidia appearing as scattered small black dots on the lesions.

  3. Control methods

  ① Scraping and treating lesions. This is generally carried out in early spring, and can also be done at any time during the growing season when lesions are found. After scraping, apply 50% thiophanate-methyl wettable powder at 50 times dilution, or 50% tuzet wettable powder at 50 times dilution, or 5-10 degrees Baume lime sulfur mixture, or 1% copper sulfate solution for disinfection, then apply Bordeaux mixture to protect the wound and scar. It is best to scrape the lesion into a spindle shape, and the scraping wound should be smooth and flat to facilitate healing. The scraping range for the lesion scar should extend about 1 cm beyond the discolored necrotic tissue.

  ② After harvest, in combination with pruning, cut off diseased and pest-infested branches, scrape off diseased bark, collect and burn them, and reduce sources of pathogen infection. In winter, whitewash the trunk to prevent rot disease caused by frost damage and insect damage.

  ③ Chemical control. Use Fuxing at 1,000 times dilution + Kangfubao at 100 times dilution for wound disinfection. Apply the first time in March-April, the second time in July-August, and the third time after harvest, which can effectively prevent the occurrence of rot disease. Commonly used disinfectants also include Junbiqing at 100-150 times dilution, Junduqing at 30-50 times dilution, etc., with good results.