According to Liu Zhenkun's research (1982), the trunk of young trees and the big branches of adult trees are generally damaged. Because of the thin cortex, the lesion is easy to penetrate into xylem and regenerate callus around, so it presents spindle-shaped lesion. At first, the affected area was dark gray and slightly swollen with water stains. When you press it with your finger, the liquid with foam flowed out, which smelled of distiller's grains. In the later stage, the diseased tissue turns brown, loses water and sinks, and scattered small black spots appear under the skin. When the lesion surrounds the branch for a week, the branch or the whole tree will die. The trunk and main branches of adult trees are diseased. Because the bark is thick, the phloem often rots, and there are no obvious symptoms outside, only black water flows out along the cracks in the trunk (color picture 60).
Walnut rot is caused by Colletotrichum juglandis. The sexual generation has not yet been discovered. The small black spot on the diseased tissue is the daughter seat. Conidia are embedded in the substrate, protrude from holes outside the substrate, and release orange curly spore horns. The base is dark brown, with one hole and many chambers, irregular shape and obvious neck length. Conidia are unicellular, colorless sausage-shaped, and the size is 3.9 ~ 6.9 μ m× 0.9 ~1.9 μ m.
Every year, the sap flows from early spring to before the trees overwinter, which is a harmful period. During the growing season, conidia can release spore horns one after another, so many infections occur. The incidence peaks in spring and autumn in a year, especially from mid-April to late May.
Walnut cysticercosis is a weak parasitic fungus, which invades from the wound, so the lesions of rotten skin disease all occur in bud marks, lenticels, knots, pruning grafting wounds, mechanical injuries, frostbite and other wounds. In addition, all the factors leading to the decline of tree potential are beneficial to the invasion and harm of walnut. Walnut is generally healthy and harmless, with few and light diseases.
Walnut trees grow in poor soil, poor drainage, high groundwater level and saline-alkali places, and their skins are badly rotten. In the place where walnut trees grow, a large number of vegetative branches and clustered branches are formed, generally frozen branches appear, and the incidence of rotten skin is high. However, if the drainage is good, the soil fertility is good, the growth and development of walnut trees are normal, and the harm of skin rot is light and the incidence rate is low.
At different growth stages, the disease resistance of walnut is obviously different. Although there are knots, bark marks, bud marks and branches at a young age, the development of the disease is inhibited because of its vigorous growth. After entering the fruiting period, due to the consumption of many nutrients, the tree body declines, which is conducive to the development of diseases. Therefore, many high-yield types of walnuts blossom and bear fruit year after year, which often leads to a sharp decline in tree vigor, rapid development of skin rot and death of the whole plant.
Plastic pruning and grafting are mostly due to rotten skin, which reduces the survival rate and preservation rate of walnuts. Generally, the bigger the wound, the more serious the infection. If the adult walnut tree is transplanted and dry-cut, if the wound protection and disinfection measures are not taken, it will cause serious infection and lead to the death of the whole tree. Disinfecting tools and wounds during transplantation can significantly reduce injuries.
Prevention and control methods: (1) Improve management level. The newly planted walnut orchard should be intercropped with forest and farmers, with sufficient base fertilizer and reasonable irrigation. Stop water in time at the late growth stage. Irrigation should be carried out in winter to prevent streaking and physiological drought in early spring. After entering the fruiting period, more organic fertilizer should be applied to enhance the tree vigor and improve the disease resistance. (2) Strengthen prevention and protection to reduce infection. Clean up diseased branches and trees at any time. Paint tree trunks in early spring and late autumn, especially in areas prone to freezing injury. Newly planted young trees should be bandaged and painted for wintering and cold protection. (3) Spray brush protection. You can choose 50% thiophanate methyl and 65% zineb. Newly planted young trees should be brushed in early spring and late autumn, and the concentration of the medicine should be diluted by 50 ~ 100 times, 200 ~ 300 times for grafting wounds and 50 ~ 100 times for healing wounds. Trim the wound and spray it for protection. The concentration is 100 ~ 500 times. (4) Scrape off old skin and remove diseased spots. Scraping the old skin means exposing the new skin slightly, and all the diseased spots that have rotted through the cortex should be scraped off. The method is to cut from the outer edge of the lesion 1 ~ 2 cm to make it spindle-shaped. If healthy tissue is exposed, it should be cut open. Immediately after scraping, brush 50 ~ 100 times of liquid medicine (same as before) and mixed yellow mud liquid to protect the wound. This measure can be carried out throughout the growing season, but early spring is better. Scraping the old skin can eradicate the scar hidden under the cortex, expose the ulcer, fully contact and absorb the liquid medicine, and achieve the expected prevention and treatment effect. Old peeling can also promote the growth and development of trees and restore their vitality. This is an extremely important measure for high yield of fruit trees and should be vigorously promoted.