A Detailed Guide to Identifying and Controlling Walnut Tree Diseases
Pruning walnut trees while the branches are dormant causes wounds on the branches that bleed sap, leading to a serious loss of nutrients from the tree, weakened vigor, and dieback of branches. To prevent sap bleeding caused by pruning walnut trees, you should know the right pruning period and prune at the proper time. In spring, after the branches and leaves of the walnut tree have sprouted and the leaves have unfurled, this...
Branch dieback is a fungal disease that mainly affects the branches and trunk of walnut trees, causing dead branches and dead wood, and in severe cases can cause a large number of branches to die, greatly affecting yield. The disease begins on young, tender short branches and then spreads downward to the main trunk. The cortex of affected branches is initially dark gray, then turns light reddish-brown, and finally becomes dark gray, with many small black dots forming (the fungal acervuli). The leaves on diseased branches gradually turn yellow, fall off, and the branches die. When humidity is high, large numbers of short black column-like structures (the conidia) grow on the diseased area. The pathogen overwinters as mycelium and acervuli in the diseased tissue, and in the following spring, when conditions are suitable, produces conidia that spread via wind, rain, and insects. The disease generally begins in May-June, with July-August being the peak period; branches with weak growth and years with severe frost damage or spring drought show more severe disease.Control Methods
(1) Agricultural control. Apply more organic fertilizer to strengthen tree vigor and improve disease resistance; prune out diseased branches promptly during the growing season and burn them; whitewash the trunk in autumn to prevent frost damage; if the main trunk is affected, scrape off the lesions and disinfect the wounds with 1% copper sulfate solution, then apply a wound protectant.
(2) Chemical control. From June to August, spray with 70% thiophanate-methyl wettable powder at 800-1000 times dilution or mancozeb wettable powder at 400-500 times dilution, spraying once every 10 days for 3-4 consecutive sprays, with good results.
Rot canker, also called black water disease, is a fungal disease that mainly harms the branches and trunk of walnut trees, blocking nutrient transport, causing weakened tree growth and reduced fruiting, and in severe cases killing the entire tree. Symptoms vary with tree age and the affected part. On the main trunk and scaffold branches of young trees, the lesions are initially dark gray, water-soaked, slightly swollen, and the diseased bark turns dark brown with a distiller's grain smell. After the diseased tissue loses water and sinks, many small black dots are scattered over the lesions. The lesions spread vertically and horizontally along the trunk, and in the later stage the bark splits lengthwise and exudes black water; when a lesion encircles the trunk, the main branch or the entire young tree dies. After the main trunk of a mature tree is infected, it is initially hidden in the bark with no obvious external symptoms; when the lesions merge and expand, the bark splits and exudes black water, which shines after drying as if brushed with a layer of black lacquer. After vegetative branches or 2-3 year old lateral branches are infected, the branches gradually lose green color, the cortex and xylem separate, and they lose water and dry up. The pathogen overwinters as mycelium or stroma and pycnidia in the diseased tissue. In the following spring, after the sap begins to flow, conidia are produced and spread by wind, rain, and insects, invading through wounds. The main period for lesion expansion and pathogen activity is from mid-April to late May, with a second disease peak in August, continuing until it stops before overwintering. Generally, walnut trees on poor soil, with poor drainage, under extensive management, or suffering frost damage and drought-related water loss are prone to the disease.
Control Methods
Strengthen cultivation management, deep plow and expand the planting holes, apply more organic fertilizer, rationally apply nitrogen, phosphorus, and potassium fertilizers and trace element fertilizers to raise the tree's nutrient level and enhance cold resistance and disease resistance. Along with pruning, promptly remove diseased branches, dead branches, and dead trees from the orchard and burn them in a centralized manner. Whitewash the trunk before winter to prevent frost damage. In early spring and during the growing season, promptly scrape and treat lesions, then after scraping apply 50% thiophanate-methyl at 50 times dilution or 5-10 degrees Baume lime sulfur mixture to protect the wounds.
Canker is a fungal disease that mainly harms the main trunk, young shoots, and fruit of young walnut trees. It first appears as brown to black, nearly round lesions 0.1-2 cm in diameter, mostly occurring on the trunk and the base of main and lateral branches. Some expand into diamond-shaped or elongated lesions. On young and smooth bark, the lesions are water-soaked or appear as obvious blisters; after rupturing, they exude brown sticky fluid that turns dark brown on exposure to air, after which many small black dots are scattered over the diseased area, and in severe cases the lesions join to form diamond-shaped or elongated patches. When the diseased area expands to encircle a branch or trunk, shoot dieback, branch death, or death of the whole tree occurs. In autumn the epidermis of the diseased area splits. On older bark, the lesions are mostly water-soaked with a dark brown center, and the diseased area rots down to the xylem. After the fruit is infected, brown round spots of varying sizes appear, and the fruit drops early, shrivels, or rots black. The pathogen overwinters as mycelium in the diseased tissue, and in the following spring when the temperature reaches 11.4-15.3°C it begins to be active; in late May when the temperature is around 28°C, conidia form in large numbers and spread via wind and rain, reaching a disease peak; in summer when the temperature exceeds 30°C, the disease basically stops spreading, and after autumn when temperature and humidity are suitable, the disease develops again, but not as severely as in spring. Orchards with poor soil, heavy clay soil, poor drainage, a high water table, or extensive management, no fertilization, no pruning, and many wounds caused by frost damage and insect damage are prone to the disease. In addition, the sunny side of the trunk is more often affected.
Control Methods
Apply more organic fertilizer or plant green manure crops to improve the soil and strengthen tree vigor. Prune and train the trees rationally to improve the canopy structure and increase light use efficiency. Whitewash the trunk in autumn and winter to prevent sunscald and frost damage. During the disease period, spraying 70% thiophanate-methyl at 800 times dilution, or scoring the diseased bark with a knife down to the xylem and then applying 3-5 degrees Baume lime sulfur mixture, both have some effect.