Occurrence Characteristics and Control Methods of Walnut Canker

  1. Distribution and occurrence characteristics. It is distributed in most walnut-producing areas. The pathogen mainly overwinters as mycelium within the diseased tissue of the current year. In April of the following year, when the temperature rises to 11.4-15.3°C, it begins to become active. In late May, conidia are formed in large numbers and spread by wind, rain, and other means, mostly invading through wounds (frost injury, sunscald, mechanical injury, or insect injury, etc.), and the disease reaches its peak. In late June, the spread of the disease basically stops. After autumn begins, when temperature and humidity are suitable, the disease develops again, but not as severely as in spring. The incubation period of the disease is 15-60 days. In areas with poor soil, heavy clay texture, poor drainage, high groundwater levels, and poor tree growth, the disease occurs commonly and severely.

  2. Pathogen and damage symptoms. It is a fungal disease that mainly damages the main trunk, young branches, and fruit of young walnut trees. It can cause premature fruit drop, reduce fruit quality and yield, and in severe cases cause weakened plant growth, dead branches, or death of the entire plant.

  It damages the trunk and main and lateral branches of walnut seedlings and large trees, mostly occurring within 0.5-1.0 m of the trunk base. Initially, it appears as brown to black nearly circular lesions, 0.1-2 cm in diameter, some of which 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 upon exposure to air; afterward, many small black dots are scattered over the diseased area. In severe cases, the lesions connect to form diamond-shaped or elongated patches, soak into the surrounding tissue, and make the entire lesion appear water-soaked, dark brown in the center and light brown around the edges, with no obvious margin. In the later stage, the lesions shrivel and sink, with a small longitudinal crack in the center, on which many small black dots are scattered, which are the pathogen's conidiomata. The phloem and inner bark of diseased trees rot and necrose, appearing brown or dark brown, and the rotten area can sometimes extend deep into the xylem. After the fruit is affected, brown round spots of varying sizes appear, and the fruit drops early, shrivels, or turns black and rots.

  3. Control methods

  ① Prevent drought and drain waterlogging, dig ditches for drainage, and lower the groundwater level.

  ② Avoid mixed planting with susceptible Chinese wingnut, black locust, or poplar, etc., to prevent cross-infection.

  ③ Whitewash the tree trunks. Whitewash formula: quicklime 5 kg, salt 2 kg, oil 0.1 kg, water 20 L.

  ④ Chemical control. In April-May and August, spray the branches and trunk once each with 50% thiophanate-methyl wettable powder at 200 times dilution or antibiotic 402 emulsifiable concentrate at 200 times dilution. Use a knife to scrape off or score the diseased bark, reaching deep into the xylem, then apply 3-5 degrees Baume lime-sulfur mixture or 2% copper sulfate solution, 10% alkaline water (sodium carbonate), 10% carbendazim at 50 times dilution, or other such agents.