Kiwifruit canker causes red spots on leaves and spots on trunks leading to dieback: how to prevent and control it
Kiwifruit canker is a bacterial disease with hidden, explosive and devastating characteristics. The disease cannot be diagnosed before external symptoms appear, and once symptoms appear, the losses cannot be made up. In mild cases, branches die back and spots form on the trunk; in severe cases, the kiwifruit tree dies.
Kiwifruit canker
(1) Disease symptoms:
It mainly damages leaves, fruit and canes, seriously affecting kiwifruit yield and quality. The disease starts from young buds on stems and canes, lenticels, leaf scars and branch forks. It is initially water-soaked, then the spots enlarge and darken, the cortex and xylem separate, and when pressed by hand it feels soft and loose. In the later stage, the affected cortex cracks in longitudinal lines, exuding white mucus that later turns reddish brown. The spots expand around the stem; when the diseased stem is cut open with a knife, the cortex and pith turn brown, and the pith is full of milky white bacterial ooze. The branches and leaves above the affected stem and canes die. If the base is affected, after the upper branches die, new shoots can sprout near the ground or from the rootstock. When leaves are affected, small red spots appear, surrounded by an indistinct yellow halo, later expanding into irregular dark green spots 2-3 mm in size; the leaf color is dark green, the yellow halo is obvious and about 2-5 mm wide. Under humid conditions, they rapidly expand into water-soaked spots, and the spots become angular due to restriction by leaf veins.
(2) Transmission routes and conditions for disease development:
Long-distance spread of kiwifruit canker occurs mainly through seedlings, scions and the like. Seedlings are more susceptible than adult trees; the older the tree, the milder the disease. Low temperature and high humidity favor disease development. In spring, when the average temperature is 10-14°C, if there are strong winds and rain or continuous high humidity and rainy days, the disease easily becomes epidemic. The new shoot growth period is the peak period for canker.
(3) Control measures:
1. Strengthen tree vigor and improve soil fertility. Apply more organic fertilizer and improve the soil so that it is loose and fertile, which helps the kiwifruit root system expand and grow deep. Promote formula fertilization. Kiwifruit should bear fruit in real time, with a reasonable load and scientific management, maintaining vigorous tree vigor and improving disease resistance.
2. Cultivate disease-resistant varieties: attach importance to selecting, breeding and planting disease-resistant varieties, gradually eliminate susceptible varieties, and fundamentally improve the resistance of excellent kiwifruit varieties to canker.
3. Strictly prohibit the introduction of seedlings from epidemic areas, and carry out seedling treatment for introduced seedlings. The disinfection method is: use an agricultural streptomycin solution containing 700 units per milliliter with 1% alcohol added as an auxiliary agent, and disinfect for 1.5 hours.
Kiwifruit canker control
4. Reduce the routes of pathogen invasion. Prune in December. Disinfect pruning tools with 90% alcohol, disinfecting once after pruning each plant. Make cuts smooth and flat to reduce large wounds. After winter pruning is completed, promptly spray to "seal the three openings" (fruit stalk openings, leaf stalk openings and pruning cuts), and take diseased dead branches and fallen leaves out of the orchard to be burned in a centralized manner. In combination with pruning, remove diseased and pest-infested branches, diseased leaves, leggy branches, drooping branches, etc. All branches along which bacterial ooze has flowed should be cut off to reduce sources of infection. From the end of February to early-to-mid March is the bleeding period for kiwifruit plants, and pruning should not be done again. During the peak period of spring canker, conduct regular inspections. Once severely infected kiwifruit plants are found, remove and burn them promptly to control the spread of the pathogen.
5. Chemical control. After harvest or before winter, in combination with orchard pruning, generally spray 1-2 times with 3-5 degrees Baumé lime sulfur mixture or 1:1:100 Bordeaux mixture; from the beginning of spring to before bud break, spray 1:1:100 Bordeaux mixture or 50% copper succinate (DT) wettable powder 500 times dilution; after bud break to the end of flowering, spray 50% Jiaruinong (kasugamycin • copper hydroxide) wettable powder 500-800 times dilution and other agents, once every 10 days.
When disease is found, promptly scrape the trunk and apply medicine and spray: first scrape off the diseased spots (take the diseased bark and sawdust out of the orchard and burn them). When approaching healthy bark, disinfect the scraper, then scrape off part of the healthy bark. Then apply agents such as 95% Xijunling original powder 500 times dilution, or 60% Baijuntong 30 times dilution, or 5% Junduqing liquid formulation 50 times dilution to the trunk, applying 4-5 times, once every 7 days. At the same time, spray 4-5 times with agents such as 95% Xijunling original powder 2000 times dilution plus penetrant, or 5% Junduqing liquid formulation 300 times dilution plus penetrant, once every 7 days.
Longitudinal scoring, smearing and spraying control: use a disinfected small knife to make longitudinal cuts on the canker spots on kiwifruit canes, with the cuts larger than the spots, then smear with the agent, smearing 4-5 times, once every 7 days. At the same time, combine with spraying 4-5 times, once every 7 days, using agents such as 95% CT Xijunling original powder 2000 times dilution plus penetrant, or 5% Junduqing liquid formulation 300 times dilution plus penetrant.