What are the control methods for citrus canker?
Symptom characteristics
Damage to leaves, twigs and fruit. The common feature of the symptoms is that all affected parts show corky, raised lesions, with the center of the lesion cracking in a "crater" pattern; the difference is that the corky lesions on leaves penetrate both the front and back of the leaf, with a yellow halo around the lesion, while the yellow halo around lesions on fruit and twigs is absent or inconspicuous.
Pathogen and disease characteristics
The pathogen is a bacterium, called Xanthomonas campestris pv. citri, the pomelo pathovar. The pathogen mainly overwinters latent in diseased leaf, twig and fruit tissues, especially lesions on autumn shoots, which are the main overwintering sites and become the main primary infection source of the disease the following year. In the following spring, when temperature and moisture are suitable, the pathogen oozes from the lesions and spreads through wind and rain, insects, contact between branches and leaves, and farming operations, infecting through stomata, lenticels and wounds. Long-distance spread occurs through the transport of contaminated seedlings, scions and fruit. High temperature and abundant rain, especially in years and seasons with frequent typhoons and heavy rainstorms, favor disease development; under suitable temperature (25-30°C) conditions, rainy days and rainfall are positively correlated with disease incidence; orchards with excessive nitrogen fertilizer, severe citrus leafminer outbreaks, poor summer shoot control, uneven autumn shoot emergence, and mixed variety planting have more severe disease; seedlings and young trees are more severely affected than older trees; sweet oranges are the most susceptible, followed by pomelos and lemons, then mandarins, with tangerines being more resistant, and kumquats being the most resistant; the pathogen only infects young tender tissues at a certain stage of development (after stomata have formed).
Control methods
(1) Strict seedling quarantine and cultivation of disease-free seedlings. Seedlings and scions introduced from elsewhere should be soaked in 700 units/ml streptomycin + 1% alcohol for 30-60 minutes, or in 0.12 mercury bichloride or 0.3% ferrous sulfate for 10 minutes. Disease-free nurseries should be 3 kilometers away from citrus orchards. (2) In winter and spring, combine with pruning, thoroughly clean the orchard, and immediately spray the ground and trees once for protection (30% copper oxychloride or “Kaoshan” suspension concentrate at 800-1000 times dilution). (3) Focus on bud removal and shoot control to reduce disease incidence. For young trees, remove buds and control shoots so that summer and autumn shoots emerge uniformly, facilitating spraying to protect the shoots; for young mature trees, remove buds to control summer shoots and reduce disease incidence. (4) Apply fertilizer rationally, using fertilizer to control shoots; control pests such as citrus leafminer and swallowtail butterflies, and release autumn shoots when leafminer populations are low; in areas with frequent typhoons, establish shelterbelts around orchards, all of which can reduce disease incidence. (5) Spray to protect shoots and fruit. Spray the first time when new shoots are 1.5-3 cm long, and the second time when leaves turn green; for mature fruiting trees, focus on protecting young fruit, spraying once each at 10 days, 30 days and 50 days after petal fall. Choose “Kaoshan” suspension concentrate at 1000-1500 times dilution, or 77% Kocide, 30% copper oxychloride suspension concentrate at 800-1000 times dilution, or agricultural streptomycin 400-800 units/ml + 0.1% alcohol, or Chuanhua-018 wettable powder at 600 times dilution, or 50% Kasumin wettable powder at 800-1000 times dilution, or 0.5-1:2:100 Bordeaux mixture, or 50% amobam aqueous solution at 500-800 times dilution. You can also try 20% Youlüte soluble powder at 400-500 times dilution, or 20% quinone wettable powder at 1000-1500 times dilution. Note: Continuous spraying of Bordeaux mixture can easily induce outbreaks of rust mites and other mite pests, so it should be applied together with acaricides for safety.