What are the control methods for citrus canker?

Citrus canker Xanthomonas citri (Hasse) Dowson is a bacterial disease that seriously damages citrus, and has been listed as a plant quarantine object at home and abroad. After plants are infected, the tree vigor weakens, and leaves and fruits drop easily; thus reducing yield and affecting fruit quality. After seedlings and young trees are seriously damaged, the leaves fall off completely and the branches die, hindering the development of citrus production.

[Pathogen and symptoms]

Citrus pests and diseases: The pathogen of this disease is a kind of Xanthomonas, which mainly damages citrus and Poncirus of the Rutaceae family, and can also infect Fortunella. The pathogen can infect the leaves, twigs and fruits of citrus. In the early stage of leaf damage, needle-sized yellow oil-soaked spots appear, which after expansion become round spots, raised on both the front and back sides of the leaf, suberized, rough, with the center cracking in a crater shape, and a yellow halo around the lesion; the damage on twigs and fruits is similar to that on leaves, the lesions on twigs have no yellow halo and often join into irregular shapes, the lesions on green fruits have a yellow halo, and the halo disappears after the fruit ripens.

[Conditions for disease occurrence]

Citrus pests and diseases: The pathogenic bacteria lurk on diseased branches, diseased leaves and diseased fruits, and in the spring rainy season of the following year, with the help of wind and rain, are spread through mutual contact of branches and leaves, invading through wounds, stomata or lenticels. The transport of diseased seedlings can spread the disease over long distances. High temperature and abundant rain are conducive to the epidemic of this disease, and orchards with excessive application of nitrogen fertilizer, many excessive shoots, and improper pruning are also prone to the disease. May to September each year is the peak period of disease occurrence, and generally summer shoots are more severely damaged. In addition, different varieties also differ in disease resistance; oranges are relatively susceptible, pomelos are next, and mandarins are the least affected.

[Control methods]

1. Strict quarantine, prohibiting the transport of seedlings, seeds, etc. from diseased areas.

2. Sterilize contaminated seeds by soaking them in hot water at 55-56°C for 50 minutes.

3. Clean the orchard in winter, prune diseased branches and leaves and burn them in a concentrated manner, and spray 0•8-1 Baume lime sulfur mixture.

4. For young trees, spray once each when the spring, summer and autumn shoots unfold leaves and when the leaves turn green; for fruiting trees, also spray once each 10 days, 30 days and 50 days after flower drop. The agents can be 30% copper oxychloride 700-fold dilution, 72% agricultural streptomycin 4000-fold dilution, or 50% DT wettable powder 500-fold dilution.