Citrus canker

Xanthomonas axonopodis

Leaf Lesions: Citrus canker lesions start as pinpoint spots and attain a maximum size of 2 to 10 mm diameter. The eventual size of the lesions depends mainly on the age of the host tissue at the time of infection and on the citrus cultivar. Lesions become visible about 7 to 10 days after infection on the underside of leaves and soon thereafter on the upper surface. The young lesions are raised or ‘pustular’ on both surfaces of the leaf, but particularly on the lower leaf surface. The pustules eventually become corky and crateriform with a raised margin and sunken center. A characteristic symptom of the disease on leaves is the yellow halo that surrounds lesions. A more reliable diagnostic symptom of citrus canker is the water-soaked margin that develops around the necrotic tissue, which is easily detected with transmitted light.

Fruit and Stem Lesions: Citrus canker lesions on fruit and stems extend to 1 mm in depth, and are superficially similar to those on leaves. On fruit, the lesions can vary in size because the rind is susceptible for a longer time than for leaves and more than one infection cycle can occur. Infection of fruit may cause premature fruit drop but if the fruit remain on the tree until maturity such fruit have reduced fresh fruit marketability. Usually the internal quality of fruit is not affected, but occasionally individual lesions penetrate the rind deeply enough to expose the interior of the fruit to secondary infection by decay organisms. On stems, lesions can remain viable for several seasons. Thus, stem lesions can support long-term survival of the bacteria.

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