Integrated Plant Genetics Inc.
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Gainesville, FL 32653

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Citrus Canker In-Depth

I. Economic Hosts

II. Pathogens

III. Disease

IV. Symptoms and Signs:

Citrus canker disease has been historically described as having different "forms", including Asiatic citrus canker, false citrus canker and Mexican lime cancrosis. However, these three "forms" are not distinctive in terms of disease phenotype, and have not been distinguished based upon host symptoms. Although phylogenetically different strains of Xanthomonas cause citrus canker, the symptoms elicited on susceptible hosts are the same, and the bacterial signs are the same. Since the differences are in terms of microbial strain distinctions but not in terms of disease symptoms, use of the term "form" of disease may be inappropriate. Citrus canker disease is described as follows.

On leaves, first appearance is as oily looking, 2 - 10 mm circular spots, usually on the abaxial surface (reflecting stomatal entry following rain dispersal). Lesions are often similarly sized. Later, both epidermal surfaces may become ruptured by tissue hyperplasia induced by the pathogen.

Tell-tale Signs
On leaves, stems, thorns and fruit, circular lesions become raised and blister-like, growing into white or yellow spongy pustules. These pustules then darken and thicken into a light tan to brown corky canker, which is rough to the touch.

On stems, pustules may coalesce to split the epidermis along the stem length, and occasionally girdling of young stems may occur. Older lesions on leaves and fruit tend to have more elevated margins and are at times surrounded by a yellow chlorotic halo (that may disappear) and a sunken center. Sunken craters are especially noticeable on fruits, but the lesions do not penetrate far into the rind. Defoliation and premature abscission of affected fruit occurs on heavily infected trees.

A Closer Look
An essential diagnostic symptom is citrus tissue hyperplasia (excessive mitotic cell divisions), resulting in cankers. In typical natural infestations, all young aerial parts of the Rutaceous host can be conspicuously affected.

Cankers may be present on leaves, stems and fruit of mature trees; canker symptoms on leaves and fruit can be readily obtained in artificial inoculations. If cankers are not present on leaves, stems and fruit of mature trees, or if leaves and fruit of susceptible citrus species do not develop cankers following artificial inoculation, a diagnosis of citrus canker is not indicated. This may seem obvious, but a fungal disease that did not affect fruit was misdiagnosed in Mexico in 1982 as a "form" of citrus canker, and opportunistic leaf spotting xanthomonads that did not cause cankers or affect the fruit of mature trees were misdiagnosed in Florida in 1984 as yet another "form" of citrus canker (refer below).

Occurrence of lesions is seasonal, coinciding with periods of heavy rainfall, high temperatures and growth flushes. These factors generally coincide with early summer in citrus growing regions where rainfall increases as temperatures increase. Citrus canker is unlikely to be found in regions where rainfall decreases as temperatures increase.

Signs of the pathogen should be evident, even in older lesions, as masses of rod-shaped bacteria streaming from the edges of thinly cut lesion sections. If bacterial streaming is not observed, a diagnosis of citrus canker is not indicated.

Some confusion is in the literature because the canker-like disease "mancha foliar", caused by Alternaria limicola [11,16] was found in Mexico in 1982 and reported to be "citrus bacteriosis" and a "form" of citrus canker [12]. A USDA embargo on Mexican citrus was the result, despite the fact that Koch's postulates were inconsistent, lesions were not found on fruit, lesions developed most strongly in the dry season, and bacterial streaming was not observed [18]. There are no Xanthomonas strains known to cause the symptoms described as "citrus bacteriosis".

Subtle Differences
Unlike Citrus Canker, 'nursery leaf spot' is indicated by flat lesions on the leaf, rather than elevated pustules, and an absence of fruit or stem lesions.

Some confusion is in the literature because the opportunistic leaf-spotting disease "citrus bacterial leaf spot", or "nursery leaf spot" caused by X. campestris pv. citrumelo [6,7,9] was found in Florida in 1984 and reported to be a new "form" of citrus canker [15]. A USDA quarantine and grower compensation program was the result, despite the fact that cankers were never observed, and mature trees in groves were not affected. Lesions were found only sporadically in citrus nurseries. By contrast with citrus canker disease, bacterial streaming is not observed from older lesions of citrus bacterial spot, even when caused by the most "aggressive" strains of X. campestris pv. citrumelo.

V. Isolation

VI. Identification

VII. Pathogenicity

VIII. Storage of Organism

IX. Reported Host Range

X. Geographical Range and Spread

XI. Suggested Taxonomic Keys

XII. References

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Copyright © October 2001 Integrated Plant Genetics, Inc. -- All Rights Reserved

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