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Symptoms and Signs

Citrus canker can be a serious disease where rainfall and warm temperatures are frequent during periods of shoot emergence and early fruit development. This is especially the case where tropical storms are prevalent. Citrus canker is mostly a leaf-spotting and fruit rind-blemishing disease, but when conditions are highly favorable for infection, infections cause defoliation (Figure 2), shoot dieback, and fruit drop.

Figure 2

Leaf Lesions: Citrus canker lesions start as pinpoint spots and attain a maximum size of 2 to 10 mm diameter (Figure 3). 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 (Figure 4). 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 (Figure 5). A more reliable diagnostic symptom of citrus canker is the water-soaked margin that develops around the necrotic tissue (Figure 6), which is easily detected with transmitted light.

Figure 3	Figure 4
Figure 5	Figure 6

Fruit and Stem Lesions: Citrus canker lesions on fruit (Figure 7) and stems (Figure 8) 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 (Figure 9). 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 (Figure 10). On stems, lesions can remain viable for several seasons. Thus, stem lesions can support long-term survival of the bacteria.

Figure 7	Figure 8
Figure 9	Figure 10

Leafminer Interaction: The Asian leafminer (Phyllocnistis citrella) can infest leaves, stems, and fruit and greatly increase the number of individual lesions which quickly coalesce and form large irregular shaped lesions that follow the outlines of the feeding galleries (Figure 11). Leafminers feed on the epidermis just below the leaf cuticle. Numerous cracks occur in the cuticle covering leafminer galleries providing means for bacteria to penetrate directly into the palisade parenchyma and spongy mesophyll which are highly susceptible to infection. Citrus foliar wounds normally callus within 1-2 days, however, the extensive wounds composed of the entire leafminer feeding galleries do not callus for 10-12 days, greatly extending the period of susceptibility of galleries to infection. Leafminer infestations can be very prevalent and severe producing hundreds if not thousands of potential infection courts on individual trees. When bacterial dispersal events occur in the presence of the leafminer, not only is inoculum production greatly exacerbated, but so is the potential for infection over the entire dispersal range.

Figure 11

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