Molecular Plant Pathology – Pathogen Profiles

Xanthomonas axonopodis pv.citri: factors affecting successful eradication of citrus canker

James H, Graham1, Tim R. Gottwald2, Jaime Cubero1 and Diann S. Achor1

1 Citrus Research and Education Center, University of Florida, 700 Experiment Station Road, Lake Alfred, FL 33850, USA
2 USDA-ARS, Horticultural Research Laboratory 2001 South Rock Road, Ft. Pierce, FL 34945, USA

Taxonomic status:Bacteria, Proteobacteria, gamma subdivision, Xanthomodales, Xanthomonas group, axonopodis DNA homology group, X. axonopodis pv. citri (Hasse).

Microbiological properties:Gram negative, slender, rodshaped, aerobic, motile by a single polar flagellum, produces slow growing, non-mucoid colonies in culture, ecologically obligate plant parasite.

Host range:Causal agent of Asiatic citrus canker on most Citrus spp. and close relatives of Citrusin the family Rutaceae.

Disease symptoms: Distinctively raised, necrotic lesions on fruits, stems and leaves.

Epidemiology: Bacteria exude from lesions during wet weather and are disseminated by splash dispersal at short range, windblown rain at medium to long range and human assisted movement at all ranges.

Crop loss: Severe infections cause defoliation, blemished fruit, premature fruit drop, die-back of twigs and general debilitation of the tree.

Distribution: Citrus canker is not present in all subtropical to tropical regions of citriculture in the world, so considerable regulatory efforts are expended to prevent the introduction and spread of X. axonopodis pv. citri into areas in the Americas, Australia and elsewhere, with climates conducive to the disease.

Importance: Limited strategies exist for suppression of citrus canker on more susceptible cultivars. Blemished fruit are unmarketable and exposed fruit are restricted in market access. The economic impact of loss of markets is much greater than that from yield and quality reductions of the crop.

Scanning electron microscopy of infections by Xanthomonas axonopodis pv. citri on the abaxial leaf surface of grapefruit (Citrus paradisi). (A) Bacterial egress from a stomatal opening 5 days after infiltration of the leaf mesophyll with 105 cfu/mL. (B) Eruption of infected mesophyll tissues through the epidermis to form a pustule 30 days after inoculation. (C) Bacteria embedded in a matrix of extracellular polysaccharide (EPS) from the surface of the lesion in (B)