Xanthomonas citri

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Xanthomonas citri negatively stained (click on image to enlarge it)
Source: Florida Department of Agriculture and Consumer Services - IPM Images
symptoms of citrus canker on citrus fruit (click on image to enlarge it)
Source: Florida Department of Agriculture and Consumer Services - IPM Images

Xanthomonas citri (ex Hasse 1915) Gabriel et al. 1989

This bacterium is wide-spread, it causes citrus canker and a number of diseases on other crops. Several subspecies and pathovars have been described and the following are covered on separate pages:
Xanthomonas citri subsp. malvacearum - (bacterial blight of cotton)
Xanthomonas citri pv. mangiferaeindicae - (mango bacterial canker)
Xanthomonas citri pv. viticola - (bacterial canker of grapevine)

Another form also infects beans. It has been described previously as "Xanthomonas fuscans subsp. fuscans" or "Xanthomonas campestris pv. phaseoli var. fuscans", but belongs to Xanthomonas citri (Constantin et al., 2016).


Xanthomonas citri subsp. citri (ex Hasse 1915) Gabriel et al. 1989 - (citrus canker)

Economically, citrus-infecting forms of Xanthomonas citri are the most important and widespread ones, especially the Asian form (pathotype A). This form is apparently native to tropical and subtropical parts of Asia, but absent and a quarantine pathogen in Europe. Infected fruits have a reduced market value or may drop prematurely.

The bacterium overwinters in lesions of leaves and shoots and causes new infections under warm and moist conditions. New infections start by bacteria entering through stomata or small wounds. These start as pin-point spots on the lower leaf surface around 1 week after the infection. As the lesions enlarge they become tan or brown, corky and finally crater-like with a raised margin and a sunken centre. Cankers also appear on shoots and trunks. Short- and medium distance spread occurs through rain splashes and strong winds which can carry the bacteria up to 50 km under extreme conditions. Long-distance spread involves the movement of infected planting material or infected fruits.

Vernacular names
• Deutsch: Bakterienkrebs der Zitrusarten
• English: citrus canker
Asiatic citrus canker
bacterial canker of citrus
• Español: cáncer de los agrios
• Français: chancre des citrus
• Português: cancro cítrico

Control is practised by planting resistant cultivars and by applying copper bactericides. The disease has been eradicated in some areas. Windbreaks have been also suggested as a means of disrupting the transmission through rain splashes.

Pathotype B of citrus canker causes a less severe disease and pathotype C only infect Key lime (Citrus aurantifolia) and produce a hypersensitive reaction in grapefruit (Citrus paradisi) leaves. This pathotype has been named Xanthomonas fuscans subsp. aurantifolii. Xanthomonas fuscans is here treated as a synonym of Xanthomonas citri (e.g. see Constantin et al., 2016).

Synonyms:
Xanthomonas axonopodis pv. citri
Xanthomonas campestris pv. citri
Xanthomonas fuscans subsp. aurantifolii
Xanthomonas smithii subsp. citri

For reviews of citrus canker see: