Aqueous extracts (2%) of three plants; roselle (Hibiscus subdariffa Linn.), cinnamon (Cinnamomum verum J. Presl.) and clove (Syzygium aromaticum Linn.) were assessed for control of citrus canker disease in lime (Citrus aurantifolia Swingle) caused by Xanthomonas axonopodis pv. citri (Xac) under in vitro and in vivo conditions. Mancozeb fungicide (1.5%) was used as a positive control. In the first experiment, the effect of these three plant extracts was investigated in vitro through inhibition zone technique. Results indicated that roselle and cinnamon extracts caused significantly less inhibitory effect (38.5% and 25%, respectively) on canker severity compared to the fungicide Mancozeb (control), but they were significantly (P ≥ 0.05) more efficacious than clove extract. Clove extract showed little or no in vitro inhibitory activity. In the second experiment conducted in a greenhouse with natural infection of Xac on lime seedlings, aqueous extracts (2%) of roselle, cinnamon and clove significantly (P ≥ 0.05) exhibited an inhibitory effect against the disease severity with a decline from 40.4 to 45.2%. Roselle extract was the most effective for control of citrus canker (45.2%) followed by cinnamon and clove which recorded only 40.4% disease control after three weeks compared to the control treatment.
Citrus bacterial canker disease (CBCD) caused by Xanthomonas citri subsp. citri (Xcc), is one of the most destructive diseases to the citrus plantations worldwide, newly invaded, and threatened citriculture in Sudan. Occurrence and spread of CBCD in Sudan have been surveyed in two states, representing the main citrus producing states in Sudan. Field surveys were conducted during January 2015 in commercial citrus orchards and nurseries at the two locations. Symptomatology, host range study, physiological and biochemical characterization of the isolated pathogen were also carried out to obtain important clues on pathogen identification. In addition, the pathogenicity test was performed on detached leaves of several selected citrus varieties such as grapefruit, orange and lime to establish the identity of the presumptive Xanthomonas citri subsp. citri (Xcc). The disease on lime orchards in northern and southern Sawagi (Kassala State) recorded a disease incidence of 66.6% and 18%, respectively. In nurseries, the disease incidence attained 51.7% and 53.6% in northern Sawagi and southern Sawagi, respectively, while in Khartoum State the disease was unexpectedly detected at a considerably high incidence of 45%, but at only one nursery, no disease detected in Khartoum State` s orchards during these surveys. Lime trees displayed typical symptoms of CBCD, but nearby canker-susceptible citrus species, such as grapefruit (C. paradisi) and sweet orange (C. sinensis) were unaffected. Typical symptoms of CBCD were noticed on leaves, twigs, fruits and branches. The pathogenicity tests of the recovered canker isolates induced typical lesions on local lime only, but produced atypical lesions on other citrus varieties. All the biochemical and physiological characteristics obtained from the re-isolations were also indicative of the presence of Xcc. As important perspectives, it appeared that these citrus canker isolates were distinctive and specific on lime. They were very similar to the pathotype and they attained an epidemic level in Kassala State.
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