Abstract
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.
Highlights
Citrus (Rutaceae family) is considered as one of the most important commercial fruit crops
The survey conducted in commercial citrus orchards and nurseries in both Kassala and Khartoum States indicated the occurrence of citrus bacterial canker (CBC) on lime (Citrus aurantifolia Swingle), but not on the other surveyed citrus varieties
While the typical disease symptoms in Kassala State were evident in both citrus orchards and the nursery lime seedlings, they were only displayed in the nursery stock and absent in orchards in Khartoum State
Summary
Citrus (Rutaceae family) is considered as one of the most important commercial fruit crops. With all its vast area, wide range of soils, diverse climatic conditions, and ample water resource possesses great potentials for citrus production. The commercial citrus production in Sudan spreads all over the country, mainly along the narrow strips of alluvial soils of the main River Nile, Blue Nile, and White Nile. It is extending to the banks of annual valleys and rivers and upper terraces in which underground water is available for irrigation [1]. The important citrus groups have grown commercially in Sudan include: Small fruited acid lime (Citrus aurantifolia Swingle), grapefruit (C. paradise Macfad), sweet orange (C. cinensis Osbeck), and Mandarins (C. reticulate Blanco). Each group is composed of several varieties and selections (Ali-Dinar, 1984)
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