University of Khartoum

Prevalence of Citrus Bacterial Canker in Gadaref State and Evaluation of Some Toxicants for its Control

Prevalence of Citrus Bacterial Canker in Gadaref State and Evaluation of Some Toxicants for its Control

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Title: Prevalence of Citrus Bacterial Canker in Gadaref State and Evaluation of Some Toxicants for its Control
Author: Awdelseid, Atef Hemaida Mohammed
Abstract: As the State of Gadaref is neighbouring the hot spot of citrus bacterial canker (CBC) in the horn of Africa, the objective of this study was to diagnose and survey the extent of the natural spread of this disease in Gadaref State and assess its distribution in the commercial citrus orchards and nurseries, and in the home backyard citrus trees. It was also intended to evaluate the efficacies of several toxicants that can selectively be used as a practical component of integrated disease management program. Extensive surveys were conducted in seven localities in Gadaref State in commercial orchards and when available, in nurseries and home trees towards the end of the rainy season in year 2014. Symptomatology and disease development were assessed in a total of 800 lime and ~ 200 trees of other citrus varieties. The laboratory tests consisting of pathogen isolation and its phenotypic characterization as well as in vitro evaluation of five chemicals for canker bacteria control were conducted at the Faculty of Agriculture, University of Khartoum. While the pathogenicity tests on seedlings of selected citrus varieties were performed in a confined nursery. Typical symptoms were produced naturally on affected lime trees only, which consisted of erumpent corky lesions on leaves and fruits. In severe infections, frequent fruit drop, extensive defoliation, and consequently a twig die-back were evident. The disease was found in five out of seven localities, where significant (P ≤ 0.05) differences in disease development were recorded among localities and between planting types. Whereas the most severe citrus canker was recorded in commercial orchards in Alquresha locality bordering with Ethiopia, the least severity was encountered in nurseries at Showak north of Gadaref. The disease was found to be restricted to lime only, while the most canker susceptible species (Grapefruit and Orange) were not infected, even if in close proximity to symptomatic lime trees. Typical CBC lesions were reproduced in response to pathogenicity tests on lime seedlings only, whereas other test seedlings reacted with atypical, if any, lesions. As far as in vitro efficacies of various toxicants are concerned, significant (P ≤ 0.05) differences were demonstrated among tested chemicals and their concentrations. Mancozeb and Carbendazim fungicides were more potent than other fungicides tested. Based on these results, in addition to the biochemical characteristics of the bacterial isolates recovered from typical canker lesions, it was concluded that Xanthomonas citri subsp. citri (Xcc) was the incitant of the citrus canker disease encountered in Gadaref State. However, because of the atypical combination of host range and symptomatology, these isolates of citrus canker bacteria appeared to resemble closely the strains of the pathotype A*. The favourable climatic factors combined with absence of quarantine measures, poor sanitation and inefficient cultural practices in nurseries and orchards are probably held responsible for the wide-spread CBC epidemic in Gadaref State. It is envisaged that establishing the correct pathotype strains and supplementing the use of tolerant cultivars with integrated system of compatible cultural practices, chemical application, phytosanitary and quarantine measures will form the basis for a holistic approach for CBC management.
URI: http://khartoumspace.uofk.edu/123456789/25539


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