A Survey for 'Candidatus Liberibacter' Species in South Africa Confirms the Presence of Only 'Ca. L. africanus' in Commercial Citrus.

Greening disease of citrus is a serious disease known in South Africa since the late 1920s. In South Africa, it is associated with infection by 'Candidatus Liberibacter africanus', a heat sensitive, phloem-limited, noncultured alpha-proteobacterium. Huanglongbing (HLB), a similar, but more devastating disease that was described initially from China but which now occurs in several citrus producing countries, is associated with a different Liberibacter species, 'Ca. L. asiaticus'. A 'Ca. L. africanus' subspecies, 'Ca. L. africanus subsp. capensis', has been found only in South Africa infecting an indigenous Rutaceous species, Calodendrum capense (Cape Chestnut), in the Western Cape in 1995. The discovery of a new Liberibacter species in Brazil, 'Ca. L. americanus', and the spread of 'Ca. L. asiaticus' to a number of additional countries over the last few years prompted us to assess whether only 'Ca. L. africanus' is present in commercial citrus orchards in South Africa. Samples displaying greening or similar symptoms were collected from 249 citrus trees from 57 orchards distributed throughout the greening affected citrus production areas of South Africa. Multiplex polymerase chain reaction (PCR) was performed on DNA extracts to detect the known citrus Liberibacters. Amplicons were obtained from 197 samples. None of the samples yielded a 1,027-bp amplicon indicative of 'Ca. L. americanus' infection. The amplicons of 84 samples were sequenced, and all were identical to the cognate 'Ca. L. africanus' Nelspruit sequence in GenBank. No instance of 'Ca. L. asiaticus' or 'Ca. L. africanus subsp. capensis' sequence was found. Geographically representative samples that tested negative for Liberibacter also tested negative for phytoplasmas based on real-time PCR results. Based on the results of this survey, it is concluded that to date only 'Ca. L. africanus' is associated with citrus greening in commercial citrus in South Africa.

Genomic characterization of a liberibacter present in an ornamental rutaceous tree, Calodendrum capense, in the Western Cape Province of South Africa. Proposal of 'Candidatus Liberibacter africanus subsp. capensis'.

In 1994, the uncultured phloem-restricted bacteria of citrus huanglongbing (ex-greening) disease in Asia and Africa were characterized as 'Candidatus Liberobacter asiaticum' and 'Candidatus Liberobacter africanum', respectively. Following the rules of the International Code of Nomenclature of Bacteria, the two bacterial species have now been renamed 'Candidatus Liberibacter asiaticus' and 'Candidatus Liberibacter africanus'. A third liberibacter was detected by PCR in an ornamental rutaceous tree, Cape chestnut (Calodendrum capense), in South Africa. The new liberibacter was characterized by serology and from the sequences of its 16S rDNA, intergenic 16S/23S rDNA and ribosomal protein genes of the beta operon. Phylogenetic analysis showed that the liberibacter present in C. capense differed from the two previously described liberibacter species from citrus and that it was more closely related to 'Candidatus Liberibacter africanus' than to 'Candidatus Liberibacter asiaticus'. It is proposed that the liberibacter from C capense be assigned a subspecies status, 'Candidatus Liberibacter africanus subsp. capensis'.

Comparative Efficacy of Preplant Fumigation and Postplant Chemical Treatment of Replant Citrus Trees in an Orchard Infested with Tylenchulus semipenetrans.

Preplant soil fumigation with methyl bromide at 49 and 98 g/m2 was compared with various postplant nematicide and fungicide programs in a replant citrus orchard infested with the citrus nematode Tylenchulus semipenetrans and the fungal pathogens Phytophthora nicotianae, Fusarium solani, and F. oxysporum. Postplant treatments comprised multiple applications of fenamiphos at 4.0 g a.i./m2 soil, aldicarb at 4.5 g a.i./m2 soil, metalaxyl at 4 g a.i./m2 soil, fose-tyl-Al stem painting at 400 g a.i./liter, and combinations of fenamiphos + fosetyl-Al and aldi-carb + fosetyl-Al at the same rates as for single treatments. P. nicotianae could not be detected in the entire experimental site after replanting, but populations of F. solani and F. oxysporum showed only a temporary decline following site preparation. T. semipenetrans did not re-establish in any of the treatments within the first 2 years. Numbers of juveniles remained low in most treatments during the third year, but thereafter both juveniles and females increased significantly in all except the fumigated plots. Female populations on roots of citrus trees planted in fumigated soil remained suppressed for 8 years and the trees developed more vigorously and produced higher yields and larger fruit than those in non-fumigated soil. Compared with the control, net income for the period 4 to 8 years after planting increased by 101 and 46% in plots fumigated with 49 and 98 g/m2, respectively. With the exception of aldicarb, all other treatments showed net losses.