Bacterial Spot of Tomato and Pepper in Africa: Diversity, Emergence of T5 Race, and Management

Abstract

Bacterial spot disease was first reported from South Africa by Ethel M. Doidge in 1920. In the ensuing century after the initial discovery, the pathogen has gained global attention in plant pathology research, providing insights into host-pathogen interactions, pathogen evolution, and effector discovery, such as the first discovery of transcription activation-like effectors, among many others. Four distinct genetic groups, including Xanthomonas euvesicatoria (proposed name: X. euvesicatoria pv. euvesicatoria), Xanthomonas perforans (proposed name: X. euvesicatoria pv. perforans), Xanthomonas gardneri (proposed name: Xanthomonas hortorum pv. gardneri), and Xanthomonas vesicatoria, are known to cause bacterial spot disease. Recently, a new race of a bacterial spot pathogen, race T5, which is a product of recombination between at least two Xanthomonas species, was reported in Nigeria. In this review, our focus is on the progress made on the African continent, vis-à-vis progress made in the global bacterial spot research community to provide a body of information useful for researchers in understanding the diversity, evolutionary changes, and management of the disease in Africa.

Keywords: Africa; Management; Xanthomonas; bacterial spot; pepper (Capsicum annum L.); tomato.

FIGURE 1

(A) Pepper field in northwestern Nigeria infected with bacterial spot disease. Most pepper race 6 Nigerian strains (Jibrin et al., 2014) were isolated from this field. (B) Tomato field in Tanzania infected with bacteria spot disease. (C) Pith necrosis observed in tomato stem artificially inoculated with Xanthomonas perforans strain Xp91-118. (D) Quadrant streaking to isolate pure culture of bacterial spot strains from field samples of tomato. (E) Pure culture obtained after sub-culturing strain from (D). (F) Typical harvest of tomato in Nigeria. Diseased fruits are separated from healthy fruits before marketing.

FIGURE 2

Map of Africa showing countries where bacterial spot disease has been reported on tomato and/or pepper. Xe, Xp, Xg, and Xv represent X. euvesicatoria (X. euvesicatoria pv. euvesicatoria), X. perforans (X. euvesicatoria pv. perforans), X. hortorum pv. gardneri, and X. vesicatoria, respectively. Xa represents X. arboricola reported only in Tanzania. Races include T1, T2, T3, T5, T2P1, T2P2, and T2P3. Countries with black dots are those for which bacterial spot have been reported, but neither species nor race was identified. Map for this figure was obtained from https://commons.wikimedia.org/wiki/File:Canary_ Islands_in_Africa_(-mini_map_-rivers).svg (date of access: 6 September 2021).

FIGURE 3

Comparisons of reference strains of each bacterial spot species against sequenced representative strains from Africa. From innermost to outermost ring: Xanthomonas euvesicatoria 85-10, X. euvesicatoria NI38, X. perforans 91-118, X. perforans NI1, X. gardneri ATCC19865, X. gardneri JS749-3, and X. vesicatoria ATCC35937. There was no representative African X. vesicatoria strain, as no known strain has been sequenced. Varying color gradients in percentage identities are based on BLAST match of a minimum of 50% percentage identity (1e-5) of genomic regions against genome of strain 85-10 as shown in key. Because BLAST matches are calculated using 85-10 as reference sequences, regions that are absent from genome of strain 85-10 but present in one or more of other genomes are not displayed. Vertical lines in key are, respectively, colored according to color of each genome in ring. Circular genome was produced using BLAST Ring Image Generator (BRIG; Alikhan et al., 2011).

FIGURE 4

A diagrammatic representation of secretion systems and LPS clusters in reference strains compared with sequenced African strains. Shapes and color codes follow methodology of Potnis et al. (2011) with modifications. For T1SS, dark blue color in shape shows presence of homologs of Ax21 and RaxST; white space indicates absence of RaxST. T2SS systems show presence of both Xps and Xcs systems in reference strains and African strains. T3SS shows similarity in hrp cluster. Unique X. gardneri hrp cluster is in faint blue. Type IVA shows Dot/Icm cluster in red and Vir system in deep blue. Unique type IVB cluster in X. gardneri is also shown, whereas type IVC is shown. T5SS shows that NI1 lacks homologs of genes encoding FhaB protein, whereas one copy is present in X. gardneri. X. gardneri lacks T6SS. LPS cluster of NI1 is most unique and is similar to that of X. translucence pv. translucence (Jibrin et al., 2018b).