Cassava diseases caused by Xanthomonas phaseoli pv. manihotis and Xanthomonas cassavae

Abstract

Xanthomonas phaseoli pv. manihotis (Xpm) and X. cassavae (Xc) are two bacterial pathogens attacking cassava. Cassava bacterial blight (CBB) is a systemic disease caused by Xpm, which might have dramatic effects on plant growth and crop production. Cassava bacterial necrosis is a nonvascular disease caused by Xc with foliar symptoms similar to CBB, but its impacts on the plant vigour and the crop are limited. In this review, we describe the epidemiology and ecology of the two pathogens, the impacts and management of the diseases, and the main research achievements for each pathosystem. Because Xc data are sparse, our main focus is on Xpm and CBB.

Keywords: Xanthomonas; cassava; cassava bacterial blight; cassava bacterial necrosis; quantitative resistance.

FIGURE 1

Aetiology, ecology, and distribution of Xanthomonas phaseoli pv. manihotis (Xpm) and Xanthomonas cassavae (Xc). (a) Cassava bacterial blight disease cycle. Dashed arrows indicate processes that are only relevant for traditional farming systems, that is, where unmanaged sexual reproduction may lead to the incorporation of seedlings into local germplasm. The inset indicates environmental and ecological factors that affect the spread or development of disease. (b) Angular leaf spots caused by Xpm. (c) Blight (solid white arrows) and leaflet curling (dashed white arrows) caused by Xpm. (d) Collapsed petioles from wilted leaves (solid black arrows) and gum exudation (dashed black arrows) from stems caused by Xpm infection. (e) Shoot apex wilting and dieback caused by Xpm. (f) Typical colonies of Xpm on LPGA medium. (g) Typical colonies of Xc on LPGA medium. (h) Leaf spots caused by Xc. (i) Worldwide cassava production (FAO, 2020) by country (missing data for South Africa, Guam, and Palau) for 2018 and distribution of Xpm and Xc (CABI, 2020)

FIGURE 2

Graphical meta‐analysis of incidence, severity, and losses reported for cassava bacterial blight (CBB). (a) Histogram of the incidence ranges reported by 11 studies (Table S1). The colour scale correlates with the frequency of the reported range. (b) Violin plots showing severity ranges recorded by 15 studies (Table S2) according to measurement timepoints after planting. The confounded variable groups data from reports that did not include timepoint information. A harmonized severity grading scale is presented at the right side of the plot. The dashed line indicates that values above 2 reflect systemic disease. (c) Boxplots showing fresh root yield losses ranges described by seven reports (some studies reported several ranges, Table S3) according to the designated resistance status of the plants. The confounded variable groups data from reports that did not include the designated resistance status of the evaluated plants

FIGURE 3

Taxonomic position of Xanthomonas phaseoli pv. manihotis (Xpm) and Xanthomonas cassavae (Xc). Phylogeny of 30 representative Xanthomonas, including 27 species and four pathovars of X. phaseoli (Table S4). Strain code is indicated in parentheses. Xpm and Xc are highlighted in purple and blue, respectively. Phylogeny was constructed from RefSeq complete genomes using the bioinformatic workflow PhaME (Shakya et al., 2020). Core genome alignments resulted in 155,507 aligned single nucleotide polymorphism (SNP) positions that covered coding and noncoding regions. Trees were reconstructed with the GTRGAMMAI model of RAxML and consensus tree was calculated from 100 bootstraps; results higher than 80 are shown above branches

FIGURE 4

Roles of Xops, host determinants of susceptibility, and defence responses in the cassava–Xanthomonas phaseoli pv. manihotis (Xpm) interaction. Xop effectors (coloured circles) are injected by Xpm into the plant cytoplasm, where they interfere with host cellular processes. Type III effectors (T3Es) are grouped (shading) according to their predicted involvement in PAMP‐triggered immunity (PTI), effector‐triggered immunity (ETI), and/or other virulence‐related phenotypes (see section 6.3). The upper section of the depicted nucleus outlines the contribution of TAL effectors to cassava susceptibility, in which transcriptional activation of the S gene MeSWEET10a leads to an over‐accumulation of sugar transporters (purple, anchored to the membrane) and sugar export (see section 6.3). The RXam1 RLK (lilac, in the membrane) and the RXam2 NBS‐LRR (blue, in the cytoplasm) are associated with the defence response triggered against Xpm, but their matching elicitors are unknown. Calcium signalling and autophagy elements are represented in the cytoplasm as part of the anti‐Xpm defence mechanisms. The roles of miRNA and transcription factors are schematized in the nucleus (see section 7.5). H₂O₂ and O₂ ⁻ represent reactive oxygen species (ROS) accumulation. SA, salicylic acid