First Report of Xanthomonas citri pv. citri Pathotype A Causing Asiatic Citrus Canker in Madagascar

Abstract

Asiatic citrus canker (ACC), caused by Xanthomonas citri pv. citri, is a threat to citrus industries worldwide. X. citri pv. citri is delineated into three pathotypes differing in pathogenicity towards Citrus species and hybrids. The pathogen negatively impacts production in tropical and subtropical areas, resulting in direct losses (yield decrease due to premature fruit drop and/or defoliation, alteration of fruit quality, and increased production costs) and indirect losses (Graham et al. 2004). Several surveys on various citrus pests and pathogens have been conducted in Madagascar over the last two decades, with no signs of Asiatic canker at any time or place. Typical canker-like symptoms were first observed in May 2024 in a single site of the Boeny region of Madagascar, with low incidence and severity. Bacterial isolation and identification were performed according to the protocols described in EPPO standard PM7/44(2) (EPPO 2023). Three strains were purified and further characterized (LW023-1, LW023-2 and LW023-3). Unlike the negative control (sterile 0.01M Tris buffer, pH 7.2), all three strains, along with strain IAPAR306 (positive control), produced typical XAC1051 amplicons by real-time PCR (EPPO 2023). Based on MLVA-31, which targets 31 minisatellites, Madagascar strains were assigned to lineage one, composed of pathotype A strains (Pruvost et al. 2014). All strains were inoculated onto Mexican lime (C. x aurantiifolia) SRA 140 and sweet orange ‘Washington Navel’ SRA 102 (C. x sinensis). Inoculations consisted of infiltrations of bacterial suspensions (three replicates each consisting of eight infiltrations) containing approximately 1 x 105 cells/mL in Tris buffer. Plants were incubated at 30°C during the day and 26 °C at night, with a 12/12-hour light/dark cycle and a relative humidity of approximately 80%. All inoculated leaves but the negative control produced typical erumpent, canker-like lesions at inoculation sites on both citrus species six to seven days post inoculation, consistent with their assignment to lineage 1 pathotype A (i.e., the most virulent strains). In planta population sizes were determined as reported earlier (Bui Thi Ngoc et al. 2010) using KC semi-selective medium (EPPO 2023). Canker lesions produced from inoculations with the Madagascar strains 16 days post inoculation hosted ≥ 1.7 x 108 cells/g and ≥ 2.4 x 108 cells/g on lime and sweet orange, respectively, which is typical of compatible interactions (Bui Thi Ngoc et al. 2010). Boiled suspensions produced from bacterial colonies (n = 6, one from each replicate) were assayed by real-time PCR (EPPO 2023) and produced the expected amplicon, fulfilling Koch's postulates. This is the first description of X. citri pv. citri in Madagascar. Recent surveys conducted in 2025 in ca. 60 citrus orchards and nurseries covering the main citrus cultivation areas of the Boeny region, a major Mexican lime production area indicate that ACC is widely distributed. This suggests that integrated pest management is the most suitable control strategy for this area (Graham et al. 2004). Based on observed premature fruit drop (ca. 20 to 40%), a high significance of ACC is anticipated, as management options will increase production costs and only partly temper yield decrease. Additionally, surveys in other citrus-growing regions are urgently needed, as concerns exist about the long-distance movement of contaminated nursery plants through trade.

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