First report of cassava witches’ broom disease and <i>Ceratobasidium theobromae</i> in the Americas

Abstract

Cassava (Manihot esculenta) is a primary source of carbohydrates for millions of people in the tropics. In the Americas, cassava also holds deep cultural and economic significance for indigenous peoples (Parmar et al., 2017). However, the increased commercialisation and connectivity between cassava-producing regions has heightened the risks of the introduction of diseases that threaten this food security crop. Symptoms of dwarfism, weak sprouts with short petioles and vascular necrosis were observed since 2023 in cassava-growing communes in French Guiana (Table 1). These symptoms resembled those of cassava witches’ broom disease (CWBD) which has previously been reported only in Southeast Asia (Pardo et al., 2023) (Fig. 1). Recently, CWBD was associated with Ceratobasidium theobromae (Leiva et al., 2023; Landicho et al., 2024), the fungus causing vascular streak dieback of cacao (Keane et al., 1972). To characterise the disease, on 13 May, 2024, we visited Saint-Georges-de-l'Oyapock (N3.731375, -W51.788637), the commune that had the largest number of affected fields in our previous survey (Table 1; Fig. 1). Petioles of the top part of cassava plants with and without symptoms were collected following a diagonal transect along the field (code name GUF-114). Petioles were wrapped in paper towels, placed in plastic bags, and kept at 4°C for 24 hours before processing. For fungus isolation, petioles were disinfected using 1% hypochlorite followed by 75% ethanol following the protocol described by Gil-Ordóñez et al. (2024). For DNA extraction dry petioles were transferred into 2 mL tubes and ground in liquid nitrogen before using a DNeasy Plant Mini Kit (Qiagen, Germany). The rDNA internal transcribed spacer region was amplified using ITS4 and ITS5 primers (Landicho et al., 2024). A phylogenetic tree was generated using the Maximum Likelihood method (1000 replicates) with related sequences available in GenBank. PCR specific to C. theobromae targeting the CAMK gene, was conducted as described by Leiva et al. (2023). Characteristic flat and yellowish mycelia containing septa at every 90° branch were observed after incubating the petioles for three days at 25°C (Fig. 2). However, no basidiospores were produced which could be used to perform pathogenicity tests and attempts to infect plants using mycelia have so far failed, as observed in cacao (Keane et al., 1972). The ITS sequence (ITS GUF-114, GenBank Accession No. PQ475929) had 98.6% identity with C. theobromae isolated from cacao in India (KU310940) and 98.4% identity with C. theobromae isolate LAO1 isolated from cassava in Laos (PQ412515) (Fig. 3). The CAMK sequence (CAMK GUF-114: PQ384454) amplified in all diseased plants in the current study was >99% identical to C. theobromae isolates from Southeast Asia (OQ863061-OQ863086). This is the first report of CWBD and C. theobromae in the Americas and a phytosanitary alert that requires urgent regional action. Thanks to the Alliance of Bioversity and CIAT for financial support through its institutional fund for strategic research activities and to the U.S. Agency for International Development.

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