Detection, isolation, and identification of Rhizoctonia theobromae associated with cassava witches’ broom disease in the Philippines

Cassava (Manihot esculenta Crantz) is an important food security root crop in the tropics, including the Philippines. However, its production is increasingly threatened by the Cassava witches' broom disease. This study detected, isolated, and identified Rhizoctonia theobromae (syn. Ceratobasidium theobromae), a newly associated fungus of cassava. Species-specific PCR detected R. theobromae in 96 out of 97 symptomatic cassava samples collected between 2017 and 2022. Morphological characterization of isolates revealed cottony white to yellowish mycelia, right-angled branching, dolipore septa, and monilioid hyphae, features typical of Rhizoctonia/Ceratobasidium. Although successful isolation was achieved during early crop stage (2 months after planting), subsequent subculturing was unsuccessful due to poor fungal growth, thus reproducing symptoms via inoculation and fulfilling Koch's postulate were difficult. Sequence analysis of the internal transcribed spacer region and the 28S large subunit ribosomal RNA gene confirmed the identity of the fungus, showing 99.7–100 % similarity to previously reported R. theobromae isolates from vascular streak dieback-diseased cacao in Indonesia and vascular streak dieback-diseased redbud in the United States, but slightly lower similarity (98.4–98.8 %) to those cassava-associated isolates in Laos, Cambodia, and Vietnam. Haplotype diversity analysis revealed low genetic diversity among isolates (π = 0.0047), suggesting a genetically stable population and neutrality tests showed no significant results. These findings confirm the strong association of R. theobromae with cassava witches' broom disease in the Philippines, highlighting its potential threat to cassava production in the region. The study emphasizes the need for revisiting existing management strategies to safeguard the cassava industry.