IPPC Pest Alert – Rhizoctonia theobromae (Cantharellales: Ceratobasidiaceae)

(Other scientific names: Ceratobasidium theobromae, Oncobasidium theobromae, Thanatephorus theobromae)

Date of Issue: 27 November 2025

Figure 1: Vascular streak dieback (VSD) symptoms on cacao caused by Rhizoctonia theobromae. Credits: Ali, S.S., Asman, A., Shao, J., et al. (2019). Draft genome sequence of fastidious pathogen Ceratobasidium theobromae, which causes vascular-streak dieback in Theobroma cacao. Fungal Biology and Biotechnology 6:14. Licensed under CC BY 4.0.

Figure 2: Cassava Witches' Broom Disease (CWBD) symptoms in French Guiana caused by Rhizoctonia theobromae (a, b) Broom-like growths; (c) Vascular necrosis in an infected stem cross-section. Credit: Pardo, J.M., Gil-Ordóñez, A., Leiva, A.M., et al. (2024). First report of cassava witches' broom disease and Ceratobasidium theobromae in the Americas. New Disease Reports 50(1): e70002. Licensed under CC BY 4.0.

Reason for this alert

Rhizoctonia theobromae is a fungal pathogen of significant economic concern that has demonstrated recent intercontinental spread. Rhizoctonia theobromae is native to Southeast Asia and Melanesia, where it has long been established on cacao^5,10. It has also been consistently detected and localized in cassava plants affected by Cassava Witches’ Broom Disease (CWBD) and it is strongly suggested by recent scientific evidence as the causal agent of the disease^1,4. It has recently jumped from its native range in Asia to South America, where it was first detected in 2023 on Cassava^2,3,13. Considering its potential to spread through asymptomatic planting material⁴ and the substantial economic damage it causes, the Pest Outbreak and Alert Response Systems (POARS) Steering Group assessed and qualified R. theobromae as an emerging pest of global concern.

Based on the assessment conducted the pest meets the criteria for (i) recent geographical spread, with outbreaks across multiple, non-contiguous regions; (ii) limited but expanding distribution in endangered areas; (iii) substantial economic impact, with yield losses up to 50% in cassava and 30–40% in cacao; (iv) high likelihood of further introduction, primarily through asymptomatic infected planting material; (v) significant risk management challenges, due to latent infections, diagnostic difficulties, and limited field-level control options.

Distribution

• Asia (native region): Rhizoctonia theobromae is the confirmed causal agent of Vascular Streak Dieback (VSD) on cacao in Indonesia, Malaysia, and Vietnam¹⁰, and strongly supported as the causal agent of Cassava Witches' Broom Disease (CWBD) on cassava in Lao PDR⁴ and the Philippines¹².

• Latin America and the Caribbean (Recent introduction): First detected in 2023 on cassava (CWBD) in French Guiana² and Brazil³.

• Pacific (native region): Papua New Guinea⁵.

Hosts

• Cacao (Theobroma cacao).

• Cassava (Manihot esculenta) consistently detected in plants affected by Cassava Witches’ Broom Disease (CWBD) and considered a strongly associated host.

Impact

In cacao, Rhizoctonia theobromae is the causal agent of Vascular Streak Dieback (VSD), characterized by leaf chlorosis, vascular discoloration (streaking), branch dieback, and potential tree mortality. It is a major limiting factor for cacao production in Southeast Asia and Melanesia⁵.

In cassava, recent scientific evidence strongly supports R. theobromae as the causal agent of Cassava Witches’ Broom Disease (CWBD) in key production areas such as Lao PDR⁴ and the Philippines². In other parts of Southeast Asia, the fungus has been consistently detected and localized in diseased plants and is regarded as a strongly associated or putative causal agent^1,4. Although the etiology of CWBD was historically attributed to other causes, the latest primary research consistently points to R. theobromae as the dominant pathogen associated with the disease^1,4,12.

The economic impacts reported for CWBD are substantial, with yield losses of up to 50% due to reductions in root volume and starch content^1,8. These losses pose a direct threat not only to the commercial viability of the starch industry but also to food security, particularly for indigenous communities in newly affected areas of South America, where cassava is a primary food source³.

Pathways

The primary pathway for the international spread and entry of R. theobromae into new areas is the trade and movement of infected plants for planting, particularly cacao seedlings and cassava stem cuttings^2,3. In cassava, the fungus has been detected and localized within vascular tissues of plants affected by Cassava Witches’ Broom Disease (CWBD) ⁴. Although its causal role remains under investigation, these findings indicate that asymptomatic planting material could serve as a potential carrier. Because the fungus occurs at very low titers and is often undetectable even in symptomatic tissues⁹, it may escape standard phytosanitary inspection procedures. Detection is further hindered by its fastidious growth, which complicates isolation and culturing in routine diagnostics⁹.

Once introduced into a new area, spread occurs primarily through human-assisted movement of infected or contaminated planting material between farms and regions, a common practice in both cassava and cacao production systems. Natural spread is believed to occur through wind-dispersed basidiospores released from infected leaf scars or petiole surfaces under humid conditions⁵. However, current evidence indicates this mechanism is limited to short distances (less than ~100 m) and contributes mainly to localized, within-plantation spread rather than long-distance or regional dissemination.

Expert Working Group (EWG) Activation

Given the global significance of Rhizoctonia theobromae and the need for coordinated technical guidance, the IPPC will establish a dedicated Expert Working Group (EWG) to develop global materials to support contracting parties in prevention, preparedness, and response. The IPPC Secretariat will issue a public call for experts to ensure broad, multidisciplinary participation in this process.

Disclaimer

IPPC Pest Alerts are an early warning tool for emerging pests identified through routine POARS horizon scanning, which includes scientific and technical sources and specialized databases. The information contained herein is considered current as of the date of issue.

While produced by the IPPC Secretariat, these alerts are intended for informational purposes only and do not constitute an official declaration of pest status by the IPPC or FAO. In some cases, the sources of information might not have been confirmed with the corresponding National Plant Protection Organization (NPPO) at the time of publication. The IPPC Secretariat and the FAO assume no liability for the accuracy or completeness of this information, nor for any actions taken in reliance thereon. This alert should be used with these limitations in mind.

If you have any questions or comments about this alert, please e-mail us at [email protected].

References

¹ Leiva, A.M., Pardo, J.M., Arinaitwe, W., et al. 2023. Ceratobasidium sp. is associated with cassava witches’ broom disease, a re-emerging threat to cassava cultivation in Southeast Asia. Scientific Reports, 13: 22500.
² Pardo, J.M., Gil-Ordóñez, A., Leiva, A.M., et al. 2024. First report of cassava witches’ broom disease and Ceratobasidium theobromae in the Americas. New Disease Reports, 50(1): e70002.
³ Oliveira, S.A.S., Sheat, S., Margaria, P., et al. 2025. Rhizoctonia theobromae associated with a severe witches’ broom outbreak in cassava of the Brazilian rainforest and evidence of its Southeast Asian origin. bioRxiv (preprint).
⁴ Gil-Ordóñez, A., Pardo, J.M., Sheat, S., et al. 2024. Isolation, genome analysis and tissue localization of Ceratobasidium theobromae, a new encounter pathogen of cassava in Southeast Asia. Scientific Reports, 14: 18139.
⁵ Guest, D. & Keane, P. 2007. Vascular-streak dieback: a new encounter disease of cacao in Papua new guinea and Southeast Asia caused by the obligate Basidiomycete Oncobasidium theobromae. Phytopathology, 97(12): 1654–1657.
⁶ Pardo, J.M., Chittarath, K., Vongphachanh, P., et al. 2023. Cassava witches’ broom disease in Southeast Asia: A review of its distribution and associated symptoms. Plants, 12(11): 2217.
⁷ Harni, R., Amaria, W., Mahsunah, a. & Lakani, I. 2019. Effects of Trichoderma metabolites and botanical fungicides against VSD in cacao. Jurnal Tanaman Industri dan Penyegar, 6(3): 109–118.
⁸ Landicho, D. 2024. Addressing cassava witches’ broom disease impacts (media brief). DOST-PCAARD.
⁹ Ali, S.S., Asman, A., Shao, J.Y., et al. 2019. Draft genome sequence of fastidious pathogen Ceratobasidium theobromae, which causes vascular-streak dieback in Theobroma cacao. Fungal Biology and Biotechnology, 6: 14.
¹⁰ Samuels, G.J., Ismaiel, A., Rosmana, A., et al. 2012. Vascular Streak Dieback of cacao in Southeast Asia and Melanesia: in planta detection of the pathogen and a new taxonomy. Fungal Biology, 116(1): 11-23.
¹¹ McMahon, P. & Purwantara, A. 2016. Vascular streak dieback (Ceratobasidium theobromae): History and biology. In B. A. Bailey & L. W. Meinhardt (Eds.), Cacao diseases: A history of old enemies and new encounters (pp. 307–335). Springer.
¹² Landicho, D.M., Montañez, R.J.M., Manayon, V.S., et al. 2025. Detection, isolation, and identification of Rhizoctonia theobromae associated with cassava witches’ broom disease in the Philippines. Physiological and Molecular Plant Pathology, 140: 102872.
¹³ Embrapa. 2024. Embrapa identifies the first case of witches' broom in cassava in Brazil. Official Communication. Available at: https://www.embrapa.br/en/busca-de-noticias/-/noticia/91690274/embrapa-identifies-first-case-of-witches-broom-in-cassava-in-brazil.

First confirmed report of Clavibacter nebraskensis outside North America

Date of Issue: 03/03/2025

Trigger alert event:

NRO: Pest report: Clavibacter nebraskensis (Goss's wilt of maize) has been detected in four provinces in South Africa

Distribution*: Africa: South Africa. North America: Canada, Mexico, United States of America (EPPO Global Database). Major economic host: Maize (Eichenlaub, R., & Gartemann, 2011; Lang et al., 2017). Current impact: Severe infections have been documented to cause yield losses of up to 50% in susceptible maize varieties, especially under favorable environmental conditions such as high humidity and warm temperatures (Jackson-Ziems et al., 2014). Between 2012 and 2015, estimated total yield losses due to Goss’s Wilt in the U.S. and Canada exceeded 1.27 million tonnes, making it one of the most destructive maize diseases in the northern U.S. and Ontario (Wise et al., 2019).

© Osdaghi, E., Robertson, A.E., Jackson-Ziems, T.A., Abachi, H., Li, X., & Harveson, R.M., 2022. Licensed under CC 4.0. Clavibacter nebraskensis causing Goss's wilt of maize, 2024

Environmental conditions favoring spread:

The pathogen thrives in warm temperatures and high humidity, which are common in maize-growing regions (Wise et al., 2019).

Possible pathways of spread for Clavibacter nebraskensis:

The pathogen spreads through both natural and human-mediated pathways. Naturally, the pathogen can spread via wind-driven rain, contaminated soil, and infected crop residues, which serve as reservoirs for future infections. Human activities, such as the movement of contaminated agricultural equipment and the trade of infected plant material, facilitate long-distance spread. Seeds spreading is possible. However, the spreading rates of seeds are considered low (Flores-Lopez et al., 2024). The risk of these pathways needs to be carefully determined.

*Disclaimer:

IPPC Pest Alerts are news items obtained from public sources. They do not serve as official communication from the IPPC or FAO. The "IPPC Pest Alerts" is an early warning tool for potential emerging plant pests. In some cases, information within alerts is not confirmed with the corresponding National Plant Protection Organization. They are provided solely as an early warning to IPPC Contracting Parties and should be used with this disclaimer in mind.

If you have any questions or comments for us about this alert, please e-mail us at [email protected]

References:

¹ Eichenlaub, R., & Gartemann, K. H. (2011). The Clavibacter michiganensis subspecies: Molecular investigation of gram-positive bacterial plant pathogens. Annual Review of Phytopathology, 49(1), 445-464.
² EPPO Global Database. (2025). Clavibacter nebraskensis (CORBNE) – Distribution. European and Mediterranean Plant Protection Organization. Retrieved 02/28/2025, from https://gd.eppo.int/taxon/CORBNE/distribution
³ European and Mediterranean Plant Protection Organization (EPPO). (2025). Clavibacter nebraskensis – Alert list. Retrieved 02/28/2025, from https://www.eppo.int/ACTIVITIES/plant_quarantine/alert_list_bacteria/Clavibacter_nebraskensis
⁴ Flores-López, L. F., Olalde-Portugal, V., Vidaver, A. K., Morales-Galván, Ó., Hernández-Rosales, M., & Huerta, A. I. (2024). Unlocking a Mystery: Characterizing the First Appearance of Clavibacter nebraskensis in Mexican Cornfields. Plant Disease, 108(5), 1374–1381. https://doi.org/10.1094/PDIS-08-23-1493-RE
⁵ Jackson-Ziems, T. A., Harveson, R. M., & Vidaver, A. K. (2012). Goss's bacterial wilt and leaf blight of corn. Plant Health Progress, 13(1), 1-7. https://doi.org/10.1094/PHP-RS-12-0160
⁶ Wise, K. A., Smith, D. L., Freije, A. N., & Mueller, D. S. (2019). An overview of Goss’s bacterial wilt and blight. Crop Protection Network. https://cropprotectionnetwork.org/publications/an-overview-of-gosss-bacterial-wilt-and-blight
⁷ Lang, J. M., DuCharme, E., Ibarra Caballero, J., Luna, E., Hartman, T., Ortiz-Castro, M., ... & Jackson-Ziems, T. A. (2017). Detection and characterization of Clavibacter nebraskensis causing Goss's bacterial wilt and blight of corn in Mexico. Plant Disease, 101(5), 836-844.
⁸ Osdaghi, E., Robertson, A. E., Jackson-Ziems, T. A., Abachi, H., Li, X., & Harveson, R. M. (2022). Clavibacter nebraskensis causing Goss's wilt of maize: Five decades of detaining the enemy in the New World. Molecular Plant Pathology. https://doi.org/10.1111/mpp.13268