Cassava witches’ broom disease (CWBD) is one of the main diseases of cassava in Southeast Asia (SEA). Affected cassava plants show reduced internodal length and proliferation of leaves (phyllody) in the middle and top part of the plant, which results in reduced root yields of 50% or more. It is thought to be caused by phytoplasma; however, despite its widespread distribution in SEA still little is known about CWBD pathology. The overarching goal of this study was to review and corroborate published information on CWBD biology and epidemiology considering recent field observations. We report the following: (1) CWBD symptoms are conserved and persistent in SEA and are distinct from what has been reported as witches’ broom in Argentina and Brazil. (2) In comparison with cassava mosaic disease, another major disease of cassava in SEA, symptoms of CWBD develop later. (3) Phytoplasma detected in CWBD-affected plants belong to different ribosomal groups and there is no association study available indicating phytoplasma as the causing agent of CWBD. These findings are essential clues for designing surveillance and management strategies and for future studies to better understand the biology, tissue localization and spatial spread of CWBD in SEA and other potential risk areas.

Cassava frogskin disease (CFSD) is a graft-transmissible disease of cassava reported for the first time in the 1970s, in Colombia. The disease is characterized by the formation of longitudinal lip-like fissures on the peel of the cassava storage roots and a progressive reduction in fresh weight and starch content. Since its first report, different pathogens have been identified in CFSD-affected plants and improved sequencing technologies have unraveled complex mixed infections building up in plants with severe root symptoms. The re-emergence of the disease in Colombia during 2019–2020 is again threatening the food security of low-income farmers and the growing local cassava starch industry. Here, we review some results obtained over several years of CFSD pathology research at CIAT, and provide insights on the biology of the disease coming from works on symptoms’ characterization, associated pathogens, means of transmission, carbohydrate accumulation, and management. We expect this work will contribute to a better understanding of the disease, which will reflect on lowering its impact in the Americas and minimize the risk of its spread elsewhere.

Con el objetivo de identificar las especies de Phytophthora asociadas a la mazorca negra y al cáncer del tallo del cacao en Antioquia, Colombia, se realizó la caracterización e identificación morfológica y molecular de 120 aislados de Phytophthora spp. obtenidos de mazorcas, tallos y muestras de suelo en 11 municipios de este departamento. Se evaluaron caracteres morfológicos en esporangios y clamidosporas, la formación de estructuras sexuales, el patrón y la tasa de crecimiento de las colonias y las temperaturas cardinales. La identificación molecular se llevó a cabo mediante PCR convencional con cebadores género-específicos y qPCR. Los 120 obtenidos se identificaron como Phytophthora, 119 como P. palmivora y la cepa CH6-3S no coincidió con ninguna especie de Phytophthora previamente reportada como patógena de cacao. Un modelo de escalamiento multidimensional basado en atributos morfológicos mostró una amplia diversidad en el tamaño y forma de los esporangios (ocho morfotipos); sin embargo, la variabilidad de los aislados fue baja (17 %) y los subgrupos formados no mostraron relación con las localidades, ni con el tipo de tejido de donde se obtuvieron.

Frogskin is the most limiting disease of cassava crops in Colombia, causing losses in production up to 90%. Since this disease was associatated with 16SrIII phytoplasma presence, a study was carried out to isolate this phytoplasma using liquid and solid culture media. Root, petiol, stem, leaf and cutting tissues of cassava affected by frogsking were employed as source materials. Molecular and microscopy techniques were applied to verify the phytoplasma growth and to discard other microorganism´s presence. The results showed that the media consistently allow phytoplasma growth, and colonies in solid medium were obtained. PCR, qPCR and sequencing tests confirmed the presence of 16SrIII group phytoplasmas in both liquid and solid culture media. Additionally, the isolation of a pigeon pea witches' broom phytoplasma strain (group 16SrIX) was obtained from stems, petioles and flowers of symptomatic Catharanthus roseus confirming the effectiveness of the medium in the phytoplasma isolation and culture. This is the first isolation of field-collected phytoplasma strains in groups 16SrIII and 16SrIX in America that confirm and corroborate the previous results in phytoplasma cultivation achieved on micropropagated and field-collected phytoplasma infected samples. En Colombia, el ‘‘cuero de sapo’’ es la enfermedad más limitante del cultivo de yuca, que ocasiona pérdidas en producción de raíces hasta del 90%. La presente investigación tuvo como objetivo, el aislamiento in vitro del fitoplasma asociado a cuero de sapo. Para ello, se emplearon medios de cultivo líquido y sólido, usando tejidos de raíces, peciolos, tallos, hojas y semillas de yuca, afectada por la enfermedad. Pruebas de PCR, qPCR, secuenciación, microscopia de luz y microscopia electrónica de transmisión fueron aplicadas, para verificar el crecimiento de fitoplasmas y descartar la presencia de otros microrganismos. Los resultados muestran que los medios permiten, consistentemente, el crecimiento de fitoplasmas, obteniendo colonias en medio sólido a partir de medio líquido. Las pruebas de PCR, qPCR y secuenciación confirmaron presencia de Cassava frogskin phytoplasma del grupo 16SrIII, en los dos medios de cultivo. Además, a partir de las colonias, se lograron fotografías de células con morfología y tamaño similares a las fitoplasmáticas. Es la primera vez, en el mundo, que se consolida información suficiente del aislamiento de fitoplasmas en medio artificial. Adicionalmente, se logró el aislamiento de Pigeon pea witches´ broom phytoplasma del grupo IX, a partir de tallos, peciolos y flores de vinca (Catharanthus roseus), con síntomas asociados a fitoplasmas. Este proceso permitió corroborar la efectividad del medio y la morfología de las células fitoplasmáticas, bajo microscopia electrónica.

Vegetatively propagated crops, including cassava, sweet potato, banana, and potato, are susceptible to mixed-pathogen infections resulting from the continuous use of clonal planting material and infrequent seed replacement. A diverse array of viruses, bacteria, and fungi can accumulate within these materials over successive cultivation cycles, precipitating seed degeneration and complex disease syndromes that complicate diagnosis and management. Mixed infections frequently trigger synergistic interactions that exacerbate disease severity and yield losses. This review synthesizes data on mixed-pathogen complexes in vegetatively propagated crops, with particular focus on vascular and systemically colonizing pathogens and analyzing starch crops to highlight the epidemiological, biological, and ecological drivers of synergism and antagonism. Furthermore, the review examines host defense responses during coinfection, including the modulation of plant immune pathways, and evaluates how interpathogen dynamics influence pathological outcomes. Although advancements in molecular diagnostics—notably next-generation sequencing and metagenomics—have revolutionized the detection of mixed infections, they have also introduced challenges in differentiating causal agents from commensal microorganisms. Finally, we discuss the implications for integrated disease management, emphasizing clean seed systems, resistance breeding, and phenotyping strategies tailored to multipathogen environments. The dynamics of mixed infections is critical for resilient and sustainable management strategies amidst increasingly complex agricultural and climatic shifts.

The fastidious basidiomycete Rhizoctonia (Ceratobasidium) theobromae is a biotrophic pathogen that causes Vascular-Streak Dieback (VSD) of Theobroma cacao (cocoa). The fungus has also been identified as the cause of an emergent disease known as Cassava Witches Broom Disease (CWBD) raising concerns that the pathogen is spreading to alternative hosts and to new regions. Interestingly, while VSD of cocoa and CWBD are reported as co-present in several countries, there is currently no evidence for cross-infection between species. The fungus is difficult to culture in vitro due its slow growth and Koch’s postulates have not been definitive on either host. The complete fungus life cycle therefore remains enigmatic, though studies have progressed knowledge on pathology within the both the cocoa and cassava hosts. We have conducted limited field trials and sequenced mating (MAT) and ITS loci of isolates from various infected hosts and regions. We hypothesize that (i) genetic variation at MAT loci correlates with region or host (ii) long amplicon ITS sequences between isolates are more definitive for polymorphisms (iii) life-cycle traits of R. theobromae may be inferred from MAT loci (iv) cassava grown under VSD infected cocoa will be infected and develop symptoms of CWBD. We did not find any cross-infection in field trials, and we show that the pathogen is highly homozygous, despite undergoing meiosis, indicating a predominantly homothallic life cycle. Our data indicate that the pathogen is likely host specific and regionally divergent and suggests that host specificity on cocoa and cassava evolved by selection from a common ancestor rather than a host jump.