Fungi Associated with Disease and Weevil Damage of Amaranthus cruentus in South Africa

Agronomic trials were conducted in central SouthAfrica to establish Amaranthus cruentus as a commercial grain crop. Knowledge of potential pest-and pathogen-related problems that might arise during cultivation is essential for future industry development. Previous studies linked fungi belonging to the genera Fusarium and Alternaria with weevil damage of leafy amaranth (A. hybridus). The aim of the present study was to conduct a disease survey to identify possible fungal association with weevils (Coleoptera: Curculionidae) and weevil damage in A. cruentus fields at Potchefstroom and Taung in the North-West Province, South Africa. Isolations were made from lesion margins, larval galleries of the weevil Hypolixus haerens, and stem lesions associated with the weevils Athesapeuta dodonis and Baris amaranthi. Weevil larvae associated with lesions as well as adults collected from plants in amaranth plots were euthanased and plated out onto fungal growth media. Fungi arising from the weevils were purified and identified to genus level based on morphology. Alternaria was predominantly isolated from leaf spots in contrast to Fusarium species that were isolated from root rot, cankered roots and stems. Fusarium species were by a considerable margin the dominant group associated with weevil damage and stem lesions, as well as larval tunnels, larval bodies and free living adults. Species of Cladosporium and Alternaria were also isolated from adult weevils with significant consistency. Other less dominant fungi isolated from adult weevils and larval tunnels included species of Aspergillus, Epicoccum, Humicola, Mucor, Penicillium, Rhizopus and Trichoderma. It is evident that Fusarium and Alternaria are the genera that host the most important and potentially dominant fungal pathogen taxa linked with grain amaranth disease symptoms and associated weevil pests. As such they will have to be investigated further in future.