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Identification of Fungi and Fungal Pathogens Associated with Hypolixus haerens and Decayed and Cankered Stems of Amaranthus hybridus

Discoloration, cankers, and decay in branches, stems, and root collars of Amaranthus hybridus were observed in Bloemfontein, South Africa. Examination of symptomatic stems revealed larval galleries of the pigweed weevil (Hypolixus haerens). The objectives of this study were to: identify the most common fungal species associated with this damage, determine if the adult pigweed weevil might be a vector for the fungi, and test if the associated fungi can cause the stem canker disease observed in the field. The most common fungal species isolated were Fusarium subglutinans from discolored tissues adjacent to insect galleries (42%), F. subglutinans from weevil larvae (29%), the Alternaria tenuissima group from adult weevils (31%), and the A. tenuissima group from cankered stems (40%). Three of the seven most common fungal species produced cankers following wounding and inoculation, with F. sambucinum and F. oxysporum being the most aggressive. Although fungal species compositions differed (P < 0.01) among the four tissue/insect stage combinations tested, all four had the same major fungal species, suggesting the pigweed weevil as a vector for the Fusarium pathogens. There is significant potential for yield loss affiliated with this insect-fungal association. The identification of this insect-fungal relationship and the pathogens involved in disease set the stage for further research on the etiology and disease management of this important insect-fungal relationship.