First Report of Fusarium sambucinum, F. oxysporum, and F. subglutinans Associated with Stem Decay of Amaranthus hybridus in South Africa

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.

Genetic Variation Among Fusarium oxysporum Isolates Associated with Root Rot of Amaranthus hybridus in South Africa

Thirty isolates of Fusarium oxysporum were recovered from six substrates: (i) tap roots of Amaranthus hybridus showing symptoms of root rot, (ii) side roots of A. hybridus showing symptoms of root rot, (iii) soil surrounding plants of A. hybridus, (iv) pigweed weevils (Hypolixus haerens) that feed on A. hybridus, (v) rhizosphere of maize plants growing adjacent to a field of amaranth, and (vi) rhizosphere of dry bean plants growing adjacent to a field of amaranth. The isolates were characterized by means of pathogenicity tests, isozyme analysis, and vegetative compatibility group (VCG) tests. In the pathogenicity tests, toothpick tips were infested with F. oxysporum and inserted into amaranth stems. All 30 isolates were pathogenic on A. hybridus, with significant differences in pathogenicity based on lesion length measured 4 weeks after inoculation. The isolates were grouped into nine VCGs by complementation tests using nitrate nonutilizing mutants. Self-incompatibility was not observed for any of the isolates. The most common VCG was VCG1, which comprised 20 of the 30 isolates tested. The second most common group was VCG3, which included three isolates, while the remaining seven VCGs each consisted of a single isolate. The results indicate that the population of the amaranth root rot pathogen examined in this study is relatively homogeneous. Results of the isozyme analysis supported the results of VCG tests. Three major groups were delineated within the 30 isolates of F. oxysporum following cluster analysis of electrophoretic phenotypic values for seven isozymes (EST, IDH, G6PDH, ACP, PEP1, PEP2, and PEP3) tested. No relationship was found between isozyme phenotype and the substrate from which the isolates were recovered.