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Infection, Colonization, and Disease of Amaranthus hybridus Leaves by the Alternaria tenuissima Group

With the increased use of Amaranthus hybridus as a leafy-vegetable crop in Africa and the recent identification of Alternaria leaf spot on this host in southern Africa, the role of this potentially damaging pathogen was investigated. The goals of this study were to test the pathogenicity of the Alternaria tenuissima group, determine how these fungi infect Amaranthus hybridus leaves, and examine the colonization pattern within host tissues. Asymptomatic leaves of Amaranthus hybridus were collected from two field sites in South Africa. Eight A. tenuissima group isolates collected from these leaves were used in inoculation experiments conducted in both greenhouse and growth chamber studies. Scanning electron microscopy revealed A. tenuissima-like conidia germinating on leaf surfaces and mycelia entering leaves only through stomata of both field-collected and artificially inoculated leaves. Unwounded, inoculated leaves had no symptoms, and light-microscopy observations of both asymptomatic field-collected and unwounded and inoculated leaves revealed hyphae in mesophyll tissue growing intercellularly with no host cell penetration or host-cell response. Seven of the eight isolates produced brown to black, circular to oval, necrotic lesions only at the wound site of injured and inoculated leaves. These results confirm that isolates of the A. tenuissima group can infect and colonize Amaranthus hybridus leaves in a manner consistent with other endophytic fungi, and suggest that these fungi can act as latent leaf pathogens when the host is altered by wounding.