November 2002

Infection, Colonization, and Disease of Amaranthus hybridus Leaves by the Alternaria tenuissima Group. J. T. Blodgett, former Post-Doctoral Fellow, and W. J. Swart, Professor, Department of Plant Science, University of the Free State, P.O. Box 339, Bloemfontein 9300, South Africa. Plant Dis. D-2002-0912-01R, 2002 (online). Accepted for publication 30 May 2002.

Smooth amaranthus (Amaranthus hybridus) is a nutritious leafy vegetable well suited for cultivation in semi-arid regions throughout the world. Recently, a potentially damaging disease, Alternaria leaf spot caused by fungi in the Alternaria tenuissima group, was described on this host in South Africa. Other than diseased leaves, the Alternaria leaf spot pathogen was frequently isolated from healthy appearing leaves collected from agricultural fields. Therefore we investigated whether the Alternaria leaf spot pathogen isolated from asymptomatic leaves can cause disease, and examined how the fungus infects and colonizes amaranthus leaves. We found that isolates of this fungal group infect and colonize unwounded leaves, yet the leaves appear completely healthy. However, when leaves are wounded, this fungus can produce brown to black, circular to oval, necrotic lesions at the wound site. Thus, wounding can trigger disease expression. This study also introduces a method to screen isolates of this Alternaria group quickly, and potentially to screen species and varieties of Amaranthus for resistance to this pathogen.