Amaranthus hybridus is an important alternative leafy-vegetable crop with the potential for increased commercial production in southern Africa and other semi-arid regions of the world (2). In May 1998, extensive leaf spotting was observed on A. hybridus at Potchefstroom, South Africa. Many of the leaves had symptoms and most were severe. Symptoms were dark brown to black, circular to oval, necrotic lesions with a diameter ranging from <1 mm up to 7 mm. Larger lesions had tan centers. Tissues adjacent to the leaf spots remained green. Alternaria tenuissima was isolated from 43% of 40 lesions sampled making up 89% of the isolates recovered. A. tenuissima was isolated from asymptomatic leaves of 5-month-old A. hybridus plants sampled from the same site in April 1997 (1). The foliar symptoms observed on A. hybridus in Potchefstroom were reproduced by inoculating wounded leaves of A. hybridus with single-spore A. tenuissima isolates obtained from asymptomatic leaves collected at Potchefstroom. Eight isolates were selected for pathogenicity tests conducted in a growth chamber. A. hybridus leaves grown from seed in a greenhouse were placed in moist chambers and wounded with a needle (0.5 mm) at leaf center; one 5-mm-diameter, colonized potato dextrose agar plug of each of the isolates was placed on the center of each leaf. A sterile plug was used as a control. Moist chambers were placed in a growth chamber set at 25°C day and 20°C night and provided artificial light for 16 h per day. In a greenhouse (average temperature 25°C day, 17°C night), a conidial suspension (105 conidia per ml of sterile, distilled water) was applied to an individual leaf of each of 14 plants with an atomizer. Sterile, distilled water was applied to control leaves. Leaves were then wounded with a needle (0.5 mm) at leaf center. Treatments were assigned randomly and the experiments were repeated. Symptoms were first observed at 14 and 18 days (growth chamber and greenhouse, respectively). Seven of the eight isolates caused necrotic lesions with an average diameter of 3 mm (both growth chamber and greenhouse). Symptoms were observed on an average of 56 and 82% of the inoculated leaves (growth chamber and greenhouse, respectively). The range of symptoms was the same as that observed in the field, but symptoms were only observed at the wounds. Controls remained green and showed no symptoms. A. tenuissima was recovered from necrotic lesions of surface-disinfested, inoculated leaves (average 93 and 68%; growth chamber and greenhouse, respectively), never recovered from growth chamber controls, and seldom recovered from greenhouse controls (5%). These results suggest that A. tenuissima is a leaf-spot pathogen of A. hybridus and wounding might trigger disease expression. A minimal amount of leaf spotting of this leafy-vegetable crop can cause total crop loss.
References: (1) J. T. Blodgett et al. S. Afr. J. Sci. 94:xviii, 1998. (2) W. J. Swart et al. S. Afr. J. Sci. 93:xxii, 1997.
