Emex spinosa (L.) is a common weed in cereal crops and pastures in northern Tunisia. The build up of the seed bank from a cropping-grazing farming system makes chemical and cultural controls inefficient. Biological control as part of integrated weed management may improve weed control. Diseased seedlings were collected from several locations in northern Tunisia during field surveys. Symptoms were small, circular, light brown leaf spots varying in size (1.75 to 3.5 mm in diameter) with a definite dark brown border on both sides of leaves that wilted and died. Microscopic observations showed conidiophores and conidia within and around the spots. Pure cultures from single conidia were obtained on carrot leaf extract agar. The fungus was identified as Cercospora tripolitana on the basis of identification keys described by Chupp (1). Conidia, borne on unbranched, fasciculated conidiophores, were elongate, hyaline, multiseptate, 110 to 150 × 1.8 to 3.7 μm (average 130 to 2.5 μm), and had truncate bases. For pathogenicity testing, six plants were sprayed with a spore suspension of 6 × 105 conidia/ml. Controls were sprayed with sterile distilled water. Plants were placed in a growth chamber at 22°C, 95% relative humidity, and 18/6 h of light/dark and monitored for symptoms. Ten days after inoculation, symptoms identical to those observed in the field were observed on inoculated plants. Control plants did not develop any symptoms. Four weeks later, diseased leaves turned yellow and died. The fungus was reisolated from symptomatic plants according to Koch's postulates. Although C. tripolitana has been previously reported in North and South Africa, to our knowledge, this is the first report of the fungus as a pathogen on E. spinosa under Tunisian agroecological conditions, making it a promising candidate for weed control.
Reference: (1) C. Chupp. A Monograph of the Fungus Genus Cercospora. C. Chupp, Ithaca, New York, 1953.
