First report of anthracnose caused by Colletotrichum gloeosporioides on onion (Allium cepa) in Benin

Onion is the third most important vegetable after tomato and pepper for income generation by farmers in Bénin, West Africa (Agossou et al., 2001; MAEP, 2006). Since 1995, a serious disease of onion has been reported by farmers across cultivated fields in southern Bénin. All of the farmers interviewed in a survey performed in 2001 by the "Laboratoire de Défense des Cultures" claimed severe crop losses from plants exhibiting symptoms of Colletotrichum gloeosporioides. Fifty-five percent of farmers reported they had abandoned onion production as a consequence of this disease. Disease symptoms consist of white, oval sunken spots on the leaves at the onset of infection followed by the appearance of concentric rings of orange acervuli in sunken necrotic spots. Dieback was observed on severely infected leaves, which lead to the collapse of plants.Twisting and curling of leaves appeared in many plants (Ebenebe, 1980).

In 2004, onion plants exhibiting symptoms of anthracnose were collected from onion fields in Grand-Popo. A fungus was repeatedly recovered from infected leaves onto potato dextrose agar medium (PDA). This was identified as Colletotrichum gloeosporioides based on morphological and cultural characteristics. The fungus initially produced white to grey mycelia that became dark brown with age. Acervuli were numerous, dark brown to black, globose to irregular-shaped and glabrous. Conidia were hyaline, unicellular, aseptate, and oval to cylindrical with rounded ends and were 10-20 µm long and 3-5 µm wide (CABI, 2007). The culture was given the reference #680 and stored at IITA. The identification was confirmed by DSMB, Germany and the Global Plant Clinic, UK. Two separate pathogenicity tests (spores and hyphae) were performed from single-spore fungal isolates grown at 25°C for 10 days on PDA under fluorescent light. In each case ten onion plants cv. 'Violet de Galmi' (37 days old, bearing five to six leaves) grown under glass (Sattar et al., 2006) were inoculated as appropriate: with spore inoculation 30 ml aqueous suspension containing 9.5 x 10⁵ spores/ml were inoculated into each onion plant; with hyphal inoculation each plant was wounded at three locations on upper leaf surface using a sterilised pin, a piece of PDA (2 mm²) from the hyphal leading edge of the 10 day old colony then being applied over the wound. Control plants were similarly sprayed with sterilised water or wounded and covered with sterile PDA (2 mm²). Plants were kept in a glass house at 30 ± 2°C. After 7 to 10 days,typical symptoms, as observed in the field, developed on plants inoculated with either spores or hyphae, while no symptoms were observed on control plants that remained healthy. Koch's postulates were confirmed by re-isolating the same fungus from symptom-bearing leaves. Although, C. gloeosporioides has previously been found in West Africa on a range of plants, and the disease of onions is found in other part of the world, this is the first report of this fungal pathogen affecting A. cepa in Bénin.