A disease of shallot onions, Allium cepa var. ascalonicum, that caused yield losses of up to 20 to 30% in some fields was reported from Kalpitiya Peninsula in the North Western Province of Sri Lanka in the late 1980s. Disease symptoms consisting of chlorosis followed by curling and twisting of leaves and abnormal elongation of the neck region appeared after germination of the onion bulbs, subsequently causing plant collapse. Symptomatic plants occurred randomly in most fields and the disease was prevalent throughout the year. Tissue pieces from leaves and the neck region of symptomatic plants plated on potato dextrose agar containing 1% streptomycin (PDAS) produced purplish pink fungal colonies identified as Fusarium oxysporum. Pathogenicity tests were performed with single-spore fungal isolates grown at 25°C for 14 days on PDAS. Shallot onion cv. Vathalan was inoculated by soaking bulbs, pierced with a sterilized needle, in a spore suspension containing 2 × 106 spores per ml for 10 min. Control bulbs were wounded, and soaked for 10 min in sterilized water. Bulbs were then planted in sterile sand and maintained in a green house at 25 to 28°C. After 2 to 3 weeks, typical symptoms, as observed in the field, developed in plants produced from bulbs inoculated with the F. oxysporum isolate. Symptoms were not observed on plants from the control bulbs. Koch's postulates were confirmed by reisolating the same fungus from the neck region of diseased plants. Onion twister disease described by Ebenebe (1) also had been observed in this region in Sri Lanka during 1992-1993. There were differences between these two diseases in disease symptoms and disease development in the field. Acer-vuli of Colletotrichum spp. were always detected in lesions of the neck region, as well as on leaf blades, of plants severely affected by onion twister disease. This disease developed from foci in fields that spread over time. Moreover, onion twister disease occurred only from October until January, coinciding with the rainy season.
Reference: (1) A. C. Ebenebe. Plant Dis. 64:1030, 1980.