ARET, Annual Report 2001, p. 121-5.

Efficacy of acibenzolar-S-methyl (Bion 50 WG) on the control of wildfire and angular leaf spot diseases in tobacco

ARET, Agricultural Research and Extension Trust, Lilongwe, Malawi
Wildfire and angular leaf spot, both bacterial diseases of tobacco are caused by two different strains of the same bacterium, Pseudomonas syringae pv tabaci. The wildfire strain causes necrotic lesions on leaves surrounded by yellow, water socked halos while the angular leaf spot strain causes water soaked black necrotic spots with angular shaped edges. When infection is high, both diseases cause significant damage to tobacco leaves and reduce the economic value of the leaves. The disease is most common in Lilongwe West, Mchinji, Kasungu, Dowa and in some areas in Mzimba South. Disease development in these areas is favoured by wet weather. Both diseases can be controlled by using resistant varieties, practising crop rotations, crop hygiene and chemical sprays of Cupric Hydroxide, Copper Oxychloride and Kocide. A chemical called Bion 50 WG has shown some potential in controlling these diseases in Malawi because when it was sprayed on a tobacco crop under observation trial for 2 seasons, little or no wild-fire and angular leaf spot was observed in the sprayed plot. The objective of this trial was therefore to test the efficacy of Bion 50 WG on the control of wildfire and angular leaf spot diseases in tobacco in a replicated trial which was divided into three components, depended on the management levels of the farmer and type of tobacco. The trial was conducted at two sites - Kampini (Lilongwe) and Magomero (Dedza) where outbreaks of wildfire were observed for the past two years. Malawi Western variety was used because of its susceptibility to wildfire. The trial was conducted in two phases, the first phase was in the nursery while the second phase was in the field. The chances of the variety being infected were increased by using the farmer's seedlings established from recycled seed. Six chemical treatments: control (water), Copper oxychloride, Antracol, Bion 50 WG (Rate 1), Bion 50 WG (Rate 2), and Bion 50 WG (Rate 3) were laid out in a randomised complete block design both in the nursery and field. In the nursery each seedbed was 5m long by 1.2m wide while in the field each plot consisted of three ridges of 10m long spaced at 0.9m and plants were spaced at 0.9m. Both nursery and field layouts were replicated three times. All the cultural practices were done as per recommendation of Dark-fired tobacco. Alternaria brown spot and angular leaf spot diseases were observed at Magomero in the field and the severity of the diseases was low. Wildfire and angular leaf spot diseases were observed at Kampini, and the latter disease was more severe than the former. At Kampini, the middle level of Bion application was more effective in controlling the disease than the lowest and higher levels of application including Copper oxychloride and control in the following year.