5.6.6
FLUSULFAMIDE: NEW OPPORTUNITIES FOR THE CONTROL OF CLUBROOT (PLASMODIOPHORA BRASSICAE)

GR DIXON1, JE THOMAS2, DM KENYON2, J DAVIES3 and SJP OXLEY4

1Department of Bioscience & Biotechnology, University of Stathclyde, Glasgow Gl 1XQ, UK; 2National Institute of Agricultural Botany, Huntingdon Road, Cambridge CB3 OQE, UK; 3Horticulture Research International, Stockbridge House, Cawood, Selby, Yorkshire Y08 OTZ, UK; 4SAC - Edinburgh, West Mains Road, Edinburgh EH9 3JG, UK

Background and objectives
The soilborne fungus, Plasmodiophora brassicae, causal agent of clubroot disease of Brassica crops has, until now, been without an effective and environmentally acceptable means of direct fungicidal control. This situation poses major problems for Brassica crop production which is increasing in world-wide nutritional and economic significance. Factors such as greater intensity of production and severely diminished opportunities for rotational control have resulted in substantial epidemics being reported in Asia, Australia, Europe, North America and South Africa over the past decade. There is, therefore, an urgent need for an effective, safe and reliable fungicide with which to combat this pathogen.

Materials and methods
The novel sulphonamide, flusulfamide (MTF 651) has been evaluated for its ability to control P. brassicae at several sites representing a range of soil types, Brassica crops and climatic conditions distributed throughout Britain over the last five years (1993-97). Each trial has been grown from transplanting to maturity using appropriately standardised local commercial husbandry, been fully replicated in four or six randomised blocks and evaluations of disease intensity and yield taken from graded produce at maturity. Flusulfamide has been applied prior to planting as either dust or soluble concentrate formulations containing 511/o ai at 0.6 to 1.2 kg/ha.

Results and conclusions
Control of clubroot has been achieved in cole Brassica crops ranging from cauliflower and calabrese (green broccoli) to summer maturing cabbage and spring greens following a single pre-planting application of flusulfamide at 0.6 kg/ha ai. Higher rates of application increased control of clubroot at some locations, but not sufficiently to demand their use as a commercial recommendation. Symptom intensity has been reduced, in several trials, by more than 70% with significantly fewer roots affected by severe or moderately severe symptoms. Yield from plots treated with flusulfamide was increased concomitantly with decreased clubbing. Cabbage yields, for example, have been consistently increased following treatment by more than 50% accompanied by much improved product quality. The chemical is soil-incorporated into the top 10 cm of cropping beds in advance of transplanting. Efficacy is retained from planting to harvest following one application and there appears to be no diminution in disease control when the chemical is tank-rnixed with trifluralin herbicide. Phytotoxicity was absent from all trials. Some site and seasonal variations in efficacy have been detected which are being investigated in more detail. Glasshouse tests of soil samples taken at intervals from treated field plots and planted with susceptible Brassica seedlings indicated that the field inoculum potential of P. brassicae declined after treatment with flusulfamide.

References
1. Dixon GR, Craig MA, Burgess PJ, Thomas J, 1994. Proceedings of the Brighton Crop Protection Conference, 541-548.
2. Dixon GR, Page L, 1998. Acta Horticulturae (in press).