Thiram steep seed treatment to control Phoma betae and improve the establishment of the sugar-beet plant stand

Abstract For several years there has been increasing pressure to replace diethyl mercuric phosphate (EMP) for control of Phoma betae on sugar-beet seed and a need to achieve quicker emergence and more established plants. Steeping seed in a 0·2% suspension of thiram achieved these objectives. However, in 12 experiments testing factorial combinations of steep duration (0–24 h) and temperature (20–30°C), it was seldom possible to discriminate between the results of treatment for 4–16 h at 30°C, 8–16 h at 25°C, or 16–24 h at 20°C. These thiram treatments resulted in about 5% more plants than an EMP steep for 20 minutes at ambient temperature. Extending the thiram steep to 24 h at 30°C (as previously considered essential) was consistently less beneficial. In 278 simple comparisons, thiram-steeped seed (16 h at 30°C or 16 h at 25°C) gave more plants than EMP in over 80% of the fields; the average improvement was 3%, and this was probably an underestimate. The response to thiram tended to be greatest following early sowings on medium and heavy soils. Thiram-steeped seed resulted in more plants than EMP-treated seed for at least 3 years after the treatments had been applied. The intention is to replace EMP with a thiram-steep treatment in England in 1989.