Fungicide seed treatments can protect onion seedlings from damping-off, caused by Pythium spp. and onion smut, caused by Urocystis cepulae Frost. The standard seed treatment in Canada has been PRO-GRO, a combination of carboxin and thiram. However, under high disease pressure, this treatment appeared to be losing effectiveness and growers began adding mancozeb in the seed furrow. Greenhouse efficacy tests were conducted to identify new effective fungicides and rates, in order to reduce the number of treatments evaluated in the field and increase the number of times a trial could be repeated. Trials were conducted in the greenhouse and field in 2000 and 2001. In the greenhouse, 200 cell black plastic trays were filled with field soil (60% organic matter, pH 6.4), naturally infested with the pathogens. Onion, cv. Gazette was seeded, one seed per cell, with four replicate trays per treatment. Trays were held in the dark at 13-16degreesC until seedling emergence and then placed in a greenhouse. Emergence and the incidence of damping-off and onion smut were recorded. Treatments with Charter (triticonazole 2.4%, 1 mg/100 g seed) or Vortex (LO258, 5, 10, 25, 50, 100 or 200 mg/100 g seed), in combination with Raxil (tebuconazole 28.3% at 100, 200 or 400 mg), and Allegiance (metalaxyl 28%, 30 mg), were compared to the standard, PRO-GRO (30% carboxin, 50% thiram, 2000 mg), and an untreated check. Treatments were applied with film coat. Field trials, with the best treatments determined in the greenhouse tests, consisted of cv. Gazette seeded 46 seeds/m, in 5 in rows, in early May each year. A treatment of 6.6 kg ai/ha of mancozeb, applied in the seed furrow, was included. Incidence of onion smut on untreated checks was 24-63% in the greenhouse and 19-51% in the field. The incidence of damping-off was low in the greenhouse (3%) and was not observed in the field. Charter was less effective than Raxil in greenhouse trials. PRO-GRO was less effective in field trials than in the greenhouse. In field trials, Raxil (100 or 200 mg ai/100 g seed) plus Allegiance, plus or minus Vortex (5 or 50 mg ai/100 g seed) seed treatments were as effective as the current standard treatments of PRO-GRO plus a furrow application of mancozeb (6.6 kg ai/ha).
