5.1.7
SYNTHETIC GARLIC OIL REDUCES VIABILITY OF SCLEROTIUM CEPIVORUM SCLEROTIA AND INCIDENCE OF WHITE ROT ON ONIONS

MR MCDONALD and MHY HOVIUS

Muck Crops Research Station, University of Guelph, Guelph, ON, L0G 1J0, Canada

Background and objectives
Allium white rot (Sclerotium cepivorum) is a major threat to the onion industry in Canada, where most of the crop is grown on organic (muck) soils. Disease incidence can exceed 65% in commercial onion fields. Recommended management practices, such as dichloran application and winter flooding, are only partially effective. Materials such as onion oil and synthetic garlic oil are known to stimulate the germination of sclerotia of S. cepivorum in the absence of a host, reducing the soil populations [1, 2]. Trials were conducted in commercial onion fields and in the greenhouse to determine the potential of germination stimulants to reduce populations of sclerotia of S. cepivorum and subsequently control white rot on onions.

Materials and methods
Two germination stimulants, diallyl disulfide (DADS) and N-dipropyl disulfide (DPDS), were applied to field plots (65% organic matter, pH 5.5-6.5) in the Holland Marsh, Ontario, known to be naturally infested with the pathogen. The materials were injected at depths of 10 and 20 cm, at rates of 10.0 l/ha in 500 l/ha water at sites 2 and 3 in 1994, and at sites 4 and 5 in 1995 and 1996. Only DADS (5.0 l/ha) was applied to site 1 in 1994. Adjacent untreated areas were used as controls. There were six replications per treatment. Onions were grown at sites 1, 2 and 3 in 1995 and sites 4 and 5 in 1997, and evaluated for white rot at harvest. Mesh sacks of 100 sclerotia of S. cepivorum were placed in pots of organic soil treated with DADS and DPDS (1 ml/m2 in 50 ml water) and recovered at monthly intervals. Sclerotia were extracted and plated on potato dextrose agar to test for viability.

Results and conclusions
Incidence of onion white rot was relatively low in both 1995 and 1997 (maximum 13.7 and 13.0%, respectively). Single treatments with DADS significantly reduced white rot on onions at sites 1 and 2 (reduction of 72 and 88%), while two applications of DADS reduced white rot by over 95%. Exposure of sclerotia to DADS and DPDS in pots of treated soil reduced viability after 2 and 3 months, but DADS was much more effective than DPDS (18 and 60% viability, respectively, compared to 99% for the water check). DADS is an effective soil treatment for the control of onion white rot in organic soils, especially when applied twice prior to onion production. The effectiveness of DPDS may be improved by increasing the rate.

References
1. Crowe FR, Debons J, Darnell T et al., 1994. Proceedings of the 5th International Workshop on Allium White Rot, 5.2 b.
2. Utkhede RS, Rahe JE, 1981. Canadian Journal of Plant Pathology 4, 79-80.