The failure of phenylamide fungicides to provide adequate control of onion downy mildew in recent years in the Lockyer where 80% of Queensland's onions are produced, has been a major concern to industry. This prompted research into the development and extension of a disease forecasting system for onion downy mildew in the Lockyer Valley.
A disease forecasting system called DOWNCAST (acronym for downy mildew forecaster) which uses critical weather factors (temperature, relative humidity, leaf wetness and rainfall), was validated over three seasons (1995 to 1997). This model was adapted for local conditions and gave reliable predictions of sporulation/infection events which formed the basis of scheduling foliar fungicide sprays. Field trials in 1995 and 1996 showed that the spray programme based on this model maintained a high level of disease control with fewer fungicide applications (50% and 30% respectively), than the standard calendar programme. Results of the 1997 field trial showed the importance of accurately pinpointing the first disease cycles to strategically time post-infectional fungicides to afford good disease control by arresting early infections. Correct timing of sprays can reduce the rate of inoculum build-up for subsequent sporualtion/infection events.
There were sufficient variations in weather parameters recorded at the three different sites chosen to warrant placement of separate weather stations throughout the district. These were located in the three major production areas, namely Gatton, Tenthill and Lowood.
Spore-trapping (using a Burkard® volumetric spore trap and trap plants) was demonstrated to be an unreliable predictor of disease on which to base the strategic timing of post-infection fungicide sprays.
From these field trials we concluded that the introduction of the forecast model as a disease prediction was feasible for the Lockyer Valley and offered a means of more accurate fungicide scheduling than calendar spraying.
Acrobat® MZ 690 (600 g/kg + 90 g/kg dimethomorph) treatments performed better than Ridomil® MZ 720 (640 g/kg mancozeb + 80 g/kg metalaxyl) in both field and glasshouse/controlled experiment experiments, where phenylamide-resistant isolates of Peronospora destructor, the causal pathogen of onion downy mildew were used.
Extensive testing of fungicides, as drench or foliar treatments in the glasshouse and controlled environment conditions showed that dimethomorph was the most suitable fungicide, with an effective post-infection activity period of 48 h, in the presence of both phenylamide resistant and tolerant isolates of P. destructor. Phosphonate fungicides (fosetyl-Al (800 g/kg) and phosphorous acid (200 mL/L)) were ineffective as was metalaxyl against phenylamide-resistant isolates.
Post-infection activity of dimethomorph was enhanced significantly by oil-based adjuvants (mineral or vegetable), especially on mature plants that have a prominent epicuticular wax layer on their foliage. Subsequent detailed scanning electron microscopy (SEM) studies showed that this enhanced uptake was due to the dramatic alteration of the epicuticular wax layer. Dimethomorph showed good anti-sporulant activity, which is essential for a fungicide to be used with a forecast system in the field.
Further research in the area of fungicide-adjuvant interactions is strongly recommended. In particular, it would be of value to determine whether mancozeb, the main protectant fimgicide used in spray programmes in the Lockyer Valley, when used with Synertrol® Oil (a canola oil adjuvant) has some curative activity.
Following these research findings Acrobat® MZ 690 was registered for use in May 1997 and subsequently Cyanamid Australia, the national manufacturer, had recommended that this fungicide be used with an oil-based adjuvant. Also, because of it only having a post-infection or 'kickback' activity of 48 h they recommended it be used on a protectant rather than a curative basis.
The forecast system developed in this research was further validated by Queensland Department of Primary Industries (QDPI) in 1998 and in 1999 the inaugural Onion Downy Mildew Forecasting Service for the Lockyer Valley was launched by QDPI on 30 June. The author was responsible for providing the forecasts on Mondays, Wednesdays and Fridays, which coincided with the high-risk period for the disease.
Close liaison by the author with industry personnel has ensured that interested parties have been kept well informed of our findings. Extension activities have included direct contact with growers, agrichemical resellers, chemical company reps. University of Queensland staff and students, QDPI etc., through field days, seminars, local and regional press releases (newspapers and radio), correspondence to registered onion growers through three extension bulletins, articles published yearly (1993 to 1999), in the national onion industry journal, Onions Australia, articles in magazines such as Good Fruit and Vegetable News and Queensland Fruit and Vegetable News, in QDPI Agrilink Onion Information Kit and three articles in the 1997 Proceedings of the 11th Biennial Conference of the Australasian Plant Pathology Society.