Downy mildews on seedlings (pansies. stocks and brassicas) are a major disease problem for the nursery industry with economic losses estimated at $4 million annually in Australia.
This project evaluated fungicides, tested seed for contamination, monitored spore release, recorded weather data, examined the effect of light on disease development and reviewed the literature on downy mildews in nurseries, to develop integrated control strategies.
These strategies, which consist of both management practices and fungicides spray programs, have been adopted by commercial nurseries in Victoria and the incidence of downy mildew has dropped by 80-100% on Brassica seedlings and by 100% on stock seedlings. Sales of stock seedlings have risen by 30%.
The nursery industry can confidently adopt the following practices to control downy mildew on nursery seedlings. Fungicides alone will not control downy mildew if disease pressure is high.
- Irrigation: Avoid watering seedlings in the mornings when spores are released and available for infection.
- Ventilation: Improve ventilation by spacing trays of seedlings and by growing fewer seedlings per punnet so leaf surfaces will dry off quickly, reducing leaf wetness and conditions which are suitable for infection.
- Hygiene: Remove sources of spores, by roguing heavily infected trays of seedlings, control weeds which may act as an alternative host for the Brassica downy mildew and avoid placing healthy seedlings near diseased seedlings.
- Nutrition: Maintain a balanced program of nutrition as a deficiency of potassium (K) will make seedlings (cauliflowers) more susceptible to the disease.
- Fungicides: If seedlings are free of the disease during summer, commence a preventative spray program in late March or early April (Melbourne) when temperatures are falling to 15°C and rainfall is on the increase.
Apply a combination of a registered systemeic fungicide (Agrechem product pending registration) as a drench once a fortnight and a weekly spray of a registered contacted fungicde e.g. Mancozeb®. till about October when temperatures are consistently above 15°C and rainfall in declining.
- Monitor: Check seedlings at least once a week to detect early signs of the disease.
Financial support for project NY9406 was provided by the Horticultural Research and Development Corporation, the Nursery Industry Association of Australia and Agriculture Victoria.
The authors thank Mr Richard Wall, the management and staff at Floriana Nursery and Mr Peter Wood, the management and staff at Woodlyn Nurseries Pty. Ltd, for use of their nurseries as field sites and for their tremendous in-kind support for the project.
The achievements of this project would not have been possible without the enthusiastic cooperation and assistance of these nurseries.
The authors thank Ian Pascoe and Martin Mebalds for their contributions and planning at the commencement of the project.
Dr. Bob Taylor (Consultant) and Ian Atkinson (NIDO), are thanked for advice on the project.
The authors acknowledge South Pacific Seedsfor donations of cauliflower seeds.
Bayer Australia Ltd. BASF Australia, Schering Pty. Ltd. and Sandoz Australia Pty. Ltd. for donations of fungicides.