1Deptartment of Microbiology, University of Durban-Westville, Private Bag X54001, Durban 4000, South Africa; 2Department of Microbiology and Plant Pathology, University of Pretoria, Pretoria, 0002, South Africa

Background and objectives
The downy mildew fungus, Peronospora parasitica (Pers. ex. Fr.) Fr is the most frequently recorded disease on horticultural and agricultural members of the genus Brassica [2]. Control of this pathogen usually involves the application of fungicidal sprays and observation of strict cultural practices. However, due to the over-usage and over-dosage of fungicides, most fungicides have lost their efficacy against the downy mildew pathogen [1]. Thus there is an urgent need for the control of this pathogen which attacks the young cabbage seedlings, which are most susceptible to infection. Biological control of the downy mildew disease on crucifers has not yet been investigated. Hence, the aim of this study was to determine effective biological control agents against the pathogen.

Materials and methods
Bacteria were isolated from growth media contaminated with P. parasitica, and leaf surfaces of P. parasitica-infected cotyledons of cabbage seedlings. The identification of microorganisms were conducted using Gram, spore and capsule stains, and API test kits. Bacterial cultures were maintained on nutrient agar plates. Seeds of cabbage cultivar Cape Spitzkool were sown in pine bark growth medium. 200 seedlings/trial were subjected to the experiment. Bacterial cultures were grown in nutrient broth. After 24 h, the cultures were applied as sprays on 10 day old cabbage seedlings. Uninoculated nutrient broth served as a control. Seven days after spraying cotyledons with the antagonists, cotyledons were inoculated with P. parasitica. Cotyledons were visually observed for signs and symptoms of disease development. Trials were repeated five times.

Results and conclusions
Bacillus sp. and Pseudomonas sp. were identified using API kits. B. cereus, B. coagulans, B. subtilus and P. aeruginosa were also isolated and identified, and applied as antagonistic organisms. Results of the spray trials using all six antagonistic organisms against the pathogen indicated only lesion development on cotyledon surfaces. Sporulation of the pathogen did not occur. Results of trials with recurrent inoculation of cotyledons with the pathogen influenced sporulation of P. parasitica on the abaxial and adaxial surfaces of the cotyledons. However, this was apparent on only 15% of the cotyledons. Spread of infection did not occur, suggesting a probable presence of the antagonistic organisms. Lesion development on the cotyledon surface is a form of host defense mechanism to invasion by the pathogen [3]. The antagonists either triggered host defense mechanisms or destroyed the pathogen in competing for nutrients. Sporulation of P. parasitica in recurrent spray trials occurred as a result of an increase in pathogen inoculum, suggesting that antagonistic organism population levels were not optimal.

It is concluded from this study that the antagonists employed have potential as biological control agents against P. parasitica. Future work would involve recurrent spray trials with the antagonistic organisms in order to maintain optimal population levels, and application of scanning electron microscopy to elucidate pathogen-antagonist interactions on the host leaf surfaces.

1. Brophy TF, Laing MD, 1992. Crop Protection 11, 160-164.
2. McMeekin D, 1960. Phytopathology 50, 93-97.
3. Thomas CE, Jourdain L, 1990. HortScience 25, 1429-1431.