BIOCONTROL OF BOTRYTIS CINEREA USING BACILLUS BREVIS IN LETTUCE GROWN UNDER PROTECTION
RC MCHUGH, K EMSLIE, N MACKAY and B SEDDON
Department of Agriculture, University of Aberdeen, MacRobert Building, Aberdeen, AB24 5UA, UK
Background and objectives
Botrytis cinerea is a major cause of prophylactic use of fungicides, such as iprodione, on protected lettuce crops. More sustainable and environmentally acceptable control may be achieved using biocontrol agents. Bacillus brevis has previously been shown to give control of Botrytis cinerea in Chinese cabbage, grown in polyethylene tunnels, equivalent to the control provided by iprodione . The Bacillus brevis wild type (WT) biocontrol agent has two modes of antagonism to Botrytis cinerea: production of the antibiotic Gramicidin S which inhibits conidial germination; and production of a biosurfactant which is thought to reduce periods of leaf wetness availability to the pathogen and thus restricts its development. A Gramicidin S negative mutant of Bacillus brevis, El, which retains biosurfactant properties gives disease control as good as the Bacillus brevis VvT in tomato crops grown under protection . This study aims to compare control of Botrytis cinerea by iprodione (Rovral), Bacillus brevis WT and the El mutant on protected lettuce in order to establish the contribution of Gramicidin S and the biosurfactant to disease control.
Materials and methods
Five week old lettuce seedlings (Clinton, butterhead type) were planted out in growbags (10 plants per growbag) in a polyethylene constructed tunnel (8.2x6.1 m). A randomised block field design was used with treatments allocated to plots (a plot consisted of two growbags; 10 plots in each of three blocks with duplicate treatments and controls in each block). Three treatments were applied to suppress Botrytis cinerea: Bacillus brevis W7 and El (both grown in Tryptone Soya broth for 14 days) and full strength Rovral. Two identical controls of sterile distilled water were applied per treatment and in the same way. Treatments were applied to run-off at 14 day- intervals from 6 days after planting to 48 days after planting using small hand held sprayers. Disease monitoring was performed weekly or twice weekly (depending on rates of disease progression) by observing percentage Botrytis cinerea leaf cover for each plant and recording disease as percentage leaf cover per plant per plot.
Results and Conclusions
Ten weeks after the start of the experiment, mean Botrytis leaf cover for control plants was 17.8±3.2%. Rovral gave best disease control with a mean Botrytis leaf cover of 1.1±0.09% (93.8% disease control). WT and El also gave good disease control with mean Botrytis leaf cover of 5.3±1.4% and 6.1±3.4%, respectively, representing 70.2% disease control using VVT and 65.7% disease control using El. The similarity in disease control given by WT and El (no significant difference at 10% level) suggests that the action of the biosurfactant is more important than that of Gramicidin S in suppression of Botrytis cinerea in these studies with lettuce. Similar results have been observed with tomato crops  where surface wetness monitoring suggested reduced leaf wetness duration in El treated plants as the basis for disease control. The large variability of disease levels (as indicated by the SEM value given) probably reflects environmental influence due to position in the polyethylene tunnel (proximity to doors, vents, etc.) and consequent spread of disease from initiation loci, in addition to the influence of external environmental factors (temperature, solar radiation, humidity, wind-s, etc.) on the microclimate in the lettuce crop within the polyethylene tunnel. Future work will continue to study the use of Bacillus brevis VVT and El in control of Botrytis cinerea in greenhouse crops and will incorporate environmental monitoring to determine the influence of environmental conditions on disease progression and efficacy of control.
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