Inoculum and disease dynamics of soil borne plant pathogens in consecutive crops : an epidemiological analysis
D. Bailey (1), P. Lucas (1), M. Gosme (1) N. Paveley (2), J. Spink (3), A. Kleczkowski (4) C. Gilligan (4)
(1) INRA - Agrocampus Rennes UMR BiO3P, BP 35327 F - 35653 Le Rheu Cedex
(2) ADAS High Mowthorpe, Duggleby, Malton, North Yorkshire YO17 8BP, UNITED KINGDOM
(3) ADAS Rosemaund, Preston Wynne, Hereford HR1 3PG, UNITED KINGDOM
(4) Department of Plant Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EA, UNITED KINGDOM
Strategies for sustainable control of economically-important soil-borne disease of crops requires a shift in current emphasis from tactical application of control in a single crop to consider the dynamics of control during a sequence of crops. The disease status in successive crops reflects a highly nonlinear, often stochastic and spatially heterogeneous amplification, decay and dispersal of inoculum during sequential parasitic and saprotrophic phases of the epidemic. Control in one season therefore influences the amount and the distribution of inoculum in the following season. In this paper we present an overview of the disease and inoculum dynamics over consecutive crops together with their control for two contrasting patho-systems; damping-off of vegetables by Rhizoctonia solani using microcosm experimentation and field epidemics of take-all caused by Gaeumannomyces graminis in a sequence of winter wheat crops. In the former, changes in the net infectivity of inoculum at the beginning of first and second crops caused a switch in epidemiological dynamics. Epidemics of first crops were dominated by secondary infection leading to amplification of inoculum so that epidemics of second crops were overwhelmingly determined by a high rate of primary infection. The biocontrol agent reduced primary infection and hence parasitic amplification of inoculum in both first and second crops but the efficiency of control dropped from 91.7% in first crops to 64.8% in second crops with sudden outbreaks of disease in second crops that had previously been disease-free. We conclude that parasitic amplification can cause a rapid build-up of disease and inoculum over consecutive crops leading to loss in the efficiency of biocontrol. This form of inoculum production is supplemented by saprotrophic infestation that can result in sudden outbreaks of disease in protected crops where control of disease had previously been fully successful. For take-all of wheat, first wheat crops were characterised by low rates of primary infection and secondary infection with little evidence of disease suppression and a low rate of inoculum decay. Second and third wheats were characterised by high rates of primary and secondary infection and with increasing evidence of disease suppression and more rapid decay of inoculum. In subsequent wheat crops, the rate of primary infection remained high but a reduction in the rate of secondary infection was detected leading to less severe epidemics and the well-known phenomenon of take-all decline.