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Canopy Size and Induced Resistance in Stylosanthes scabra Determine Anthracnose Severity at High CO₂

First and second authors: Cooperative Research Center for Tropical Plant Protection, University of Queensland, St. Lucia, Queensland 4072 Australia; second author: CSIRO Plant Industry, Queensland Bioscience Precinct, 306 Carmody Road, St. Lucia, Queensland 4067 Australia; and third author: Department of Botany, University of Queensland, St. Lucia, Queensland 4072 Australia

This study examines the relative importance of canopy size and induced resistance to Colletotrichum gloeosporioides at 350- and 700-ppm atmospheric CO₂ concentrations on susceptible Stylosanthes scabra ‘Fitzroy’ from two studies in a controlled environment facility (CEF) and in the field. Plants were grown at the two CO₂ concentrations in a repeated experiment in the CEF and inoculated at 6, 9, or 12 weeks of age. Although the physiological maturity of plants was at a similar stage for all three ages, the number of lesions per plant increased with increasing plant age at both CO₂ concentrations. At 350 ppm, the increase was associated with canopy size and increasing infection efficiency of the pathogen, but at 700 ppm, it was associated only with canopy size, because infection efficiency did not change with increasing age. A level of resistance was induced in plants at 700 ppm CO₂. In a second study, plants were raised for 12 to 14 weeks at the two CO₂ concentrations in the CEF and exposed to C. gloeosporioides inoculum in replicated field plots under ambient CO₂ over three successive years. Fitzroy developed a dense and enlarged canopy, with 28 to 46% more nodes, leaf area, and aboveground biomass at high CO₂ than at low CO₂. Up to twice as many lesions per plant were produced in the high CO₂ plants, because the enlarged canopy trapped many more pathogen spores. The transient induced resistance in high CO₂ plants failed to operate when exposed to pathogen inoculum under ambient CO₂ in the field. These results highlight the need to consider both canopy size and host resistance in assessing the influence of elevated CO₂ on plant disease.