Production and dispersal of Colletotrichum gloeosporioides spores on Stylosanthes scabra under elevated CO2

This paper reports the effect of twice-ambient (700 ppm) atmospheric CO2 concentration on infection, disease development, spore production and dispersal of the anthracnose pathogen Colletotrichum glocosporioides in susceptible (Fitzroy) and partially resistant (Seca) cultivars of the tropical pasture legume Stylosanthes scabra under controlled environment and field conditions. Reduction in plant height due to anthracnose was partially compensated for by growth enhancement at elevated CO2 in Fitzroy but not in Seca. Anthracnose severity was reduced under elevated CO2 although the reduction was only significant in Fitzroy. Delayed and reduced germination, germtube growth and appressoria production were partly responsible for the reduced severity. Despite an extended incubation period, C. gloeosporioides developed sporulating lesions faster and produced more spores per day within the same latent period at high CO2 and ambient CO2. When Fitzroy seedlings grown at 700 ppm CO2 were exposed to pathogen inoculum under field conditions, they consistently developed more severe anthracnose with more lesions than seedlings grown at ambient CO2, The environmental variable, which correlated most strongly with the dispersal and infection of C. gloeosporioides spores in the field, was relative humidity in plant canopy. We have shown that an enlarged Stylosanthes canopy under elevated CO2 can trap more spores, which can lead to more severe anthracnose under favorable weather. The implications of these findings for perennial Stylosanthes pastures are discussed. (C) 2000 Elsevier Science Ltd. All rights reserved.