ANTHRACNOSE DEVELOPMENT IN PURE AND MIXED STANDS OF THE PASTURE LEGUME STYLOSANTHES-SCABRA

Development of anthracnose, incited by Colletotrichum gloeosporioides, was examined over three summer seasons in pure and mixed stands of the tropical pasture legume Stylosanthes scabra. Three accessions and two cultivars with different levels of resistance to four races of the pathogen were used. The susceptible cultivar Fitzroy was either excluded or included in increasing proportions in four different mixtures. Significant reduction in the area under the disease progress curve (AUDPC) in mixtures over the pure stand of the components occurred only in certain mixtures in some years. In the absence of Fitzroy, AUDPC in mixture was lower than the arithmetic mean of AUDPC of the components in pure stands in only one of the three growing seasons. In the mixture containing equal proportions of all components including Fitzroy, AUDPC was similar to the mean of pure stands in all 3 yr. In mixtures with more than 50% Fitzroy, AUDPC was significantly less in 2 of the 3 yr. There was no improvement in the survival of plants of individual accessions in mixtures compared with that in pure stands. In general, Fitzroy had consistently less AUDPC in mixtures with more than 50% Fitzroy than in its pure stand, by contrast, AUDPC of the more resistant accessions did not differ significantly from their respective pure stands in any year. Severity of individual accessions did not increase significantly with increasing Fitzroy content of a mixture except for Seca and Fitzroy in 1988 and 1989, respectively. The study emphasizes the need for long-term monitoring of host and pathogen populations in perennial pastures to ascertain the role of genotype mixtures in restricting anthracnose.