PSEUDOCERCOSPORELLA-HERPOTRICHOIDES, FUSARIUM SPP AND RHIZOCTONIA-CEREALIS STEM ROT IN PURE STANDS AND INTERSPECIFIC MIXTURES OF CEREALS

Cereal species mixtures were used to study the effect of increased genetic diversity in a crop stand on major fungal pathogens causing stem-rot diseases. Mixtures of spring-barley/oats, winter barley/winter wheat and winter barley/wheat/oats were tested in field experiments between 1986 and 1990. Disease incidence of Pseudocercosporella herpotrichoides, Fusarium spp. and Rhizoctonia cerealis was greatest on barley and wheat. Lesions caused by Fusarium spp. were prevalent on oats. Stem-rot symptoms were reduced by 50% in barley/oats and barley/wheat mixtures. The reduction was comparable to the effects obtained with fungicide applications. The effects of mixed cropping of barley, wheat and oats varied greatly and the results were dependent on mixture composition. Disease reductions in barley and oats grown in mixtures were greater than those in wheat. In general, higher proportions of oats in a mixture favoured plant health by decreasing the incidence of diseases caused by P. herpotrichoides and Fusarium spp. A change of pathogen dominance often occurred after the application of fungicides in pure stands. In mixtures, however, total disease pressure was the result of an interacting complex of different pathogens that was less influenced by fungicides. The field experiments demonstrated that the level of disease reductions varied between pathogens, year and mixture composition.