CROP-ROTATION WITH PURE STANDS AND MIXTURES OF BARLEY AND WHEAT TO CONTROL STEM AND ROOT-ROT DISEASES

Crop rotations are known to be a natural barrier against a wide range of fungal soil-borne pathogens. Results shown in this paper are based on observations that were made in the third growing period of a 3-year cereal crop rotation (1989 1991). Mixtures as pre-crops and main crop s were compared with pure stands of barley and wheat also grown as pre - and main crops. After this 3-year period, field experiments showed t hat interspecific mixtures of winter barley and winter wheat reduced s tem rot caused by Pseudocercosporella herpotrichoides, Fusarium spp., Gaeumannomyces graminis and Rhizoctonia cerealis. Growing barley and w heat after a cereal mixture resulted in an increased number of healthy plant stems. The pre-crop mixture significantly reduced the disease i ncidence of Fusarium spp. on barley (> 50%) during stem elongation and ripening. Disease incidence of G. graminis on barley during ripening decreased from 44% after barley to 9% when grown after the mixture. Di fferences in take-all incidence on wheat due to different pre-crops we re not significant. The percentage of P. herpotrichoides lesions on wh eat grown in a mixture increased after pre-crops containing barley. Fu rthermore, interactions occurred between pathogens, indicating differe nces in competition. Additional effects on plant health were observed where barley-wheat mixtures were grown for 2 years. In general, the in cidence of stem base diseases was lowest in mixtures that were grown a fter a mixture. Disease reductions on wheat grown in a mixture after a mixed pre-crop were comparable to those disease reductions brought ab out by chemical fungicides applied in pure stands of wheat. Pathogen f requency on wheat hypocotyl and root tissue was estimated using a sele ctive medium: the frequency of G. graminis on wheat tissue decreased f rom 14% after the pre-crop barley and 7% after the pre-crop wheat, to 1% after the pre-crop mixture. The frequency of Fusarium spp. was gene rally not affected, but the spectrum of species changed quantitatively . A greater number of weak or non-pathogenic fungi were isolated from wheat segments when the plants were grown after a mixed pre-crop.