In the field experiment carried out in 2002-2004 the effect of undersown catch crops of Italian ryegrass and red clover and stands in crop rotations with 25, 50 and 75% of spring barley on its health was assessed. The composition of microorganisms colonizing the stem base in barley was also determined. It was indicated that catch crops did not have effect on differences in the degree of barley infestation by pathogens of net blotch, ear blight and fusarium foot rot. However, they caused increased incidence of powdery mildew and scald. Rust appeared only on barley grown without catch crops. An increased development of net blotch was observed on barley with catch crop, grown after spring wheat, and of scald – after potato. In pure stand the highest intensity of net blotch, powdery mildew, rust and leaf-strip disease was found in crop rotation with 75% of barley. This crop rotation was also favorable for development of the stem base diseases. No effect of the sowing method and stand in crop rotation on the number of microorganisms colonizing the stem base of spring barley was observed.

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