Powdery mildew disease development and its spread were evaluated in Co-cultivation of different wheat cultivars in the field . Golestan (partially susceptible) and Falat (partially resistant) , Golestan and Milan Wheat cultivars were co-cultivated in 1991- 92 and 1993 respectivly at different seed ratios of 0.100,25.75, 50.50, 75.25 and 100.0 . Disease severity and crop yield. were analyzed in a complete randomized block design using five treatments in five replicates at Sari Agricultural Research Station. The results showed a significant difference between a monoculture and mixed cultivation. The most efficient mixing ratio was at 50.50 . Disease severity increased during periods of high humidity. Co-cultivation of different wheat cultivars led to a reduction in Powdery mildew disease severity by 40% as compared to a single crop cultivation and increased the yield by 1750 Kg/hec.
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