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Foliar sprays of fungicides were evaluated in a field where winter wheat cv. Joss Cambier was subject to a severe natural infection of yellow rust (Puccinia striiformis Westend.). Almost complete freedom from the disease was achieved on the leaves from repeated applications of BAS 3170F. This material prevented the development of appreciable disease for up to four weeks after spraying. Other chemicals gave varying degrees of protection but most lacked persistence. Control of ear infection was less satisfactory. The untreated crop, which was heavily attacked by yellow rust, showed a yield reduction of 34 per cent compared with the crop receiving greatest protection. The relationship between yield and yellow rust at growth stage 11.1 was statistically significant; the loss of yield recorded was 0.022 t/ha for each one per cent increase in the level of yellow rust on the flag leaf. [ABSTRACT FROM AUTHOR]