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Dazomet, applied to the soil in September 1972, increased the dry matter yield in 1973 of perennial ryegrass (Lolium perenne L.) cv. Gremie sown that spring in Hertfordshire. The greatest increase (60 per cent) was where least nitrogen had been applied and was not apparently the result of differences in pathogen or pest incidence. Sprays of the fungicides benodanil and benomyl, and drenches of captafol, did not affect yield in 1973 or 1974, although benodanil controlled crown rust (Puccinia coronata Corda) in August and September 1973. Sprays of the acaricide endosulfan and the aphicide menazon had no detectable effects in 1973. In 1974, endosulfan sprays, either alone or " with menazon, controlled Abacarus hystrix (Nal.), the mite vector of ryegrass mosaic virus (RMV), decreased RMV incidence from 54 to 26 per cent and increased dry matter yield by six per cent. Mildew (Erysiphe graminis DC.) was observed in 1973 and leaf spot (Drechslera spp.) was common in 1973 and 1974, but infection by both diseases was only slight. The incidence of barley yellow dwarf virus was slight and not apparently affected by the treatments. Root discoloration was common but not affected by dazomet or captafol. Fusarium culmorum (W. G. Smith) Sacc. was commonly isolated from very discoloured roots and one isolate, which was weakly pathogenic to ryegrass, killed wheat seedlings. After cutting, patches of grass in plots given 150 kg nitrogen/ha/cut showed poor regrowth, but this could not be associated with pathogens. Mildew and RMV were most common on plots given most nitrogen (150 kg N/ha/cut). RMV and its vectors were apparently more detrimental during the experiment than were other pathogens. [ABSTRACT FROM AUTHOR]