Seed transmission of H. gramineum was increased by soil temperatures below 12.degree. C and reduced or prevented above 15.degree. C in field plantings of naturally infected seed [Hordeum vulgare]. The critical stage for infection of the germinating embryo began when the coleoptile reached the apex of the seed and continued until the seedling emerged from the soil. Inoculum located in the pericarp and seedcoat over the embryo was most effective in producing seed-transmitted stripe. A laboratory assay for presence of the fungus in barley seed demonstrated that the percentage of seed transmission was always less than the percentage of infected seed. Developing barley seed was susceptible to infection during any stage of development from before head emergence through the soft dough stage. Infection of developing seed occurred over a temperature range of 10.degree.-33.degree. C, and free moisture was not required.