Summary: Two sprays of streptomycin (1000 μg./ml.) considerably reduce sporulation of the hop downy mildew fungus, on leaves of primary systemically infected shoots. Streptomycin markedly inhibited germination of its sporangia Toxicity was dependent on sporangial concentration, incubation temperature and the cleanliness of the suspension but was independent of sporangial age. There were no marked differences in sensitivity to streptomycin between sporangia from five different isolates of Two commercial formulations of streptomycin were as inhibitory to sporangial germination as pure streptomycin sulphate. Sublethal concentrations of streptomycin interfered with cleavage of sporangial cytoplasm and increased the proportion of multiflagellate zoospores. The proportion of multiflagellate multinucleate zoospores liberated was dependent on streptomycin concentration. Germination of multiflagellate zoospores was observed. Streptomycin was very inhibitory to zoospore germination but had little effect on zoospore motility and germ-tube growth at concentrations which caused a high incidence of cyst lysis. Streptomycin was as toxic to sporangia of as to those of whereas sporangia of and were less sensitive to the antibiotic.


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