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Stage Specificity in Streptomycin Action Against Some Plant Pathogenic Peronosporales. Yigal Cohen, Department of Life Sciences, Bar-Ilan University, Ramat-Gan, Israel, Current address of senior author: Bar-Ilan at the Department of Plant Pathology, Macdonald College of McGill University, Quebec, Canada; Meir Perl, Department of Life Sciences, Bar-Ilan University, Ramat-Gan, Israel. Phytopathology 63:1172-1180. Accepted for publication 17 March 1973. DOI: 10.1094/Phyto-63-1172.

Streptomycin inhibits sporangial germination of Plasmopara halstedii, the causal organism of sunflower downy mildew, but not that of Pseudoperonospora cubensis, the causal agent of cucumber downy mildew, nor of Phytophthora infestans, the potato late blight agent. Infection of all three hosts by their respective pathogens is inhibited by streptomycin at 10-100 µg/ml in the inocula; however, if applied 24 hr after inoculation, streptomycin has no effect either on symptom expression or on fungal development. Sporulation of all three pathogens on fully developed lesions is not affected by streptomycin. Although fungal development is stopped when streptomycin is added at the initial inoculation, a lesion restricted to the area inoculated is observed on cucumber and potato leaves (“printing”-type lesions). Mycelial growth of Ph. infestans in vitro is greatly reduced by streptomycin. No such reduction occurs if the antibiotic is added to the medium of 48 hr, or older, cultures. It is suggested that streptomycin acts on the fungus rather than on the host, and the later “resistance” to the antibiotic is attributed to some intracellular changes occurring in the fungus after the initial stages of growth.