PERSISTENCE OF STREPTOMYCIN IN PEAR AND APPLE TREES
Seventy eight and twenty six μg of streptomycin per fruit and per leaf were recovered from apple tree just after spraying (200 μg streptomycin sulfate/ml). A progressive decrease in residual quantity of streptomycin was observed within fifteen days of spray ; after 41 days no residue was detected. A smaller quantity of antibiotic could be detected in pear leaves (just after spray) and was found to be completely disappeared after 34 days. A longer persistence of streptomycin (more than 98 days) was noticed with higher concentrations of spray (400 μg/ml).
On pear trees weekly spraying of streptomycin (200 μg/ml) resulted in the deposition of antibiotic about an average of 17.31 ± 4.89 μg per leaf, varying from 6.16 to 34 μg per leaf. An average quantity of antibiotic about 9.48 μg ± 4.18 μg per leaf was detected after 7 days but it varied from less than 1 μg to 27.46 μg per leaf. No cumulative effect of weekly spraying during 13 weeks was obtained. A non systemic persistence of this antibiotic was also noticed.
Results reported here indicated that no surface residue of the antibiotic was detectable after 4–6 weeks after spraying with pratically applicable quantity (50 to 200 μg/ml).