TiO2, SiO2 and ZnO are common additives with improved applications at the nanoscale. The antibacterial activity of TiO2, which has important ecosystem health implications, is well understood. However, less attention has been paid to the antibacterial activity of SiO2 and ZnO despite them also producing reactive oxygen species. This paper explores the relative toxicity of TiO2, SiO2 and ZnO water suspensions towards bacteria (B. subtilis, E. coli) and the eukaryotic Daphnia magna. These three photosensitive nanomaterials were hazardous to all test organisms, with toxicity increasing with particle concentration. Toxicity of the three compounds decreased from ZnO to TiO2 to SiO2 and Daphnia were most susceptible to their effects. Nominal particle size did not affect the toxicity of these compounds. Antibacterial activity was noted under both dark and light conditions indicating that mechanisms additional to ROS production were responsible for growth inhibition. These results highlight the need for caution during the use and disposal of such manufactured nanomaterials to prevent unintended environmental impacts, as well as the importance of further research on the mechanisms and factors that increase toxicity to enhance risk management.

SUMMARY Glasshouse tests on the potato cultivars Majestic and Maris Bard measured the effects of single early foliar sprays of ring‐substituted phenoxyacetic acids (0.9 × 10–3 M) on the incidence of potato common scab, caused by soil‐borne Streptomyces scabies. The most effective compound was 3,5‐dichlorophenoxyacetic acid (3,5‐D), which decreased scab by about 90%; its action was preventative, early sprays being more effective than late However, it slightly decreased yield, and increased the number of tubers per plant and the proportion of deformed tubers. The trichloro‐ and tetrachloro‐ acids containing the 3,5‐dichloro‐ group also decreased scab, though to lesser extents; the most active of these was the 2,3,4,5‐tetrachloro‐ acid, which had the same side effects as 3,5‐D. The herbicide 2,4‐D almost halved scab incidence, but seriously decreased yield. Other acids tested had little or no effect on scab. In tests against S. scabies in culture, 3,5‐D was no more toxic than other disubstituted acids which were inactive, or weakly active, against the disease in vivo. 3,5‐D may be an ‘antipathogenic agent’, preventing scab development by altering tuber metabolism.

SUMMARY Glasshouse tests on potato cv. Arran Banner measured the effects of single early foliar sprays of 18 substituted benzoic and two dichloropicolinic acids on the severity of common scab, caused by soil‐borne Streptomyces scabies. The monosubstituted benzoic acids tested (25 mM) did not affect scab. However, anti‐scab action was shown by some 2,5‐disubstituted acids. Decreases in scab severity were about 35% from 1·6 mM 5‐bromo‐2‐chloro‐ and 2,5‐dimethylbenzoic acids, and over 50% from 1·6 mM 2,5‐dichloro‐, 2,5‐dibromo‐ and 5‐chloro‐2‐nitrobenzoic acids and from 0·05 mM 3,6‐dichloropicolinic acid (clopyralid), which is structurally similar to 2,5‐dichlorobenzoic acid. None of the spray treatments affected yield or shape of tubers. No visible effects on foliage were caused by two of the benzoic acids active against scab (5‐chloro‐2‐nitro‐ and 2,5‐dimethyl‐), but the other active acids caused some distortion. In tests of the six dichlorobenzoic acids against S. scabies in culture, the 2,5‐isomer (which was the most effective against the disease in plants) was one of the least toxic. With other 2,5‐disubstituted benzoic acids, it probably decreased scab indirectly by altering the response of the host to infection.

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