Phytotoxic Effects of Phenylmercuric Compounds Upon Certain Pear Varieties

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Abstract
In trials at East Mailing Research Station, extending over the years 1952 to 1959, chlorotic spotting, associated with loss of chlorophyll, and distortion of leaves occurred following post-blossom application to four pear varieties of phenylmercuric salts at 0∙003% elemental mercury concentration. On Conference, serious crop losses occurred in five successive seasons after application of 0∙005% phenylmercuric chloride (PMC), and in two seasons when the acetate, dimethyldithiocarbamate, basic nitrate, oxinate, or the salt of a “ fixtan ’’ acid was applied, each at 0∙003% elemental mercury concentration. All these organomercurials caused losses on Doyenné du Comice in both seasons, but Laxton’s Superb and Williams’ Bon Chrétien were less affected. Fruit-bud formation was reduced, on Conference only, by PMC in three successive seasons and by all the salts in one season. Reducing the dosage reduced the loss of crop, but even at 0∙0025% PMC significant reductions in crop and in fruit-bud formation occurred on Conference. Pre-blossom applications appeared to add little to the effects caused by post-blossom applications. Applying PMC in one dose at a given stage of development caused greater crop loss than applying the same dose divided equally over two successive stages. Other evidence supports the view that the effects are not cumulative. In the light of rainfall and temperature conditions recorded on the days following each post-blossom application in each year, crop loss must be expected following post-blossom application of sufficient organomercurial for scab control in any season on Conference and Doyenné du Comice ; on Conference, reduction of fruit-bud formation may also occur. Although pear leaves are obviously damaged by phenylmercuric salts, it is likely that the effects on cropping are largely due to direct absorption and/or translocation of some form of mercury into fruit-buds and fruitlets.

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