Some variants and possible errors in the test‐tube dilution and slide‐germination methods for laboratory testing of fungicides

Abstract

SUMMARYA method is given for moulding uniform Perspex (polymethyl methacrylate) cavity slides, which are quickly made, almost unbreakable and without joints; they can withstand strong alkalis, but not strong acids or solvents, and are useful for spore germination tests of water‐soluble fungicides. When the drops of spore suspension‐fungicide mixture in the cavities of these slides or of Böttcher's glass cavity slides are enclosed by cover‐slips, there is no meniscus effect, and in tests of long duration the same spores can be counted repeatedly.In such tests, errors may arise from sorption of the fungicide on the surfaces of pipettes and tubes (dilution stage) and on slides and cover‐slips (incubation stage); and from changes in the volume of the drops (incubation stage).Ions of many metals may be lost by sorption, and the losses are often greater on soft than Pyrex glass. Hg was lost rapidly. In a three‐stage serial dilution of HgCl2 solution in soft glassware from 60 to 1.0 p.p.m. of Hg, the loss in strength was 27%. More Hg was lost from HgCl2 than from phenyl mercuric acetate solutions. The losses on glass or Perspex slides with cover‐slips from solutions containing 1.0 p.p.m. of Hg (but no spores in suspension) were 30–35% in 15 min. and 61–74% in 24 hr. with HgCl2, but only 5–10% on glass and 35–37% on Perspex slides in 24 hr. with phenyl mercuric acetate. Increase in temperature (10–25% C.) slightly increased the loss from HgCl2 but not from phenyl mercuric acetate solutions. Perspex slides, which were used repeatedly for tests of water‐soluble mercury compounds and washed in distilled water after each test, eventually became toxic to conidia of Botrytis fabae Sardiña.When the slides, with drops, are placed in moist chambers for incubation, some evaporation is unavoidable because the air close to them is not initially saturated with water vapour. When the chambers were pre‐cooled or pre‐warmed, so that the temperature of the air in them was uniform, the loss from 0.30 ml. drops in cavity slides was not serious (about 8%, with or without cover‐slips, in 48 hr. at 10 or 25% C.). But when the temperature was not uniform, the possibilities of change in drop volume were much greater: in 48 hr., the volume of open drops could decrease by evaporation (44% loss at 10° C.) or even increase slightly by condensation (3% gain at 25° C.). When spores have to be counted on several occasions, the evaporation losses may be more serious.