CROOK ROOT OF WATERCRESS II. THE CONTROL OF THE DISEASE WITH ZINC-FRITTED GLASS AND THE MECHANISM OF ITS ACTION

Zinc, used as zinc sulphate at 0.5 p.p.m., was found to inhibit the growth of Spongospora subterranea (Wallr.) Lagerh., f.sp. nasturtii Tomlinson, the cause of crook-root disease of watercress. A relatively insoluble finely powdered glass frit containing zinc oxide (zinc frit) largely prevented infection by this fungus when added, at 0.2 g./350 ml., to water in which watercress was growing. In field trials made in 1955-56 on brown watercress at Bere Regis, Dorset, and on green watercress at Greatford, Lincolnshire, zinc frit was applied at two rates of 1 and 1/2 lb. per sq. yd. After 4-8 weeks, plants in the treated beds grew more strongly and root infection was reduced considerably. Untreated control beds remained heavily infected from September to April but, in the treated beds, the disease was greatly reduced from November to January though, by April, this effect had largely disappeared. Disease control at both places was associated with increases in total crop and individual plant weight. Field trials in 1956-57 gave similar results. When zinc frit was shaken with distilled water for 24 hr. a solution containing 0.21 m p.p.m. zinc was obtained, but if watercress plants had been grown in the water before it was shaken with the frit the solution contained 2-82 p.p.m. zinc. The treated water was found to contain 16 p.p.m. CO2 which had a solvent action on the frit. Field tests showed that plants in watercress beds liberated sufficient CO2 into the water to bring into solution a fungitoxic concentration of zinc. Observations on the crook-root fungus showed that the suppression of the disease by zinc was caused by the killing of the zoospores. The zinc content of watercress in beds treated with zinc frit was at a safe level for human consumption and the zinc content of the outflow water from such beds was non-toxic to trout (Salmo gairdnerii Richardson). A previous theory on the release of metals from glass by plants is discussed in relation to the experiments described on the release of zinc from zinc frit.