CROOK ROOT OF WATERCRESS I. FIELD ASSESSMENT OF THE DISEASE AND THE ROLE OF CALCIUM BICARBONATE

The discovery, occurrence and symptoms of crook-root disease (Spongospora subterranea (Wallr.) Lagerh. f.sp. nasturtii Tomlinson) are described. Assessment of crook root in commercial watercress beds indicated that the disease increased with increasing distance from the water source, was more severe from October to April than from May to September, and affected brown more than green watercress. The healthier condition of plants near the fresh-water inlets, compared with those growing further down the bed, was associated with the low concentration of zoospores. Water from the river Dene, Wellesbourne, contained a factor inhibiting crook root, which was shown to be calcium bicarbonate. In laboratory tests, increasing concentrations of calcium bicarbonate from 62 to 540 p.p.m. gave an increasing degree of control of the disease. The same effect was shown in a small field test. The combined (Ca + Mg) bicarbonate content in eighty-seven spring and artesian waters supplying diseased watercress beds in various counties varied, with one exception, from 282 to 401 p.p.m. The only bed fed by water with a higher total bicarbonate content (525 p.p.m.) was free from crook root. It was shown that solutions containing 350-750 p.p.m. calcium bicarbonate had no effect on the germination of zoosporangia or the activity of zoospores. Certain resemblances are noted between the control of crook root by calcium bicarbonate and the control of club root of brassicas by lime.