Abstract
The cause of leaf chlorosis that occurred recently on greenhouse-grown cucumber plants in the northern district of Fukuoka Prefecture was investigated. The leaf chlorosis occurred only in greenhouses where virgin soils of granodiorite origin were used as a soil-dressing material. Soil chemical analysis revealed that these soils are rich in exchangeable Mg, particularly those excavated from the lower soil strata as compared with those of granitic and basaltic origins. The leaf chlorosis was more severe in the greenhouses with higher contents of exchangeable Mg in the cultivated soil. Leaf chlorosis was evident in leaves with the Mg content above 2.0%; the higher the Mg content, the greater was the severity of leaf chlorosis. In a preliminary water culture experiment, cucumber plants displayed leaf chlorosis similar to that observed in commercial greenhouses when the Mg concentration in the nutrient solution was above 20 me-liter'1. From these results, it is concluded the leaf chlorosis of cucumber plants that occurred in the northern district of Fukuoka Prefecture was mainly caused by excess accumulation of Mg in the leaves, which is attributed to dressing of virgin soils of granodiorite origin with having a high content of exchangeable Mg.
