Iron‐Induced Oranging Disease of Rice in Relation to Physico‐Chemical Changes in a Flooded Oxisol

Soil conditions associated with an oranging disease which occurs in flooded rice ( Oryza sativa ) grown on the Oxisols of the Llanos Orientales of Colombia were studied by measuring changes in pH, Eh, and Fe and Mn levels in soil solutions after flooding. It was found that rates of reduction and the build up of Fe in the soil solution vary greatly for different Llanos soils, between virgin and previously‐cropped soils, and between air‐dried and field‐moist soils. Levels of Fe in solution could be reduced by various water management treatments and by the addition of large amounts of lime or MnO 2 . It was observed that oranging symptoms occur as solution Fe concentration increases and that large plants were more seriously affected than small plants. Roots of affected plants were heavily coated with oxidized Fe and few active white roots formed after the increase in Fe concentration. The disease appears to be due to indirect Fe toxicity, i.e., an Fe‐induced deficiency of mainly P, K, Ca, and Mg. Due to the relatively high level of Fe in solution, the roots become coated with iron‐oxide reducing the root's capacity to absorb enough plant nutrients from an already deficient environment. Under certain conditions plants may also suffer from direct Fe‐toxicity, i.e., excessive Fe absorption by the plant, resulting in bronzing symptoms. A combination of direct and indirect Fe‐toxicity may produce plants with a wide range of intensities of oranging symptoms and brown streaks on the leaves.