Foliar application of phosphonate formulations for the control of potato tuber blight

In the field, foliar sprays of partially neutralised phosphonic acid substantially reduced tuber infection by Phytophthora infestans (potato late blight), compared with other fungicide programmes, despite the development of some foliage infection particularly on older leaves. Trials in which tubers from foliar-sprayed plants were lifted and inoculated with P. infestans demonstrated that phosphonic acid directly reduces the susceptibility of tubers to infection. Application of either 2 or 4 kg/ha markedly decreased the proportion of tubers which developed blight. A spray programme in which application of phosphonic acid is combined with a compatible foliar protectant fungicide would appear to offer the possibility of controlling both phases of the disease and could have a major impact in reducing overwinter survival of the pathogen in tubers.