Following South African reports of the effectiveness of phosphonic acid for control of non-Oomycete diseases, preliminary testing of phosphonic acid was made for control of root and foliar diseases of stonefruit, apples and celery. Phosphonic acid showed potential for control of stonefruit Armillaria root rot (caused by Armillaria luteobuballna), apple white root rot (caused by Dematophora necatrix), and apple black spot (caused by Venturia inaequalis) but was ineffective for peach brown rot (caused by Monilinia fructicola) and celery leaf spot (caused by Septoria apiicola).