Abstract: Yellows diseases have devastated crops, particularly of citrus fruit. They are associated with (and ostensibly caused by) very small mycoplasma or spiroplasma type organisms (MLO for short). Attempts at chemical control of MLOs have been unsuccessful. The present invention is based on a remarkable new finding that biological control can be achieved by introducing into the phloem of the plant a virus which is infective to the MLO. The invention is of particular interest in relation to diseases associated with Spiroplasma citri, especially in citrus plantations.