Brown Citrus Rust Mite attacks the outer exposed surface of the fruit. Fruit blemish occurs from infestation by huge numbers of mites on green, immature fruit in summer/autumn. Blemish is slightly rough, greyish-brown blemish with outside edges deeper brown. The blemish shows up prominently when the fruit colours and becomes more severe the longer the fruit remains on the tree. Rust mites can also cause bronzing of leaves and green twigs.