Citrus is attacked by innumerable pests and pathogens. Although many of the pathogens are widespread, their relative importance may vary from region to region. Thus, citrus is subject to attack by a number of diseases, each of very great regional importance, but often minor importance elsewhere. Of the insect pests, citrus being a non-rotational crop, provides an ideal host for those which are sedentary and immobile.
The most important insect pests of citrus are the scale insects. The red scale which originated in Japan is the most important pest in California, South Africa and the Mediterranean region. The black scale occurs in all the main citrus-growing regions including South America. Scale insects invade the entire tree, attacking the wood, the foliage and the fruit. With such an infestation, heavily attacked trees shed their leaves and the branches die back. The second most important pests are mealybugs, which are widely distributed throughout the world. Other major insect pests include the white fly, which occurs in the USA, the black-fly which is important in India and in the Far East, and the Mediterranean Fruit Fly which is a serious pest in the Mediterranean region and South Africa. Red spider is also serious both in the USA and South Africa. In the Mediterranean region its place is taken by the rust mites.
Of all citrus diseases, a fungal disease, brown rot gummosis, or root rot, is the most widely distributed; the two alternative names describing the two most obvious symptoms. If the attack is severe, trees may die within a year. It caused very serious losses in California and Florida and in Sicily, where the citrus trees were wiped out between 1863-70. Melanose is another fungal disease which can cause very heavy losses. It disfigures the fruit of all varieties of citrus, particularly grapefruit. Melanose is very widely distributed in regions with an early summer rainfall such as Florida. Citrus tristeza is a serious viral disease in South America, South Africa and Australia. Trees which have been attacked may die within 3 months. In Brazil and Argentina during the 1930s and 1940s citrus growers suffered the appalling loss of some 20 million trees due to this disease.
Some 30 different species of fungi are recorded as causing rotting of citrus fruits. The chief offenders are two species of Penicillium, the common green mould and the blue contact mould. The two moulds have a world-wide distribution and are found wherever there are citrus fruits; in the plantation, in the pack-house and in the markets.