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.