Citrus canker is a highly contagious bacterial infection of citrus trees causing yellow halo-like lesions or scabs on the fruit, leaves and twigs of citrus trees. Severe infections can cause leaf loss, blemished fruit, fruit drop and die back. The canker bacterium spreads easily and quickly on air currents, insects, birds and on humans by means of clothing and infected implements. There are a variety of sprays designed to protect against infection including using Liquid Copper Fungicide as a preventative treatment, especially when citrus canker has been detected in the area. Unfortunately, already infected trees are generally destroyed quickly to slow down the spread of the bacteria.
Melanose is a fungal infection of young citrus fruit, primarily but not exclusively grapefruit. The scabbed fruit rind does not affect fruit quality but it is unsightly. The disease is generally more severe in older trees over 10 years of age. As the fungus propagates in dead wood, prompt pruning is an effective way of combating this disease. Liquid Copper Fungicide say can also be used as a treatment.
Greasy spot is another fungus disease of citruses. Telltale signs include yellowish-brownish blister spots on leaves, often on the underside of the leaf. As the disease develops, the spots develop into oily looking blisters. Greasy spot can cause significant leaf loss, particularly during winter and can also infest citrus, particularly grapefruit, rind. To control Greasy Spot, regularly collect and remove any fallen leaves, thus reducing the source of new spores. Spay the tree with Liquid Copper Fungicide in June or July. A second spray application may be needed to be applied in August or September to protect late-summer growth.
There are many other environmental concerns and diseases caused by these trees.