Florida's citrus industry is the state's second largest source of revenue aside from tourism. It employs more than 76,000 people and makes more than $9 billion in profit a year. Through decades of cross-breeding and hybridization, Florida growers have managed to make Florida citrus largely frost-proof and able to resist many common diseases. Unfortunately, this has resulted in a few citrus diseases mutating to be more dangerous and contagious.
Citrus canker is caused by the bacteria Xanthomonas axonopodis. It is the most infectious and problematic disease affecting Florida citrus. Bacteria populate in moist, humid conditions, which are near-constant in Florida, and are then spread to new trees by rain blown from infected trees. The bacteria produce lesions on fruit and new leaves, the telltale canker. They kill off new flower growths, reducing the overall fruit yield of the tree. The juice from such fruit is not safe to drink and the only way to treat this disease is to cut down and burn all trees that show signs of canker. State law requires it.
Diaporthe citri is a fungal infection producing a disease known as melanose. The disease takes hold in citrus foliage first, creating brownish circular indentations which later puff out as the fungus grows. The fungus spreads to the ends of leaf stems and twigs, which can become brittle and die. It is on this dead woody material that the fungus produces spores, which rainwater or irrigation systems spread to the citrus fruit. On the fruit, the fungus creates a series of raised pustules ranging from brown to black in color that have a gritty, rasping texture to the touch.
Fruits are only susceptible to this disease between the times that the flower petals fall off to fruit maturation, which is a period of about twelve weeks. Since infected fruit cannot be cured, copper-based fungicides are used during this twelve week window to discourage infection.
Mycosphaerella citri is a fungus more commonly known as greasy spot. It first presents in leaves and causes the underside to swell while the tops of the leaves develop a yellow mottling. As the tissue within the leaves die, they curl up and turn brown and black. Typically the disease just causes the leaves to drop early, but in grapefruit trees the infection worsens. The oil glands of the grapefruit themselves become infected. Outwardly this is visible as individual tiny black spots.
Eventually the spots collect and coalesce to create rind blotch, which is a random series of linked browning spots surrounded by a pale yellow area. The dark spots are the glands that have begun to necrotize. As the name suggests, the both the rinds and the leaves of infected citrus plants will have a greasy texture due to the overproduction of oil.