Evergreen backyard orange trees are a common sight in areas where the weather is generally warm and the risk of frost is low; examples of states where these citrus plants are widespread in residential landscapes are Florida, California and Arizona. Like any other plant, orange trees are susceptible to a number of diseases--some of them are fungal while others are bacterial. Other diseases are closely associated with soil-dwelling nematodes.
Orange trees may become infected with the deadly citrus greening bacteria which are transmitted by the Asian citrus psyllid. At the onset of the bacterial disease, the oranges become misshapen in form. Leaves yellow and fall to the ground. A telltale sign is the lopsided appearance of the symptoms that gradually invade larger portions of the trees--in keeping with the spread and feeding patterns of the bacteria-carrying psyllids--until they affect the entire orange tree.
Infection of a tree results in its certain death. If the Florida citrus industry is an indicator--the University of California’s Farm Advisor Craig Kallsen advises that as of 2005 more than 60,000 acres of trees required removal--California and other citrus-growing states are also at risk. As of 2010, prevention and complete destruction of affected trees is the only way to stop the spread.
It is interesting to note that sooty mold, which is a fungal disease, is actually also an indicator of an animal pest infestation. The spores of the fungus grow on the fecal matter of whiteflies and aphids, as well as other insects which suck the sap from the leaves of orange trees. The fungal infection itself does not harm the trees, because the mold grows on top of the matter covering the leaves.
Backyard gardeners control the mold by controlling the aphids and whiteflies, as well as other animal pests that feed on the leaves’ sap. Do not mistake this mold for the greasy spot fungus that leads to copious leaf drop. Unlike the sooty mold, which covers the entirety of the leaves, the greasy spot fungus only presents with a spotty appearance on the tops of the leaves.
The soil-dwelling citrus nematode is a slow killer of backyard orange trees. The overall health of the trees determines the length of time it takes from initial infection to fatal decline. In some cases, it may take three to five years before you notice any decline at all. The nematodes infect the root systems, which gradually leads to a steady decline in orange production and quality of the fruit.
Over time, the tree fails to replace falling leaves and some--usually upper--branches become bare. Gardeners who suspect their tree may be falling victim to the citrus nematode must take soil samples from between the orange tree’s trunk and the tree’s overall drip line. A lab can confirm the harmful presence of the nematodes. Treatment with chemicals may not be possible and only an uprooting of the tree and removal of the affected soil halts the disease.