Polk County FL Horticulture: Citrus Greening

Citrus Greening is a disease transmitted to all varieties of citrus and some ornamentals (i.e. box orange and orange jasmine) in the Rutaceae family by a little insect called the Asian Citrus Psyllid (ACP). The insect was first found in Florida in 1998 and the disease, which is a bacterium, was discovered in residential trees in south Florida in 2005. The insect and the disease occur in citrus production areas all over the world and now occur in most if not all citrus production areas in the United States and Florida. The disease is present in most commercial groves and probably in most if not all homeowner citrus trees. Commercial growers in Florida are struggling to control this disease. Chemical control of the psyllid and removal of infected trees are the only methods currently available to growers to manage the spread of the disease.

How would a homeowner determine if his tree is infected? Symptoms can be found year round but are most common September through March (see below). The tree may remain symptomless for some time after it has acquired the disease. Symptoms include vein corking, fruit remain green at blossom end, leaves with yellow veins, reduced fruit size, yellow shoots, fruit and leaf drop. Blotchy mottle in leaves is a key early diagnostic symptom. The tree will gradually decline and the juice will become bitter and of no value. The tree will decline over a number of years and may eventually die.

Is there some treatment to help control this disease? There are no specific treatments for control of this disease in the home landscape or in commercial groves. If a homeowner does decide to plant a citrus tree, then it would be best to treat the soil around the tree with a systemic insecticide at planting like Bayer Advanced Citrus, Vegetable and Fruit Insecticide with imidacloprid. This treatment may control the psyllid and prevent greening infection while the tree is relatively small – 8 or 9 feet tall however, this treatment may not be effective for preventing greening infection in large mature trees.

It has been found that very healthy trees in commercial groves are able to live with the disease and be productive for a longer time than unhealthy trees. So we encourage homeowners to closely follow fertilization recommendations and make sure the tree has ample water during periods of low or no rainfall to minimize tree stress. As the greening bacterium blocks off the phloem preventing nutrient transfer to the leaves from the roots, we recommend making 2-3 foliar applications of micronutrients at full leaf expansion during leaf flushes in the spring, summer and late summer/fall. The application of horticultural oils, like Volck Oil, to control sucking insect infestations will also help minimize tree stress.

Are there any on-going non-invasive research projects for control of greening which might have homeowner application? The USDA at Ft. Pierce is currently conducting experiments using heat treatments to kill the greening bacteria in infected trees - this is intended for commercial use at this point? I don't, at the moment, see how this could be done on a commercial scale, but it might have homeowner application. A possible scenario would be to cut the tree back to a few feet off the ground then cover and apply heat to a specific temperature. However, there are many questions to be answered. What kind of tree cover to use and what type of heat source? What temperature is needed and for how long - the temperature would need to be high enough to kill the bacteria and not damage the tree? How often would some type of heat treatment be needed? A greening infected tree has the bacterium in the roots, so how fast would the upper growth be infected from the roots? And what is the potential for re-infection by wild psyllids. The USDA Lab in Ft. Pierce is offering a workshop on April 30th which I plan to attend.

Until reliable control options become available to homeowners we recommend planting some other type of fruit – there are many to choose from including pomegranate, fig, blueberry, peaches, plums, nectarines, blackberries, and many more.