Citrus greening has been described as the world's most serious citrus disease, and researchers are combining their efforts to help combat this threat to an important Florida industry. Citrus greening is a fatal disease that has never been successfully eradicated in other parts of the world.
The disease, which slowly weakens and kills all types of citrus trees, causes fruit to become lopsided and taste bitter, making it unusable. Fruit does not develop the desired color, hence the greening name.
There is no known cure for the disease, which is on the U.S. Department of Agriculture's select list of threats to plants and wildlife regulated by the Agricultural Bioterrorism Protection Act. The disease does not harm people.
The scientific name for citrus greening — huanglongbing — means “yellow shoot” which describes the symptom of a bright yellow shoot that commonly occurs on a sector of infected trees. It is a serious disease of citrus because it affects all citrus cultivars and causes decline of trees.
Citrus greening has seriously affected citrus production in a number of countries in Asia, Africa, the Indian subcontinent and the Arabian Peninsula, and was discovered in July 2004 in Brazil. Wherever the disease has appeared, citrus production has been compromised with the loss of millions of trees. It has not been reported in Australia or in the Mediterranean Basin.
In August 2005, the disease was found in the south Florida areas of Homestead and Florida City. Since that time, multiple residential and commercial citrus sites have been found infected with citrus greening. The type found is the Asian form which occurs in warm low altitude areas.
According to a recent report published by the University of Florida Institute of Food and Agricultural Sciences (IFAS), the initial or early symptoms of citrus greening on leaves are vein yellowing and asymmetrical chlorosis referred to as a blotchy mottle. The blotchy mottle symptom is the most diagnostic symptom of the disease.
Leaves might be small and upright with a variety of chlorotic patterns that often resemble mineral deficiencies such as those of zinc, iron, manganese, calcium, sulfur and/or boron. Often some of the leaves may be totally devoid of green or with only islands of green spots.
The blotchy mottle symptom also may be confused with other diseases such as stubborn, severe forms of citrus tristeza virus (CTV), Phytophthora root rot, water logging or citrus blight. Root systems of infected trees are often poorly developed and new root growth may be suppressed.
Citrus greening is difficult to manage and continued production of citrus has proven difficult and expensive in areas where it is widespread. Since it is transmitted by the Asian citrus psyllid, which is well established in Florida, there is clearly a potential for the continued spread of the disease throughout Florida citrus.
IFAS researchers say the use of clean budwood and certified healthy trees is essential in preventing the disease. Budwood sources and nursery stock should be protected from psyllid infestation by screened enclosures and the use of systemic insecticides such as imidacloprid. Some biological control of the psyllid is available, but the amount of psyllid control provided by introduced parasitoids has not been sufficient to limit disease spread.
The Asian citrus psyllid, say the scientists, feeds on many rutaceous plant species. The psyllid has a preference for the landscape ornamental, orange jessamine (Murraya paniculata). It has recently been found to be a host of the citrus greening bacterium and can serve as a host plant for the psyllid.
Another rutaceous ornamental, Severinia buxifolia or orange boxwood, is also a host for the bacterium as well as the psyllid. Movement of these ornamentals is restricted under state compliance agreements and should not be moved from areas where the disease occurs.
Scouting for greening-infected trees should be done routinely so that infected trees can be removed. It has been suggested that scouting be conducted four times per year. The frequency of scouting may be higher in areas previously determined to have positive greening trees.
Symptoms of citrus greening, say IFAS researchers, are the easiest to find from October to March. However, symptoms may be present at other times of the year. The current methods used to scout are walking, using all-terrain vehicles, and on platforms mounted on vehicles.
Symptomatic tree numbers and the rows in which they are found should be marked with colored flagging tape and GPS coordinates taken or the sites marked on a map to facilitate relocation and removal of these trees. In some cases, a greening PCR diagnosis test may be necessary to confirm the disease.
Diagnosis of citrus greening may be difficult since some nutrient deficiency symptoms and other problems are often confused with some of the symptoms associated with greening. Greening-affected leaves may accumulate starch, and an iodine-based starch test can be used to assist in determining what leaves should be sent for PCR diagnosis.
The iodine test alone is not used for greening diagnosis. However, it is useful in deciding which leaves should be sent for diagnosis.
The procedure for the test can be found on the Citrus Research and Education Center greening Web site at the address listed below. Samples of suspected greening infected trees may be sent in for PCR diagnosis to the Southern Gardens Diagnostic Laboratory or to the Southwest Florida REC in Immokalee beginning in the late spring of 2008.
The procedures for submission of suspect samples for PCR testing is available at the following Web site: http://www.flcitrusmutual.com/content/docs/issues/canker/sg_samplingform.pdf
Removing infected trees is the only way to insure they will not serve as a source of the bacteria for psyllid acquisition and subsequent transmission. Prior to removal, the infected tree should be treated with a foliar insecticide (such as Danitol, fenpropathrin) to kill all adult psyllids feeding on that tree.
Failure to control these psyllids will result in the infected psyllids dispersing to new plants once the diseased tree is removed. Pruning of symptomatic limbs has been attempted in many countries to reduce the inoculum available to the psyllids. However, because the disease is systemic, pruning has not been successful since other parts of the tree may already be infected, but not yet symptomatic.
Additionally, if a tree is still infected after pruning, the new flush produced will serve as a feeding site for adult psyllids to acquire the greening bacteria. The infected psyllids may then disperse to uninfected trees once the new flush hardens off.
Integrated pest management strategies should focus on the following: Use of disease-free nursery trees, reduction of the inoculum by frequent disease surveys, removal of symptomatic trees, and suppression of Asian citrus psyllid populations.