Huanglongbing is a bacterial disease of citrus that until recently was confined to Asia and Africa. In 2004, the first discovery of the disease in the Western Hemisphere was in Sao Paulo, Brazil. In 2005, the disease was first discovered in Florida. Since then, it has spread rapidly and now can be found in all counties in Florida that contain commercially produced citrus. HLB represents one of the strongest threats to the largest citrus-producing state in the United States. In the 2007–08 season, Hodges and Rahmani (2009) estimated that the economic value of the citrus industry on the economy of Florida was $8.9 billion.
HLB affects citrus trees by blocking the phloem or the vascular system of the tree, limiting its ability to transport nutrients within the plant. It is spread by a small leaf-feeding insect: the Asiatic Citrus Psyllid (ACP), Diaphorina citri Kuwayama. The characteristics of the disease are mottled leaves and small misshapen fruit. Increased fruit drop is associated with the disease; but even if fruit remains on the tree until harvest, the fruit is undersized and contains bitter or off-flavor juice, rendering it of no economic value.
The disease first appeared in China in the early 20th century. It was first called “yellow shoot disease.” Lin and Lin (1956) are credited with first identifying the disease and gave it the name HLB. They found the disease in several provinces of southern China.
Two approaches have evolved to combat the disease. The first approach is to immediately remove any tree exhibiting symptoms of the disease. The second approach is a nutritional-based program. The first approach is generally credited to Bové (2006). In the Bové approach, an aggressive scouting program is initiated. Any tree found that exhibits symptoms of HLB is immediately eradicated. A program to suppress the ACP population is also initiated.
One drawback of the Bové program is that infected trees may not exhibit symptoms for up to 2 years (in the case of mature trees) after becoming infected. Thus, eradicating only symptomatic trees will not eliminate the disease. Diligent implementation of this approach should suppress the level of disease inoculum over time so that annual tree losses are economically tolerable. Another disadvantage of this approach is the so-called “bad neighbor” effect. If a single grower among a contiguous planting fails to follow the Bové approach, their grove will continue to serve as a source of inoculum. Another issue is that if the level of infection is too high before the disease is discovered, it may be necessary to eradicate an entire block.
University of Florida economists have estimated that the Bové method increases per-acre grove maintenance costs by ≈$400 annually, an increase of 33% since the disease was first found in Florida (Muraro). With rapid escalation of input costs, especially fertilizer, estimated annual grove maintenance costs of Florida are now placed at nearly $2000 per acre ($4942 per ha), which does not include harvesting cost. As bearing tree numbers decrease, per-acre yields will also decrease.
A grower in southwest Florida, faced with a high of level of infection, decided to implement another approach to deal with HLB. Because HLB blocks the phloem of citrus trees, he devised an approach of feeding trees through their leaves, thus bypassing the phloem transport issue. This approach is known as enhanced foliar nutrition. Under this approach, scouting for the disease is halted, and symptomatic trees are not eradicated. This approach has shown some success in masking the effects of the disease, enabling trees to produce significant volumes of fruit. It elevates the cost of grove maintenance, ranging from $200 to $600 per acre ($494 to $1483 per ha) (Roka et al., 2010).
The purpose of this article is to examine the impact of the presence of HLB on new tree plantings in the Florida citrus industry. Sweet oranges are by far the most prevalent citrus scion grown in Florida, so the analysis is limited to sweet orange plantings. HLB impacts citrus producers through reduced yield, increased tree mortality, and increased cost of production; it is expected that the presence of HLB has had an adverse impact on growers’ willingness to invest in new plantings. The estimated impact is used in a fruit production model to project the impact on future fruit production in Florida.
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Hodges, A.W. & Rahmani, M. 2009 Economic impacts of the Florida citrus industry in 2007/08. Electronic Data Information Source (EDIS) FE802. Univ. of Fla., Gainesville, FL. June 2013. <http://edis.ifas.ufl.edu/fe802>
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