Tuesday, April 15, 2008

Liver cancer on citrus

The seminar disclaimer applies to this post.

Seminar was canceled two weeks ago. Last week’s seminar was presented by Mike Irey from the United States Sugar Corporation and Southern Gardens. Southern Gardens is the 3rd largest orange grower in Florida, with 21,000 total grove acreage, all planted with oranges. Over the years more than 21% of the acreage has been lost as a result of citrus canker and the citrus canker eradication effort.

However, if citrus canker was a hemorrhoid, citrus greening (also called huanglongbing, HLB) is the liver cancer of citrus

The disease
Picture credit.
HLB is a bacterial disease, caused by one of the two fastidious (up to now unculturable) bacteria Candidatus Liberibacter asiaticus or Candidatus Liberibacter africanus, the first one of which was found in Florida in 2005. The disease is vectored (spread) by the Asian citrus psyllid, an insect that doesn’t cause any major problems on citrus by itself. When 6 trees in Southern Gardens were identified to have HLB in October 2005, immediate inspection of all groves followed, and a control program to manage the vector, the Asian citrus psyllid was put into place.

Picture credit.
The initial HLB symptoms are misleadingly unimpressive, looking somewhat like nutritional deficiencies. Some leaves, usually near the center of the citrus tree show some mottling, but the trees themselves do not decline until much later. The spread of HLB has been quite dramatic over the years. It was first identified in Florida in 2005. In April 2006, 12 counties had positively identified trees, in January 2007 that was 14 counties, by June of 2007 there were 24 counties, and as of February 2008 all 30 citrus-producing countries had trees with HLB. The disease spreads very fast.

Crop losses include the immediate loss of removed trees, but also gradual tree decline and reduced production, fruit drop, reduced size of the fruit, and possibly juice quality issues, although the latter is still controversial. A small study with 10 pairs of trees comparing yields from symptomatic trees with non-symptomatic trees of approximately the same height, measured a 56% yield reduction in the symptomatic trees. This may be an over-estimate, but it is still a number to keep in mind.

Southern Gardens and US Sugar Corporation test samples from growers by polymerase chain reaction (PCR). They find that around 50-60% of the samples are positive. Some blocks of citrus have 80% of the trees infected, company-wide around 10% of the trees have HLB. Almost 300,000 trees (~1800 acres) have been removed so far, at a cost of more than 6 million dollars (roughly 1 million for tree removal, 2 million for tree replacement, 3 million for lost production). The costs of managing HLB disease has skyrocketed because of the ongoing tree removal, the increased insecticide applications, and currently 44 full-time scouts are employed, constantly surveying the groves in search of newly symptomatic trees.

Grower reactions have varied from doing nothing at all, to aggressive inoculum management, to abandoning groves all together. Those that wait for the silver bullet will likely be disappointed, as there will be no quick fix to the HLB problem.

Disease management
The main challenge is to make management decisions to deal with a disease when there is little knowledge, and a lot of data in the literature is anecdotal. In addition, the causal agent of HLB is a select agent, which complicates research.
Possible management options include:
- Vector control
- Tree removal
- Replanting disease-free material
- Disease-resistance plants (no know resistance to date)

Southern Gardens has opted for an open-door policy, opening their doors for hands-on training of scouts and managers, organizing grower meetings throughout the state, and field trials of experimental approaches.

Management of available inoculum through intensive surveys (full-time scouting) and aggressive removal of infected trees and control of the insect vector, have been the main focus of the approach so far.

The citrus industry has for a long time implemented an integrated pest management approach to control pests and diseases, using cultural practices, surveying, and careful weighing the necessity of chemical sprays. With the current intensive spray program to control the Asian citrus psyllid, the spray programs can no longer be considered IPM.

Management and administration costs have increased dramatically; disease testing for example, costs $6.50 per sample. If the psyllid is found in a nursery, the nursery managers risk have to worry about being assessed and quarantined, risking losses, and potentially even complete shutdown.

Even more troubling is that growers are becoming desperate, trying unproven methods. Often there is no data available from scientifically performed experiments with proper controls on products for which broad claims of efficacy are made. Meanwhile, the growers do not remove infected trees while they try out new products or approaches, risking further spread.

Long term strategies
At this point it is unclear what the best management strategy will turn out to be. One interesting observation to keep in mind in developing strategies is that it appears the disease is more prevalent at the edges of citrus blocks. Potential explanations are that the psyllid is coming from the outside, affecting the outer edges, or the psyllid is moving outward from the center of the block and is stopped by the lack of trees beyond the edge. Does it make sense to change the size and shape of blocks to reduce the edge/area ratio?

How effective are some of the alternative approaches and products that are currently available on the market? How can new strains be detected in a pathogen that is (up to now) unculturable? How does one optimize the PCR reaction used to detect the pathogen (where to set thresholds)? What time of the year is the best to sample? A particular tree may be negative if tested in one month, and positive in another. How effective (and economically feasible) is the strategy to intermingle citrus plants with guava plants, which seems to work in some countries? And can it still be considered a “citrus grove” if half the trees are guava?

Citrus greening is presenting new challenges to growers, nursery managers, plant pathologists, and lab technicians. A great deal of research is necessary to come up with answers.

Further reading
Halbert, Susan. Pest Alert. Florida Department of Agriculture and Consumer Services, Division of Plant Industry. http://www.doacs.state.fl.us/pi/chrp/greening/citrusgreeningalert.html

Langham, M.A.C. 2006. Citrus greening - The yellow dragon threatens Florida citrus. APSnet News and Views. http://www.apsnet.org/education/K-12PlantPathways/NewsViews/Archive/2006_04.html

Long, P. and Merzer, M. 2007. Florida citrus industry faces new peril. Miami Herald. Complete text online at Southern Gardens News and Press Releases: http://southerngardens.com/news/101607.html

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