Ask the question, “What is the biggest current concern for citrus producers in Florida?” and it is almost a guarantee that citrus greening or Huanglongbing (HLB) caused by a phloem-limited bacteria will be the answer. This bacterial disease transmitted by psyllids causes green, misshapen and bitter-tasting fruit, and eventually kills the citrus tree.
The U.S. Department of Agriculture, realizing that the disease is a serious threat to Florida’s $9 billion citrus industry, has provided funding for research to advance the understanding of the disease, identify effective control measures and support longer-term projects such as breeding citrus with disease resistance. According to Evan Johnson, research scientist at the University of Florida Citrus Research and Education Center (UF/IFAS), Lake Alfred, Florida, psyllids can travel over 2 kilometers regardless of wind direction and currently 90 percent to 100 percent of the psyllids carry the HLB bacteria. Psyllid control is, therefore, an important part of controlling HLB.
Though HLB is widespread in citrus groves in Florida and eradication is unlikely in the near future, experts still believe that citrus groves can be productive. Johnson said that he believes that if collaborative efforts between growers continue in an effort to manage the psyllid population, and trees are removed when they are no longer productive and replanted with new trees, the industry will be OK.
“Area-wide management of psyllids near a well-managed replanting can mean the difference between 0.5 percent infection after a few years and 18 percent infection,” Johnson said. As growers are using a fair amount of insecticide they need to reduce the chance of resistance developing, and they need to rotate chemical products with different modes of action.
Since no citrus with genes for resistance to HLB have been identified (not even in wild citrus from Asia), controlling HLB will probably depend upon transgenics. Some traditional citrus breeding is under way with the goal to develop varieties that decline more slowly when infected by HLB. Growing these trees should mean that a grove will be productive and profitable for longer. In addition, work is in progress to develop some bactericides.
Recent research from Johnson and colleagues show that roots of HLB-infected trees become infected by the bacteria way ahead of symptoms appearing in the leaves and fruit. These findings suggest that evaluating the health of the root systems of the trees may help trees survive and be productive for a longer period of time.
Recommendations to protect roots
What can the grower do to support the health of roots? Johnson’s recommendations are that growers increase the acidity of irrigation water and soil to match the optimum pH for the rootstock. In addition, watering more frequently for shorter periods may allow for more effective water update when root systems have been compromised by the bacteria. Weather conditions can determine the longevity of a grove; if a hurricane was to hit the area, infected trees probably could not withstand the wind.
Citrus is susceptible to many pathogens and also has a number of production problems. Johnson reported that they have been seeing some unexpected effects of HLB on postbloom fruit drop (PFD); a disease caused by the fungus Colletotrichum acutatum, HLB causes an earlier and extended bloom period that provides more time for C. acutatum to establish in the blooms and build up inoculum, which can then greatly reduce fruit set.
Phytophthora root rot is another disease that can be a problem, and the general control recommendation is to choose a rootstock to match your soil situation. As for scion choice, this is dictated by the market and harvest time, with Hamlin being grown for early oranges and Valencia for the later market.
Citrus producers in Florida have always had problems with nutrient management due to minerals rapidly leaching from the soils. This has resulted in foliar micronutrient sprays being used. HLB infection, however, compounds the situation, as the disease causes micronutrient deficiencies in the leaves.
Weeds are rarely a problem in the groves as the herbicides are effective at controlling the weeds, which are usually vine-type plants. The organic producer, however, could have a problem, as these vine-like weeds can readily overwhelm the trees.
Impact of citrus black spot
To add to the problems already present in the Florida citrus industry, in March 2010 a serious disease then-new to the United States was identified in Southern Florida. Citrus black spot (CBS) caused by the fungus Guignardia citricarpa, known to be one of the most devastating fungal diseases of citrus in the world, was detected during a pest survey of Valencia sweet oranges in the Immokalee area of Florida. By the first week of May 2010, the disease was found in another location, about 14 miles away. The disease causes early fruit drop, and if left uncontrolled, it will result in a significant reduction in crop yield.
CBS control includes practices to accelerate leaf litter decomposition, minimizing movement of trash from grove to grove, opening the tree canopy to reduce moisture levels in addition to monthly fungicide applications of copper and or strobilurins from early May to mid-September.
Federal quarantine regulations were quickly imposed in Florida to reduce disease spread. These regulations have to be observed by growers, harvesters, haulers and processors working with citrus fruit harvested where CBS has been detected. At the USDA APHIS website (http://www.aphis.usdahttp://.gov), searching for citrus health response programs will provide information on federal quarantine notices and orders, maps of quarantined areas in Florida and updates on scientific information.
An extensive guide for managing Citrus Black Spot is available from the website https://edis.ifas.ufl.edu/cg088.
PHOTO CREDIT: M. M. DEWDNEY, UF/IFAS.