It was big news in the citrus world last month when the Cola-Cola Company announced that it is committing $2 billion to support the planting of 25,000 acres of new orange groves in Florida. When I read this news it made me wonder if some people might read the story and imagine that there must be some new miracle cure for citrus greening disease (also known as Huanglongbing or HLB). Citrus greening has spread throughout Florida, devastated the citrus industry, and caused billions of dollars in damage. Sadly there is no miracle cure. It is only the massive scale of the new plantings that will make it possible to successfully grow citrus where citrus greening is widespread. The situation is still terribly bleak both for small citrus farms and also for home citrus growers in Florida.
Citrus greening is a bacterial disease of citrus. The disease is spread by an insect called the Asian citrus psyllid (ACP).
The below graphic will help to illustrate how new citrus groves will likely be planted in order to defend against citrus greening.
Citrus greening is also present in Brazil and the growers there have had more experience growing citrus in its presence than the growers in Florida. Florida growers have been studying the growing methods in Brazil and one of Coca-Cola's partners in Florida, Cutrale, is a big citrus grower in Brazil. The new plantings in Florida will certainly be engineered with the Brazilian experience in mind.
In Florida, the citrus industry has traditionally had many small citrus groves which are geographically separated. This made it especially easy for the ACP to penetrate the groves and spread citrus greening. Likewise, residential citrus trees are very isolated and completely vulnerable to ACP and citrus greening.
One strategy used in Brazil to combat citrus greening is to plant very large groves. These large groves have a “sacrificial border” shown in the above diagram as concentric rings of densely spaced trees. The goal is to reduce the penetration of the ACP into the interior of the grove. It is expected that trees in the border region will be lost to citrus greening and infected trees are removed as soon as citrus greening is detected. When citrus greening does make it into the interior of the grove, those trees are also removed as soon as citrus greening is detected. The border region of the grove is typically sprayed with insecticide weekly while the interior is sprayed monthly. Traps for ACP are placed in the grove and any detection of the insect typically results in immediate spraying.
In summary, the fact that the Coca-Cola Company is supporting new citrus plantings is Florida makes citrus greening no less a threat. In areas where citrus greening is present, small citrus groves are no longer viable and residential citrus trees have a very short life expectancy.
In areas such as California, Texas, and Arizona where asian citrus psyllids are present, there are measures that can be taken to prevent a devastating breakout of citrus greening.
Grafting is a particularly dangerous activity. It is thought that citrus greening may have spread throughout Florida all from a single piece of infected budwood from Asia. When grafting citrus it is critical to only use certified clean budwood from a source such as the CCPP.
It is also especially dangerous to move citrus trees. Citrus trees moved from an area where citrus greening is present could be infected and could spread citrus greening to new areas. Likewise trees moved from areas where ACP is present could be infested with ACP and could spread ACP to new areas and trigger a breakout of citrus greening.
Thank you very much to Rock Christiano of CCPP for both explaining to me how large block size can be a defense against citrus greening and also for inspiring this article. For those who would like to learn more about how Brazilian growers are coping with citrus greening, an excellent report can be found here.