Trees infected with citrus greening disease first exhibit leaf discoloration, then defoliation and twig dieback, followed by root loss and eventually death. (Photo by H.D. Catling, Bugwood.org)

By David Coyle, Ph.D.

Citrus greening, also known as Huanglongbing or HLB, is one of the worst citrus diseases in the world. Bacteria (Candidatus Liberibacter asiaticus) spread by the Asian citrus psyllid (Diaphorina citri) infect citrus trees, first causing leaf discoloration, then defoliation and twig dieback, followed by root loss and eventually tree death. Fruits are stunted, misshapen, and often have a (you guessed it) green color, making them useless, even for juice. The disease can also be moved on wood and leaf tissues but not on commercial (i.e., washed and cleaned) fruit.

David Coyle, Ph.D.

In the past decade, citrus greening has caused a 21 percent reduction in fresh citrus fruit and 72 percent decline in orange production in the U.S. Florida is the largest orange-producing state in the U.S. and has seen a 74 percent decline in citrus production. No matter which metric you use, citrus greening is a devastating disease to citrus in this country and worldwide.

HLB is currently present in Florida, Georgia, Louisiana, South Carolina, California, Texas, Puerto Rico, and the U.S. Virgin Islands. The psyllid vectors have been detected in many other southern states as well as Hawaii and the Caribbean islands.

Scientists have widely accepted management recommendations for citrus greening, including preventative pesticide sprays and removal and destruction of infected material. With a threat this severe, you’d think it would be easy to get all the growers together, on the same page, and have them implement a coordinated management plan. Turns out, however, that’s not always the case, as detailed in a new paper published last week in the open-access Journal of Integrated Pest Management.

Fruits affected by citrus greening disease are stunted, misshapen, and often have a green color, making them useless, even for juice. (Photo by Florida Division of Plant Industry, FDACS, Bugwood.org)

Many, many factors impact grower decisions when it comes time to implementing an integrated pest management plan for a crop pest. Deciding to use preventative pesticide sprays, for instance, is a classic risk/reward scenario. Preventative spraying in citrus groves means the grower takes a financial risk applying pesticide that might not be needed. It may be a waste of money—that is, unless it prevents the infection of a grove, in which case it is very worthwhile.

The same goes for the removal of infected tree material. One fascinating example in the JIPM paper was of a grower who gained notoriety (and not the good kind) for refusing to remove HLB-infested trees, instead opting to use foliar nutritional applications “under the premise those would contribute to keep trees productive.” This grower’s unwillingness to remove his trees and use of a chemical Band-Aid instead of accepted management tactics no doubt hindered efforts of neighboring growers who were doing their part to halt the spread of this devastating disease. But, in doing so, the Band-Aid approach allowed him to keep his citrus grove alive and productive, and other growers followed.

From an economic standpoint, this grower made a decision that benefitted him and him alone. However, in doing so, this grower (and other growers that took this approach) also allowed the grove to become a source of HLB bacteria and insect vectors that could infect other groves. So, should this grower have acted in a way that helped the greater good, and the good of his neighbors, by removing infected trees and thus helping to limit the spread of HLB? Or is this grower’s obligation only to his own business and family? It is a very complicated problem, and it highlights how grower choices and behaviors are fully intertwined with many different issues and outcomes. Individual grower revenue and production, and also neighbor choices, behaviors, and their revenue and production, all influence disease spread, vector population dynamics, and areawide adoption of proposed scientific solutions (e.g., IPM).

Citrus greening disease is spread by the invasive Asian citrus psyllid (Diaphorina citri). Nymphs (shown here) measure less than 2 millimeters in length in their fifth instar. (Photo by USGS Bee Inventory and Monitoring Lab, via Flickr)

Citrus greening disease is spread by the invasive Asian citrus psyllid (Diaphorina citri). Adutls (shown here) measure less than just 3 to 4 millimeters in length. (Photo by USGS Bee Inventory and Monitoring Lab, via Flickr)

Citrus greening creates numerous challenges for the adoption of prevention and collective management practices from an economic perspective. When a new pest establishes in a crop, the best course of action would be to have all growers voluntarily perform accepted management actions. But, that is the challenge: getting many individuals to act together, for the greater good, when personal or business economics often dictate what management activities take place. This example is yet another reason why it is so important to have good personal relationships between stakeholders and IPM teachers. Good relationships and trust can often overcome individual growers’ reluctance to use the best scientific management techniques, and using these is imperative if we’re to have any hope in combating new, destructive pests in our agriculture and natural resource systems.

David Coyle, Ph.D., is an assistant professor in the Department of Forestry and Environmental Conservation at Clemson University. Twitter: @drdavecoyle. Email: dcoyle@clemson.edu.