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What is wrong with my tomatoes?...And other plant disease questions

Why did orange prices increase?

Citrus greening disease, also known as Huanglongbing or Dragon Yellows, is causing devastating losses to citrus trees in Africa, Asia, Brazil, and the southern United States. Plant pathologists are working hard to combat this disease and keep oranges on our tables (and at a reasonable price)!

The Problem

Citrus prices in the United States have increased over the past year and are expected to continue rising as citrus greening disease keeps killing citrus trees throughout the southern United States [1]. Citrus greening, also called Huanglongbing or Dragon Yellows, is believed to be native to parts of Asia and Africa and was detected in Brazil and Florida in 2004 and 2005, respectively. Since arriving in Florida, it has now been found in orange groves in California in the last few years. This disease is devastating in its new hosts, where it is estimated to have caused 30-40% loss of yield and about $4 billion of loss from 2006-2011 in Florida [5,7].

image alt text Fig. 1 Orange tree infected by citrus greening disease (Source: By Mmacbeth)

Infected trees develop symptoms slowly and there are currently no cures for this disease. The most distinct symptom is called blotchy mottle, which is a patchy yellow color on the leaves that can be confused with nutrient problems due to their similar appearance (Fig. 2) [4]. As the disease progresses, the entire tree may become yellow and has reduced fruit production, often yielding fruit that are green, bitter to taste, and unsuitable for eating or juicing (Fig. 3) [4].

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Fig 2. Blotchy mottle on citrus leaves of a tree infected with citrus greening disease (Source: U.S. Department of Agriculture)

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Fig. 3 Oranges that will stay green due to citrus greening disease (Source: U.S. Department of Agriculture)

Phytopathologists, scientists who study plant diseases, are working hard to prevent the spread of this disease and to find treatments for infected trees. Their first step was to determine the cause of this disease, which turned out to be two organisms working together.

The Pathogen

Citrus greening is believed to be caused by three different species of bacteria called Candidatus Liberibacter asiaticus, Candidatus Liberibacter africanus, and Candidatus Liberibacter americanus [6]. In the United States, we believe only Candidatus Liberibacter asiaticus is present. It lives inside the Asian citrus psyllids (Diaphorina citri), small insects sometimes referred to as jumping plant lice (Figure 4) [2,6]. The Asian citrus psyllids were introduced to Florida in the early 2000s and have become an invasive species–an organism that is not native to an area but successfully colonizes it–and scientists believe this is how citrus greening started in the United States [4].

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Fig. 4 Citrus psyllid larvae feeding on citrus (Source: USGS Bee Inventory and Monitoring Lab)

The citrus psyllids feed on the citrus tree by sucking out plant nutrients and secreting a sticky sugar mass on the outside of the plants which attracts fungi to grow [3]. Plants infected with high populations of psyllids may have reduced growth or fruit yield. Additionally, as the psyllids feed on the citrus tree they transfer the Candidatus Liberibacter bacteria into the tree, causing citrus greening. Currently, there doesn’t seem to be any negative affect of the bacteria on the psyllids.

Controlling this Disease

Two strategies for controlling a disease are to stop its spread or decrease the susceptibility of its host. The first way to reduce citrus greening is to reduce the psyllids. Citrus growers are encouraged to buy disease-free trees and to remove any infected limbs or trees to prevent the spread of the insects to new trees or groves. Insecticides, applied at the right time of year, can control psyllids, with varying success [3]. Another strategy growers have been using is increasing nutrients to the trees to overcome the disease symptoms and extend the life of the infected tree [7]. Phytopathologists are also looking for disease-resistant citrus plants that they could breed with our Florida citrus trees to create disease resistant varieties that still produce the fruits we as consumers desire (such as good taste or lots of juice). However, no disease resistant varieties have been identified so far [2].

Since the plants are infected for a long time before showing symptoms, scientists think they may be able to optimize the plant’s immune response to make it better at defending itself against citrus greening and potentially keep it from becoming so sick that the fruit are affected [2]. If phytopathologists understand how the plant’s immune system responds to the disease and key factors associated with the pathogen being able to establish an infection, then we may be able to manipulate the genes controlling these interactions to stop the infection [2].

As devastating as this disease is, many resources are going towards preventing the spread of this disease to new citrus groves and finding ways to prevent citrus plants from being killed by this pathogen. Phytopathologists are using similar strategies to understand and stop other plant pathogens that can cause our grocery bills to increase, like coffee rust, black sigatoka on bananas, or wheat rust. So next time you see an orange, think of the phytopathologist working hard to keep them in our grocery stores!

[1] White, M.C. 2015. Get Ready For Higher Orange Juice Prices. Time Magazine. (

[2] da Graca, J.V., Douhan, G.W., Halbert, S.E., Keremane, M.L., Lee, R.F., Vidalakis, G., Zhao, H. 2016. Huanglongbing: An overview of a complex pathosystem ravaging the world’s citrus. Journal of Integrative Plant Biology. 58: 373-387.

[3] Kabashima, J.N., Paine, T.D., Daane, K. M., Dreistadt, S.H., 2014. How to Manage Pests in Gardens and Landscapes: Psyllids. University of California IPM.

[4] University of Florida Extension. 2016. Citrus Greening (Huanglongbing). UF/IFAS Citrus Extension.

[5] Singerman, A. and Useche, P. 2015. Impact of Citrus Greening on Citrus Operations in Florida. University of Florida IFAS Citrus Extension.

[6] Brlansky, R.H., and Rogers, M.E. 2007. Citrus Huanglongbing: Understanding the Vector-Pathogen Interaction for Disease Management. Online. APSnet Features. doi: 10.1094/APSnetFeature-2007-1207.

[7] Hodges, A.W. and Spreen, T.H. 2012. Economic Impacts of Citrus Greening (HLB) in Florida, 2006/07–2010/11. University of Florida IFAS Citrus Extension.

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