By: Jiani Chen, Megan M. Dewdney, and Mark L. Gleason
RETURN TO THE EXERCISE
The case study is designed for undergraduates at the sophomore level or above, but it does not assume that they are knowledgeable about agriculture, plant pathology or pest management. The case is designed to help students understand how plant diseases develop, how to recognize symptoms of a disease, and how growers try to solve disease problems with critical thinking.
The case study focuses on a disease of citrus trees called huanglongbing (HLB), which is one of the most devastating fruit crop diseases in the world. HLB is caused by a bacterium,
Candidatus Liberibacter asiaticus, which is carried from tree to tree by a tiny insect called the Asian citrus psyllid. Therefore, a critical step in overcoming HLB is controlling the spread of psyllids.
Derek Parker, the owner of DP Citrus and Nursery in Florida, relied mostly on insecticides to reduce the population of psyllids during the first few years after HLB was detected in his orange groves. A voluntary program called Citrus Health Management Areas (CHMAs) involving Derek and nearby citrus growers, was developed to encourage citrus growers to work together to combat HLB. Derek was considering whether to be the captain of a CHMA in order to lead other growers in the effort to slow the advance of HLB in his area. In addition, researchers found a heat therapy strategy that could prolong productivity of infected trees. Should Derek help to organize a CHMA? Should he apply the heat therapy approach into his own orchard? The case puts students in Derek’s position to enable them to weigh the alternatives and decide on a reasonable plan for managing this formidable disease.
SUGGESTIONS ON HOW TO USE THIS CASE
The case study discussion can be completed in a single class period of 45 to 90 minutes, and a suitable class size would be 5 to 50 students. We suggest that the case study material and background information be made available to students before the class period, so that they will come to class prepared to discuss the case. In our experience, requiring a short written assignment in advance about the basic facts of the case or including clicker questions at the beginning of the class helped insure that students were prepared. At the start of the class, a printed version of the case can be distributed to students to aid in the discussion.
Our three tryouts with classes at Iowa State University (ISU) each used a one-hour class period. In the first few minutes, some clicker questions can be given to students to verify that they have read the material. After that, we suggest taking about 10 minutes to introduce the case and current strategies that citrus growers in Florida are using to manage HLB. When introducing the case in our test classes, we found that initially providing students with images of HLB symptoms, management strategies (insecticide spraying, heat therapy, CHMAs, etc.), and an
introductory video helped them understand the case clearly.
Following the introduction, we suggest allowing at least 15 minutes for group discussion (ideally, 3 to 4 students per group) of questions posed in the Disease Management sections; each group should write a management plan (a list of strategies is fine). From our feedback in ISU classrooms, group collaboration was the most enjoyable
for students. After group discussion, we suggest allowing about 20 to 30 minutes for facilitated general class discussion; each group can share their plan with the rest of the class via a spokesperson or scribe. The instructors and entire class can ask questions of the groups or their spokespersons about the pros and cons of each strategy. Some creative ideas that emerged from the small-group and general discussions (many more may emerge when you use the case):
Verify that the budwood (i.e. shoots bearing buds; suitable for grafting) source trees and the propagated trees in citrus nurseries are protected under psyllid screen and clean (pathogen and psyllid-free), for example by using a Polymerase Chain Reaction (PCR)-based assay to amplify and detect DNA of
Ca. L. asiaticus. Some students also mentioned that nurseries should be regularly checked for any obvious holes or the silent carriers among asymptomatic trees (again by means of PCR-based assays of plant samples) to ensure no HLB-infected stock is being sold. (This is now law in Florida, Texas, and California; see references for more details)
Scouting typically means examining the trees in an orchard for symptoms of HLB. Since Derek has around
orange trees, checking each orange tree every month is impractical. Another challenge arises from the fact that the
Ca. L. asiaticus can infect trees without showing symptoms (a latent infection), at least initially; how can scouting identify these “silent carriers” (infected trees that do not yet show HLB symptoms)? Encourage students to seek out additional information before making a plan. For example, they could investigate:
Vector ecology: Learn about the life cycle and behavior of the Asian citrus psyllid. It has been demonstrated that early HLB infections were detected more frequently on the edges of orchards in Florida and Texas. How could this information be used to design control strategies for the Asian citrus psyllid? Environmental conditions that favor psyllid development (e.g. temperature) would also be important topics. In Florida, targeting overwintering adult psyllids with insecticide sprays was found to be effective in reducing pathogen spread.
Heat therapy: Can this strategy be consistent with long-term profitability? Should heat therapy be applied selectively according to symptom severity of each tree, or by average symptom severity of all the trees in a continuous block? It may not be feasible to encase all HLB-affected orange trees with tents in an attempt to slow down the advance of the disease. There is more than one method of applying heat therapy to trees. Explore the various methods and indicate whether you think they are likely to be more or less effective than one another and the pros and cons of each method.
In our classroom tryouts, most undergraduate students thought that CHMAs could be an important part of HLB management. Therefore, instructors can share related resources about
with students before class. Use of genetically modified (GMO) orange trees with resistance to HLB is another topic that attracted students’ interest. Most students proposed replacing conventional HLB-susceptible orange trees with GMO trees. But the marketing side - whether customers would be willing to purchase GMO oranges or orange juice - also emerged as a concern.
What are the symptoms of HLB infection on leaves, shoots, and fruit of citrus trees?
How can HLB-infected citrus trees be distinguished from nutrient-deficient trees, since some of the symptoms are similar?
Why is early detection important for managing HLB effectively? How can HLB be detected in the early stage of the disease, when the citrus trees have not shown symptoms?
Suppose Derek had identified HLB-infected citrus trees at the earliest stage of infection. What should he do? Should the HLB-positive trees be removed or retained?
There is no effective way to cure HLB. But Derek learned that GMO citrus trees have been developed which have high levels of resistance to this disease. Should he consider replacing all of his citrus trees with GMO trees? Take biological, economical, and sociological factors into consideration.
PRE-CASE CLICKER QUESTIONS
HLB is a disease affecting:
All of these hosts.
Candidatus Liberibacter asiaticus:
a. infects the phloem tissue of its plant hosts.
b. is transmitted by a psyllid.
c. will eventually kill infected trees.
d. All of the above.
In the case, Derek Parker is:
a. a researcher developing a GMO that is resistant to HLB.
b. an extension citrus production expert.
c. an orange grower.
d. a reporter writing a story about HLB disease.
Which of the following is NOT a strategy for managing HLB disease?
a. Insecticides to reduce psyllid populations
b. Scouting for HLB symptoms
c. Heat treatment of diseased trees.
d. Fertilization to help alleviate the impact of HLB on infected trees
POST-CASE CLICKER QUESTIONS
Did this case study help you understand how HLB develops and how growers try to solve a disease problem?
In a Citrus Health Management Area (CHMA):
a. growers cooperate regionally to coordinate psyllid management programs.
b. growers are supported with state-provided scouting services.
c. growers can usually lower costs when the CHMA purchases insecticides and application services.
d. all of the above.
Since there is really no effective way to cure trees infected with HLB, if trees with GMO resistance were available, would you consider replacing infected trees with GMO trees? Please explain why you would consider replacing or not.