Lauri A. Lutes and Jay W. Pscheidt
Dept. of Botany & Plant Pathology,
Oregon State University, Corvallis, OR
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ANSWERS TO CASE QUESTIONS
Part A
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Based on what you know about the systematic approach to plant disease diagnosis, what information would be useful for Dr. Martinez to ask? What resources do you have available to you to help solve this problem?
Host species/cultivar, weather conditions, onset of problem, requesting more images to capture the entire host individual or community, or a sample for diagnostic testing (e.g. fungal culturing, molecular assays) – see Possible Adaptations section.
Resources may include plant disease handbooks, APS Compendia, experienced plant pathologists, and plant diagnostic clinics.
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Based on Dr. Martinez’s conversation with the grower, is there any information still missing or confusing?
Students may not be familiar with the rootstock and scion speak for sweet cherries (e.g. ‘Bing’ on ‘Mazzard’). As pointed out in the conversation, ‘Bing’ is the scion cultivar to which the ‘Mazzard’ rootstock was grafted. Another point may be that the grower did not provide clear information on how many trees were affected.
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Would you have asked different questions or in a different way?
Students may want to ask more questions regarding severity of disease, soil type, cultural practices, etc.
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Does any of the information seem irrelevant?
The powerline, maybe. Although an argument could be made for its relevance. Growers will typically talk about many irrelevant things and it is the job of the questioner to sort through what may or may not be pertinent information.
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Why might Dr. Martinez have wanted a healthy sample?
A healthy sample could be compared to a symptomatic sample to understand if the symptomatic sample tests positive and the healthy (asymptomatic) sample tests negative. If the “healthy” sample tests positive, this could indicate a possible shock reaction or indicate that the symptomatic sample may be infected by more than one pathogen. A shock reaction is when a plant expresses symptoms at the onset of disease (for the first year or two) and then maintain a latent infection and no longer express symptoms thereafter.
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What issues do you expect could make these types of conversations challenging?
Some issues may include: communicating clearly with a non-scientist, staying focused on topic, and not seeing the actual disease in the field.
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Identify the host. (Common/Latin name)
Prunus avium cv. Bing on Mazzard rootstock, sweet cherry
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Describe symptoms and signs. (Primary/secondary symptoms and signs)
Deformed leaves, stunted growth, dark green enations around the leaf vein
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Describe the damage pattern. (Community, individual, and sample of individual)
Small community (4-5 trees), aggregated (grouped), few branches
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Identify if there is evidence of damage spread. (Date of onset, year-to-year changes)
Noticed in prior year, no notes on severity
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Identify the cultural or site conditions. (When, where, if/how was sample collected)
Producing orchard, single block, 12 years since grafting, high region in field, sample collected by field representative probably in mid-July
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Determine possible diagnosis. (Common name, scientific name)
Yellow bud mosaic/Eola rasp leaf (Tomato ringspot virus – the actual causal agent of enation-induced disease in image),
Cherry rasp leaf virus, Cherry twisted leaf associated virus, Virus-induced cherry decline (Cherry leafroll virus and
Prune dwarf virus on the same tree may cause enations).
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Indicate certainty of identification. (Certain, most likely, or tentative)
Most likely
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Identify other possible causes.
See 6
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Describe what you would do to confirm the diagnosis and distinguish similar pathogens.
Visit field to check for other symptoms or vectors, ask an expert, serological (ELISA, lateral flow) or molecular (PCR) assays
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Determine possible control measures.
Use certified virus-free nursery stock, remove infected trees, control vector
Reference(s) Used (Books including page numbers, URLs, etc.):
PNW Plant Disease Management Handbook
PART B
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Write an email to Bailey Williams explaining the finding and provide recommendations for managing this disease. Copy the field representative and agent so they can help others.
Answers will vary.
Wrap-Up Questions
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Did everyone in your group agree on a diagnosis? If not, did that impact the final disease management recommendations?
Students may generally agree on a top one or two most likely pathogens. For viruses, this may not matter for immediate management as tree removal is the best approach. However, if the viruses have different vectors or no vectors this could impact long-term management decisions.
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What was the most challenging step in this process? Do you think this would be the most challenging step in real life?
If a physical sample is not available and students are relying on images, this could be particularly challenging. This may occur in a real-life situation, but access to the field site may also be possible.
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You have a limited budget to diagnose this sample. What is the most cost-effective way to diagnosis the sample to a point in which a reliable management recommendation can be made?
Sometimes simply knowing a virus is causing the disease is enough to make management decisions. Some test kits are available commercially for testing. There are also plant clinics in each state that specialize in disease diagnosis.
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What would you do if you were unable to confidently make a diagnosis?
One could consult a plant disease expert of the specific crop for their opinion, in this case a sweet cherry virologist. It is important to be honest with the grower. A good place to search for these experts in the United States is at the state’s land grant institution or USDA research laboratory.
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Some virus strains cause more severe symptoms than others. This means that a virus strain may produce symptoms that may not have a major impact on a grower, while other virus strains could be devastating. What would your recommendations be to the grower if you found a virus, but were unable to determine if it was a severe strain?
You could discuss if different management decisions would be necessary and determine the cost versus benefits of further diagnostic testing, if available.
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The virus-like symptoms you observe on a sample do not match with any of the diseases in the exhaustive list of resources you have used. How could you explain this? How would you proceed?
While most diseases have been observed, there is still a chance for a new pathogen to be found or a known pathogen to be found on a new host. Also, many pathogens could be causing a synergistic effect, which may present itself as a newly observed symptom on the host.
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Sometimes disease management recommendations can be devastating (e.g. orchard removal) to the grower, causing them to make decisions that may not be in the best interest of the community. If infectious trees are not removed, the virus can continue to spread to nearby. How would you handle a situation in which a grower is refusing to remove virus-infected trees that are putting neighboring orchards at risk? How might your reaction change if the identification of the disease required official reporting?
Discuss the matter with the grower to help them clearly understand the situation and risk. Use all resources and connections available to support you in making your best effort to communicate with the grower. Recognize that the grower may not follow your recommendations. Official reporting requirements would help highlight the importance of tree removal and give you additional support and enforcement, if needed. In the United States, there is a clear separation between extension (the givers of information) and regulatory agencies (the enforcers); however, in other countries these roles may be combined.
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Due to the cost of purchasing certified virus-free stock for propagation, some growers prefer to obtain propagation material from family or friends. How would you make a case for investing in certified virus-free stock to someone that has used non-certified stock for years without a problem?
Clearly communicate the risk involved with this practice – you could potentially infect your newly planted orchard from the start. Recognize that the grower may not follow your recommendations.
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Initial identification of viruses in an orchard are largely done by visual symptom recognition; however, symptoms may only be present during a short time period (e.g.
Little cherry virus 2 symptoms - little, immature fruit that are not marketable - can only be seen about 2 weeks before harvest until fruit drop. Harvest is a busy time for growers, field representatives, and pickers at this time of year. What disease management and identification recommendations would you make to growers, if this virus was suspected?
If action cannot be immediately taken, marking the tree(s) of interest for later assessment is important. This could be done with flagging, painting the tree trunk, or recording GPS coordinates.
Pilot Study Summary
A pilot study was conducted during a 2-hour recitation period with 8 graduate students in BOT 552 Plant Disease Management at Oregon State University. Since these students had not yet been introduced to the diagnostics process, a brief overview was given based on the Background Information provided. All case study material was covered within the class period. This case study is intended to be used at the upper-level undergraduate and/or graduate levels. When solicited for feedback, students were requested to provide any feedback from the perspective of an undergraduate or graduate student.
The overall response from students was positive. A 4-point Likert scale was used to assess student’s perceived gain in knowledge and ability to identify the steps to systematically solve and their ability to diagnose a plant disease problem, as well as discuss the relevancy, complexity, and implications of plant pathogen diagnosis. In general, a 1-point increase from before to after the lesson was observed, indicating that the students felt they had gain knowledge from the lesson in these areas.
A comments section provided more insight into the student’s thoughts on the lessons strengths and weaknesses. Some positive comments were made regarding the detective theme, scripted conversation, ambiguous disease symptoms, group work, and a well-organized lesson complete with handouts of the background information and Parts A and B of the case.
A minor edit was made to add a time reference (early June) to the Part A introduction; however, most suggestions for improvement related to implementation. This case study was designed to be available for use by instructors with limited resources and to be an all-in-one package that does not require anything beyond a computer and internet access. The lesson was effective using only the images provided; however, based on student feedback, the lesson could be enriched by using live samples to highlight a disease of interest, as described in the “Possible Adaptations” section.
Another comment from a student regarding the email response activity prompted me to add a suggestion in the “Classroom Management” section. Due to the time it may take students to process the information – especially English Language Learners and those with different learning abilities – it is suggested that the email writing activity is a take-home assignment. It would be up to the instructor to decide if making this an individual or group assignment would make the most sense for their class.
Most students did not arrive at the correct pathogen (Tomato ringspot virus) as their most likely culprit, but all selected enation-inducing viruses as their best guesses. This was expected, as there are several enation-inducing pathogens on sweet cherry and top choices were selected merely based on an image. Some assistance was provided to students on navigating the PNW Handbook website (e.g. suggesting keywords to type in the search bar, finding the section on sweet cherry diseases). Before facilitating this activity, it is encouraged that the instructor familiarizes themselves with the resources tools they intend to use to help guide the students.
