Set by Dr Eric Boa
This is the third and the last in the current series of ABC Plant Doctor Quizzes. Try May and June’s quizzes to get the hang of how the ABC approach works. It aims to help you diagnose unfamiliar symptoms on unfamiliar hosts and get closer to the type of plant health problem being observed. Despite the prominence given to molecular methods and the rise of technological approaches to diagnosis, visual interpretation of symptoms and human reasoning remain key aspects of identifying the cause of a plant health problem.
Globally, the overwhelming majority of farmers and extension officers do not have access to diagnostic laboratories. Field diagnoses are a vital part of quick responses to plant health problems everywhere. Separating biotic and abiotic causes is a good start. Plant health problems related to abiotic causes will not respond to pesticides; poor soil fertility can be reversed. Some biotic causes will respond to pesticides but more generally highlight the importance of plant hygiene: remove dead parts, use clear seed, avoid planting the same crop in the same place year after year.
A final comment on what I mean by the ‘correct’ answer. Think of this more as a ‘reasonable’ answer based solely on what you can see and does not assume prior knowledge. Of course some of you will recognize the problems but others lacking experience will be less sure. I’m not always sure of the precise cause and welcome alternative interpretations of symptoms. Have a go. We all have to start somewhere. And many thanks to the Royal Horticultural Gardens at Wisley for most of the examples. This is no reflection on the excellent care they give to plants!
#1. Celery leaves from the RHS Gardens Wisley
#2. Potato plants at RHS Wisley
The correct answer is:
Comments:
I’m not sure if this is late blight or not. But it’s definitely biotic. The necrotic lesions have a defined border, unlike the more diffuse and spreading areas I associate with Phytophthora infestans. A colleague who worked with potatoes and helped to run a diagnostic lab in Scotland said that they often put leaves into a sealed bag overnight to see if the suspect late blight symptoms developed a furry surface.
#3. Maize growing at RHS Wisley
The correct answer is:
Varietal characteristic
Comments:
The red lines are regular and the leaves are healthy. I’m familiar with phosphorus deficiency (red to purple streaks) and this is not it – nor would I expect a nutrient deficiency at Wisley!
#4. Maize growing in an open field in South Africa
The correct answer is:
Comments:
This is phosphorus deficiency. Note the edge effect, where symptoms due to a lack of a nutrient are often most visible.
#5. Basil growing at RHS Wisley
The correct answer is:
Healthy
Comments:
A quote from the Superman films seems apt, adapted for plants: Is it a Virus? Is it a phytoplasma? Or is it simply another varietal characteristic? The label at Wisley said ‘Lettuce leaved Basil’, indicating that this is a named variety. It could still, of course, be related to a virus ‘infection’, albeit of the harmless and valued sort.
#6. Courgette growing at RHS Wisley
#7. Unknown plant (yes, also at RHS Wisley)
The correct answer is:
Confused
Comments:
I’m puzzled by this flowering plant (whose label could not be found). What is the significance of the clear boundary between the yellowed leaves and the healthy ones in the background? Uneven application of fertiliser? It doesn’t look like a virus – note the even development of the yellowing. Or is it varietal? So ‘confused’ is my best suggestion.
#8. Onions growing at RHS Wisley
The correct answer is:
Comments:
This is a rust, common on several onion relatives – the chives in my garden get it. Look out for rusty leeks bought from shops (though less common than it used to be).
#9. Sweet peppers, Georgia
The correct answer is:
Comments:
Some years ago I attended the first plant pathology congress held in Georgia (the country). A colleague kindly suggested an agricultural area outside Tbilisi where I might wander round agricultural fields. The farmers were Azeris and I was intrigued at first by an unusual problem on chillies. I explained to the farmer my interest in diseases and the owner of the field went off to collect a sweet pepper. This is it. He asked me what was wrong with the plant and instead of saying I didn’t know I waffled. It could be a disease, I ventured. Too late. I noticed a slight twinkle in his eye as he told me the answer: sunscald.
#10. Cucurbit (squash/marrow) growing at RHS Wisley
The correct answer is:
Comments:
An older leaf loses its vigour. Note the ‘classic’ indicator of lack of nutrients with the veins remaining green and the outer regions yellowing first.
Results
We hope you enjoyed the quiz and learned something!
See how your score matches up…
1-3: You need to do a few more of these quizzes to become a field diagnostics expert.
3-5: You have made a start in your field diagnostics but there is some way to go…
5-7: You are getting more right than wrong so keep on learning!
7-9: Impressive you are getting pretty experienced in your field diagnostics!
10: Well done! But make sure you do the quiz again next month.
We hope you enjoyed the quiz and learned something!
See how your score matches up…
1-3: You need to do a few more of these quizzes to become a field diagnostics expert.
3-5: You have made a start in your field diagnostics but there is some way to go…
5-7: You are getting more right than wrong so keep on learning!
7-9: Impressive you are getting pretty experienced in your field diagnostics!
10: Well done! But make sure you do the quiz again next month.
