Set by Phil Taylor (CABI).
Studying symptoms in the field is essential in the growing of a crop. A few simple rules will often allow you to make a general diagnosis and will hone your subsequent enquires if you need a more detailed diagnosis. However; there are some symptoms that appear to go against all the rules; in this month’s quiz we look at some of these exceptions.
#1. The crazy growth of this asparagus plant is typical of phytoplasma infection but that is not the case here. What do you think has caused these symptoms?
The correct answer is:
Aphid feeding
Feeding by the asparagus aphid Brachycorynella asparagi induces this proliferation of leaves
Photo credit : William M. Brown Jr., Bugwood.org
#2. The symptom of wilting and leaf edge necrosis are typical of bacterial infection but that is not the case here, what has caused these symptoms on this aubergine/egg plant?
The correct answer is:
Verticillium wilt
Verticillium albo-atrum causes a disease often referred to as “Verticillium wilt” however a wilt is often not the first symptom and may be absent all together following infection.
Photo credit; Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org
#3. The stripes on this maize is due to what may be considered an unusual cause. What has caused these symptoms?
The correct answer is:
Downy mildew.
Very few grasses are hosts to the downy mildews. An exception is maize and here you see the stripes of green and yellow typical of this disease on this host.
Photo credit Eric Haraman Malawi MOA.
#4. Viral infection commonly induce leaf distortion but that is not the case here, what has caused the distortion in this grapevine leaf?
The correct answer is:
Herbicide damage.
The herbicide 24 D is an auxin mimic and causes distortion as the plant develops. This grapevine leaf is barely recognisable due to its distortion.
Photo credit; Wiki commons.
#5. Tightly bunched buds as seen on this pea plant could be caused by many things but what it the cause here?
The correct answer is:
Genetic abnormality:
There is no pest or pathogen at work here, this symptom is due to a genetic abnormality within the plant.
#6. Tip dieback can have many causes such as ageing and mechanical damage due to wind, but this is not the case here. What has caused the tips of these rice leaves to wither and die?
The correct answer is:
Nematode infection:
This is one of the few cases where nematodes attack above ground. The rice white tip nematode (Aphelenchoides besseyi) is the exception and lives in the tips of rice leaves where the feeding causes these symptoms.
Photo credit: Donald Groth, Louisiana State University AgCenter, Bugwood.org
#7. The holes in this almond leaf look very much like insect feeding but that is not the case. What has caused the development of these symptoms?
The correct answer is:
Fungal infection
This is shot hole disease commonly seen on Prunus species. It is caused by Stigmina fungus (Wilsonomyces carpophilus). The infection elicits such an extreme reaction by the host that the infected area drops from the leaf leaving the characteristic (if you know) hole.
Photo credit: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org
#8. Extremely unusual symptoms for this organism. What has caused this dark staining in this radish?
The correct answer is:
Downy mildew infection.
This is the only case I know of where a downy mildew attack below ground. The dark staining (it is not a rot) is extremely characteristic; once you know.
Photo credit Eric Boa
#9. As for the previous question dark staining in underground storage organs. What is the cause this time?
The correct answer is:
Viral infection.
What appear to be tight constrictions around the tubers and the internal browning are very common in some parts of Africa and are due to infection by Cassava brown streak virus. The foliar symptoms are mild but the tubers are severely affected.
Photo credit Rob Reeder CABI.
#10. What has caused the symptoms on this developing cashew nut?
The correct answer is:
Insect feeding.
Insect feeders that are considered sucking pests often leave no symptoms whatsoever however the leaf footed bug (Pseudotheraptus wayi) along with many other capsids leave necrotic tissue behind after feeding as seen here.
Photo credit Rob Reeder CABI
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.
