Set by Dr Phil Taylor
This months quiz we are back to that provided by Phil Taylor at CABI. This quiz is unusual in that in each case the symptoms are all caused by downy mildews. These are common pathogens and collectively they have a wide host range. They are all obligate pathogens and are not fungi but are members of the Oomycetes (water moulds).
#1. This is a photo of downy mildew of cucumber caused by Pseudoperonospora cubensis. However, if you were not familiar with this disease what could the symptoms be mistaken for?
The correct answer is:
Downy mildew on cucumber looks like a typical viral mosaic
The mosaic caused by cucumber downy mildew is a very convincing mosaic and is often thought to be caused by a virus.
#2. This is the systemic phase of downy mildew of pea. However, if you were unfamiliar with this symptom what would a reasonable diagnosis be?
#3. This is downy mildew of basil. The symptoms are unusual for a downy mildew. What are these symptoms more typical of?
The correct answer is:
The downy mildews are closely related to Phytophthora and in this case the dark necrosis is more typical of Phytophthora infection.
Downy mildews often have a biotrophic phase which does not trigger the host defences. It appears that in the case of basil downy mildew this phase is very short!
#4. This is downy mildew of radish. A very unusual symptom of a downy mildew. What would be a reasonable diagnosis if you were unaware of this disease?
The correct answer is:
All correct
Not meaning to trick you but any of the answers could have caused this internal necrosis. This is the only example of a downy mildew attacking below ground that I am aware of.
#5. This is downy mildew of maize. Again the symptoms are not typical of a downy mildew what would be a reasonable diagnosis if you were unfamiliar with this disease?
The correct answer is:
These downy mildew symptoms could be mistaken for viral infection:
The yellowing of the leaves with the green streaks do look very much like viral symptoms. The yellowing is also typical of nutrient deficiency but note none of the nearby plants have the same symptom.
#6. The blue ringed areas on this leaf are downy mildew of grapevine. The red ringed regions appear similar but are a different problem. What is that problem?
The correct answer is:
The white surface growth of downy mildew can look similar to the eriophyid mite symptoms:
This can be confusing in the field, however the downy mildew can easily be removed from the leaf whereas the growth associated with the mites cannot be removed.
#7. This is crazy top of maize caused by a downy mildew. What would these symptoms more typically be associated with?
#8. Onion downy mildew: What are the concentric rings of and yellow and green more typically associated with?
The correct answer is:
The concentric rings seen on this onion leaf are typical of a viral ringspot.
The fluffy purple growth in the centre of the spot leaves no doubt as to the cause of the problem.
#9. This is downy mildew on Lucerne, what could the symptoms be mistaken for?
The correct answer is:
The lightly mottled regions appear similar to the feeding of mites.
The fluffy growth on the underside of the leaf left no doubt as to true cause.
#10. Downy mildew of lettuce. This photo provides a general view of the symptoms but what would be your first diagnosis based on this photo ?
The correct answer is:
Bacterial infection.
The advanced stage of the downy mildew infection has caused yellowing and necrosis. Many things could do this but for me it looks superficially like a bacterial infection.
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.
