Set by Dr Phil Taylor of CABI.
Apologies to all, there was a mistake in last month quiz. I am reliably informed that there are cases where aphids do lay eggs, certain types of aphid do lay eggs so as to overwinter.
However, the vast majority of aphids do not lay eggs and are born live.
The questions this month are all devoted to the exceptions to the rules of plant pathology. The whole of field diagnosis is based on your own experience and various rules of the kind of symptoms caused by various pests and pathogen.
There are sometimes exceptions to these rules and in these cases unless you know the cause you would not get the correct diagnosis based on previous experience.
#1. This symptom in radish is extremely characteristic, the symptoms are caused by downy mildew. Based on first principles what would you generally have thought would have caused these symptoms?
The correct answer is:
Fungal infection: As stated in the question downy mildew has caused the symptoms but fungal infection, would be the most obvious of the potential other causes.
#2. The soft downy growth on the underside of leaves with light mottling on the upper surface is typical of downy mildews but this is not a downy mildew pathogen. Based on first principles what would predict the cause to be?
The correct answer is:
Fungi: Although the symptoms look like those of a downy mildew this is a fungal pathogen (Passalora fulva). None of the other alternatives ever produce additional growth on a plant.
#3. This is the poster boy of exceptions to the rules of plant pathology. This is peach leaf curl caused by Taphrina deformans, a fungal pathogen but what are the symptoms more typical of?
The correct answer is:
Mite damage: Mites can cause gross deformities within a leaf such as that seen here. None of the other problems would ever cause a symptom such as this.
#4. This is the very characteristic symptoms of downy mildew on cucumber. Which group of pathogens are these symptoms more often associated with?
The correct answer is:
Virus: The symptoms could undoubtedly be described as a mosaic, a symptom often synonymous with viral infection. Phytoplasmas do not generally create a mosaic, they turn leaves yellow. Nematodes cause a general yellowing of the leaves and not a mosaic. Nutrient deficiencies will often cause a mottle in which the yellow and green areas are not so distinct from each other as seen here.
#5. These are the symptoms of Maize lethal necrosis disease caused by two viruses. The symptoms are very atypical of viral infection what would more typically produce these symptoms?
The correct answer is:
Fungal disease: Without prior knowledge and based on the photo alone it would be reasonable to suggest that the problem was a fungal (or bacterial) pathogen. Nematodes could cause the general dieback but look at the healthy regions on the lower leaves, this does not look like a general decline. Herbicides can cause similar symptoms but nearby plants are unaffected. A nutrient deficiency could not have caused this as the plant is too large to suddenly suffer from such severe symptoms.
#6. What appears to be a rot in these cassava tubers is induced by Cassava Brown streak mosaic virus extremely unusual symptoms of a viral pathogen. What would be more likely to have produced these symptoms?
The correct answer is:
Bacterial pathogen: Unless you had prior knowledge of this complaint it would reasonable to suggest that these symptoms were induced by a bacterial (or fungal) pathogen. Nematodes can cause this type of damage to roots but will always do so from the outside of the root and work inwards. Mineral deficiency can cause internal tissue breakdown generally it more extensive than that seen here. Phytoplasmas do not cause these symptoms.
#7. These tiny leaves growing from this bramble is the typical regrowth following glyphosate application. Which group of pathogens or pest are these symptoms more typical of?
The correct answer is:
Phytoplasma infection: Tiny leaves “little leaves” are often a symptom of phytoplasma infection. Aphids can distort leaves and it is also a common symptom of viral infection but generally neither induce the extreme reduction in size seen here.
#8. These symptoms are typical of mites but that is not the case here. What do you think is causing this light mottling?
The correct answer is:
Nothing: it is a healthy leaf: There are no symptoms to be seen; this ornamental plant has been selected to appear in this variegated form. However the speckling does look like mite damage.
#9. Galls such as these on leaves are typically formed by mites but that is not the case here. What do you think has caused these galls to form on this grapevine leaf?
The correct answer is:
An aphid type insect called Phylloxera:
Very unusual symptom this pest has devastated the grape industry in the late 19th century.
Nematodes very rarely attack leaves and when they do the do not form galls. Bacterial galls are rarely found on leaves and they turn brown as they develop. Downy mildews do not form galls.
#10. This is the most well known of all the exceptions to the rules. This is blossom end rot caused by erratic watering leading to a localised calcium shortage at the distal end of the fruit. However what pathogen or pest would typically cause these symptoms?
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.
