#1. What is causing these pale spots on this bean leaf?
The correct answer is:
Rust fungus
The brown rusty coloured pustules are clearly visible in the more advanced stages. The small pale dots are also infection sites but the pathogen has not yet broken through the leaf epidermis (the tough layer of cells that creates the leaf surface).
The incorrect answers were:
Sucking insect probing
Many sucking pests, aphids, mealybugs, scale etc., do not create any damage at the site of penetration. Some pests do leave marks, but often they are necrotic and not pale like this.
Viral infection; a local lesion host
A local lesion host is a concept familiar to many who have worked with viruses but it is not often seen in the field. The rust pustules give away the real cause.
Herbicide spray damage
Symptoms can appear spattered across the leaf like this, but normally those that create an extremely localised response (e.g., paraquat) produce necrotic spots, not pale ones such as these.
#2. These bumps are a symptom called oedema. What is the cause?
The correct answer is:
Physiological problem
The cause of oedema is still a mystery. There is no pathogen or pest involved and it is entirely physiological. The uptake of water seems to be the issue.
The incorrect answers were:
Insect feeding
Sap feeding insects do not often cause feeding marks, but when they do, they are often necrotic.
Fungal infection
Rust fungi can sometimes cause a slight blister before they burst the surface, but they would never get this large, nor so uniform.
Mineral deficiency
There are occasional unusual symptoms caused by a lack of nutrients (blossom end rot is a good example) but this is not one of them.
#3. This photo shows a spider catching a hoverfly. Hoverflies are considered to be beneficial insects as they eat pests. How should we view spiders?
The correct answer is:
They are beneficial themselves as they prey upon pests.
Spiders are very rarely pests and are generally seen as useful members of the crop fauna as they will predate pests.
The incorrect answers were:
They reduce the beneficial insect numbers and should be controlled.
They can predate beneficial insects as seen in the photo, but this is unusual, and will generally consume pests.
They are generally pests themselves and should be controlled.
There are no examples of spiders as pests of crops (mites are related to spiders but are not spiders).
They have almost no effect on the pest status of a crop.
This is not true and they can reduce pest numbers considerably.
#4. Which of these leaves is showing typical symptoms of late blight as caused by Phytophthora infestans?
The correct answer is:
A
The necrotic region with very little yellowing is typical of late blight on tomato. The white bloom of sporulation is seen around the edge of the necrotic area. (Spreading of necrosis into the leaf, with a band of sporulation bordering it, is extremely characteristic of late blight infection).
The incorrect answers were:
B
This mottling is more typical of a mineral deficiency; possibly manganese deficiency in this case.
C
The tips of the leaves have become necrotic, but there is no sign of any sporulation.
D
The yellowing around the necrotic regions is not typical of late blight.
E
The tan necrotic regions are not typical of late blight under normal conditions.
#5. Wild oats in an oat crop; what is it about wild oats that make them such a problem?
The correct answer is:
They take up a lot of room and will crowd out the crop.
The main problem with wild oats among the crop is that; they are extremely large plants that grow very quickly, will shade the crop and remove water and nutrients from the soil. They also produce thousands of seeds which can be dormant, making them a very difficult weed to control.
The incorrect answers were:
The seeds contaminate the harvest, reducing its quality.
Unless the harvest is being saved for seed, this is not a problem.
They carry diseases over season to season, making crop rotation redundant.
This may be true to a limited extent, but is not foremost in farmers minds when they are battling wild oats.
The seeds are shed before the combine harvests the crop and do not contribute to the yield.
This is usually true, but it is not the missing yield that is the problem.
#6. This is Late blight on tomato again, but looking rather different from usual. The lesions are dry, tan coloured and no longer seem to be spreading. Why have these symptoms developed?
The correct answer is:
High temperatures and dry weather after the initial infection.
Foliar-attacking Phytophthora require warm and wet weather to thrive. In this case, the weather became very hot and very dry, and the Phytophthora has died. The lesions have been taken over by a saprobe and is sporulating in some cases.
The incorrect answers were:
This is a resistant variety; this is a resistant reaction.
No; this was a very susceptible reaction and, if the weather had not changed, the whole leaf would have died.
Biological control of the pathogen is taking place.
Saprobes have moved in on the dead tissue but that is not what killed the leaf tissue (or killed the Phytophthora)
These are early symptoms and will develop into more characteristic symptoms with time.
No; early symptoms of Phytophthora infection are pale, sunken patches on the leaf which rapidly turn black.
#7. Phosphate deficiency can often cause plants to turn red. Why is it clear that this plant is not suffering from phosphate deficiency?
The correct answer is:
The upper leaves are red, but the lower leaves remained green.
The symptoms of phosphate deficiency, like the other major nutrients (nitrogen and potassium), are always seen in the lower leaves first. This reddening is due to stress; caused by a stem borer living in the stem at the point where the plant turns red. Note that the other plants around are not showing any reddening.
The incorrect answers were:
The leaves are the wrong shade of red.
The red colour develops due to stress and the diagnosis on the hue of the colour is not really possible.
The plant is not stunted.
Phosphate deficiency does usually stunt plants but not always.
Maize is the exception and does not turn red in response to phosphate deficiency.
This is not true.
#8. The central portion of this tomato stem has died and within it are black lumps. This leaves little doubt as to the cause. What is the cause?
The correct answer is:
Sclerotinia sclerotiorum
A very common fungal disease, which often attacks the base of plants; it is one of the few plant pathogens that will develop an extensive network of hyphae outside the plant.
The incorrect answers were:
Bacterial wilt
Bacterial wilt can cause wilting and necrosis above the site of infection (it usually spreads to all of the plant quickly) but it never produces these black structures seen within the stem.
Phytophthora infestans (Late blight)
The upper leaves have died but the lower leaves show symptoms which would be very unusual for this pathogen. Phytophthora resting spores are too small see with the naked eye and P. infestans does not produce them in many countries.
The symptoms of TSWV are mostly ringspots and rarely a wilt. Viruses never produce structures visible to the naked eye.
#9. 'Bacterial streaming test' is a good technique to diagnose bacterial wilt diseases. The cut stem is hung in a glass of water and the cut-end viewed. What time scale is required to see the effect?
The correct answer is:
About 3 – 10 minutes should be enough.
When the water is extremely still and viewed against a black background, fine wisps of milky fluid can be seen emerging from the cut end of the stem. It is sometimes easier to take a photo and view the photo than to see it with your eye.
The incorrect answers were:
Viewed very quickly; a few seconds and you will have missed it.
This is too short. You need to allow time for the bacteria to begin to flow.
Overnight is best as this allows the bacteria to really flood out.
The bacteria will have exited the stem by this time and you will not see them in the water.
3 – 4 hours is about right.
This is too long. The bacteria will have exited the stem by this time and you will not see them in the water.
#10. When applying a foliar fungicide as a spray, the amount of active chemical ingredient per area is important, but what about the amount of water that carries it?
The correct answer is:
The dilution can be varied within certain parameters.
The correct amount of active ingredient should be applied to the correct location on the crop and, within certain limits, the amount of water – as a carrier fluid – can be varied.
The incorrect answers were:
The dilution of the chemical is unimportant provided the correct amount of active ingredient is applied.
This is true to a point, but once the spray becomes too dilute, the spray will run off leaves and onto the soil.
The dilution of the chemical has to be very exact to ensure best results.
The dilution of chemical does not always have to be exact and, usually, there is some sort of flexibility in the range of dilutions possible.
The more concentrated the solution the better and dilution is done only to make the concentrate go further.
A more concentrated fungicide will generally have greater capacity to kill the fungus, but it would be un-economic, illegal and would scorch the crop.
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.
