#1. These wheat leaves have withered from the tip, what is the likely cause of such a symptom?
The correct answer is:
Abiotic scorch of some sort.
Many things may cause the tips of leaves to die back like this, but the tan colour of the dead region and extreme transition between the dead and healthy tissue indicates scorch. In this case, the scorch is due to ammonium nitrate solution having been applied when the sun was too strong.
The incorrect answers were:
Nutrient deficiency: much yellowing occurs. The sharp transition from healthy to dead tissue is not typical of nutrient deficiencies.
Insect larvae within the stem: Insect larvae can cause problems in wheat but the symptoms are a general withering of the plant and not the sharp transition between healthy and dead tissue.
Bacterial infection: Bacterial infection generally appears wet and, whereas the dead areas here appear very dry, you would rarely see such a marked transition between healthy and dead tissue.
#2. Another abiotic factor is the problem on this bean leaf. What do you think has caused the characteristic stipples (red marks) and flecking (larger brown-yellow marks)? What is the most likely cause?
The correct answer is:
Ozone
Ozone is an under-appreciated threat to agriculture, especially in polluted areas. The symptoms are generally seen on older leaves and beans are particularly susceptible. (Photo; UKCEH)
The incorrect answers were:
Water logging; roots immersed in water for prolonged periods will damage many crop plants. The symptoms are usually yellowing and wilting.
Hydrogen sulphide; can be a problem in rice crops but not usually elsewhere.
Excessive humidity; this is one of the causes that can lead to oedema and not the reddening and flecking seen here.
#3. What is the correct term for these symptoms?
The correct answer is:
The incorrect answers were:
Mottle: the sharp lines between the yellow are green areas on the leaf make this a mosaic. If they were blurred and the contrast in colour was not so great, it may be considered a mottle.
Yellowed : this is a term used to describe a leaf where the whole leaf has lost its green colour.
Speckled : this is a symptom that can look similar to a mosaic but the effect is due to the empty surface cells reflecting light (empty due to feeding of a mite or thrips) and the leaf has not changed colour beneath.
#4. Irrigation systems which are poorly installed and maintained can lead to this white material appearing on the soil surface. What is the material?
The correct answer is:
Salts rising up from below and crystalizing on the soil surface.
If an irrigation system provides all the water for a crop and the water has the slightest mineral content, salts will build up over time on the surface of the soil. When installing an irrigation system, it is often necessary to install drainage too so that the soil could be flushed through occasionally.
The incorrect answers were:
Fungal mycelium growing due to the excessive moisture; this only occasionally happens in extremely humid conditions, not where an irrigation system is necessary.
Bleaching of the humus from the normal brown to white; even in the strongest sun, light does not bleach soil.
Fine root hairs searching for water on the very surface of the soil; root hairs extend only one millimetre from the root at most and will never grow to the surface.
#5. Potatoes are grown from “seed” potatoes; small tubers that may have begun to sprout. If the seed potatoes are large, what technique could a gardener or farmer employ?
The correct answer is:
Cut the potatoes in half and plant both halves thereby doubling your numbers of seed potatoes.
Provided it is dried off before planting, the large wound created when cutting a seed potato does not let in pathogens and is a well-known technique with potato growers (for horticultural reasons, cut them in half lengthways so both halves get the rose and stem ends of the potato).
The incorrect answers were:
Plant them as they are; do not cut the potatoes, the wound will let in opportunistic pathogens from the soil: they could be planted as they are but cutting them is an option (see above).
Slash the skin of the potato to shock it into growing: this is not a recognised technique and does not work.
Do not plant large seed potatoes as they will not sprout; go and buy some smaller ones: you may want to consider eating the larger potatoes but they will grow.
#6. What is the most likely cause of the variation in colour seen here in this field of wheat?
The correct answer is:
Uneven fertiliser distribution
The straight line between the stunted yellow crop and the lush green crop indicate that the difference is due to a mechanical effect. The general yellowing of the crop on the left is typical of wheat hungry for nitrogen, whereas the dark green crop on the right appears to have had too much nitrogen and that is exactly what happened.
The incorrect answers were:
Nematodes attacking the roots; nematodes can cause stunting and yellowing of plants but attack is never uniform as seen here, nor in a straight line.
Variation in soil type; the soil can have a significant effect on crop growth and variation between soils can be dramatic, but they are never in a completely straight line.
Viral infection; Viruses can cause yellowing and stunting as seen here but they do not form lines like this.
#7. Which of the following statements is true?
The correct answer is:
Most viruses have a specific relationship with their vectors: the relationship between the vector and the virus is complex and incompletely understood. This relationship is often very specific.
The incorrect answers were:
All plant viruses can be transmitted by insects; not true, although many viruses are transmitted by insects, other vectors include fungi, nematodes, seed or mechanical means.
All plant feeding insects can transmit viruses; not true, some groups are very commonly seen as virus vectors (such as aphids) and many cannot transmit viruses at all.
Laboratory work with micro manipulators can mimic the transmission of all insect transmitted viruses; there is a great deal more to the transmission of viruses than simple mechanical contamination of the mouthparts and micro manipulators cannot replicate this.
#8. What is the relationship between the true fungi and the oomycetes (water moulds)?
The correct answer is:
Genetically the two groups are completely unrelated.
Despite the many similarities between these two groups (long filamentous hyphae and spore formation), it is now known that they are completely different organisms, with different biochemical structure and evolutionary lineages.
IMAGE: Cucumber leaves attacked by a powdery mildew (fungus) left and a downy mildew (oomycete) right
The incorrect answers were:
The oomycetes are considered as the “lower fungi; this would have been the correct answer 40 years ago but it has become clear that oomycetes are not fungi and are not related to fungi.
The two groups occupy different habitats and there is very little similarity between these groups; the oomycetes and the fungi occupy very similar habitats and look very similar.
Superficially and genetically, they are closely related but oomycetes are not technically fungi; they do appear superficially similar (and the oomycetes are often referred to as “fungus-like organism”) but they are not genetically related.
#9. It is well known that aphids are common vectors of plant viruses but how long after feeding on an infected plant does the aphid retain the ability to inoculate other plants?
The correct answer is:
The length of time the aphid remains able to transmit the virus depends on the virus/vector combination. The ability of the aphid to transmit the virus, which is technically the length of time it remains viliferous, depends on the virus and species of aphid. The aphid may remain viliferous for a few minutes or it may be for weeks.
The incorrect answers were:
The aphid will always carry the virus (and will inoculate other plants) for the remainder of its life; this is unusual in aphid/virus combinations but it is true in some cases.
The aphid will lose the virus and will not spread it after a few hours; this is true in many aphid virus combinations.
The aphid will lose the virus and will not spread it after a few days; this is true in many aphid virus combinations
#10. What effect do medically important antibiotics have on bacterial plant pathogens?
The correct answer is:
They provide good control of bacterial pathogens:
Streptomycin, Tetracycline and other medically important antibiotics are effective in controlling bacterial plant diseases but their use is prohibited in many countries due to fears that the resistance developing in the environment could spread to human and animal pathogens.
The incorrect answers were:
They have no effect on bacterial pathogens that affect plants: medically important antibiotics are very effective in controlling bacterial plant pathogens but should not be used.
They will not control bacterial plant pathogens but do control viruses: this is not true -there are no known chemicals that will control viruses once within a plant.
They will remove the natural biological control on leaves and will actually make the situation worse: the antibiotic application will reduce the natural bacterial populations but if the disease is present, it will have greater effect on the pathogen population.
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.
