BSPP ‘Plant Doctor’ Quiz #8 – November 2021

The correct answer is:

Bacterial leaf spotting:

What appear to be infection points spreading from the leaf edges could easily be mistaken for bacterial infection.

The incorrect answers were:

Viral infection; viruses rarely cause localised symptoms such as this and the symptoms are usually leaf distortion and mosaic.

Nematode infection; the most common symptom of nematode infection is usually a wilt, only later do the leaves begin to show other symptoms generally caused by the stress of the wilting.

Mite damage; the speckling that is usually the first sign of mite infestation may start in localised patches but will spread before those patches become very severe.

Photo courtesy of  Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org

The correct answer is:

It means that the pathogen kills the host tissue before colonising it.

Necrotroph is the opposite of a biotroph; a necrotrophic pathogen will use an arsenal of toxins and enzymes to overpower the hosts defences and kill the tissue ahead of the advancing fungal mycelium.

The incorrect answers were:

It describes the fungal group that the pathogen is in; the fact that the fungus is a necrotroph has no bearing on the classification (for example genus/species) of the pathogen.

It means that the pathogen will only grow on material that has died previously; fungi that grow on dead material are generally described as saprobes or saprophytes.  They are generally not pathogens but are opportunistic / imposter pathogens.

It means that the pathogen will rot the host plant; almost all necrotrophs do rot the host tissue, as it is often killed with host cell wall degrading enzymes, but that is not what the term means.

Photo courtesy of : Howard F. Schwartz, Colorado State University, Bugwood.org

The correct answer is:

To prevent the sexual cycle of the pathogen proceeding:

This photo is of oospores; they are sexual spores of the oomycetes (water moulds). In some pathogens (such as P.infestans which causes late blight of potato or tomato) oospores will only form if there are and – strains present. Many countries only have one of the two mating types, thus preventing sexual spores being produced.

The incorrect answers were:

It is good to keep the amount of pathogen low in whatever way you can; whilst this is true, the amount of additional pathogen arriving on imports would be miniscule relative to the established population.

It will allow imports to be excluded so that local farmers can be supported; under WTO rules it is not easy to exclude produce except for phytosanitary reasons, but this is not the reason from a plant pathology viewpoint.

 The identification of the pathogen may not have been accurate and another pathogen may gain access to the country; this is unlikely as phytosanitary inspectors are well trained and are supported by expert scientists who test potential new pathogens found on imported material.

Photo courtesy of : Paul Bachi, University of Kentucky Research and Education Center, Bugwood.org

The correct answer is:

The means by which it is spread:

The means by which the pathogen moves from one plant to another can be determined (to some extent) by the development of the disease as it spreads through the crop (detailed diagnosis would still be required to determine what the pathogen is).

The incorrect answers were:

How long it can remain dormant in the field; it would be very difficult to determine this based on the pathogen spread as, once the infection is established in the field, the dormancy is no longer an issue.

Whether it is biotroph or a necrotroph; these are descriptions of how the pathogen interacts with the host and not the way in which it spreads.

Which type of pathogen group it is; there are many different pathogen groups and many of them spread in many different ways, it is not possible to infer the type of pathogen from its spread.

Photo courtesy of Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org

The correct answer is:

Using molecular experimental protocols, extract infected plant tissue and amplify the nucleic acids for analysis.

To amplify up the nucleic acid (usually RNA but may be DNA) and analyse this is the way in which most laboratories now identify plant viruses.

The incorrect answers were:

Squash a small amount of tissue, mount this with a cover slip and observe under a high-power microscope; this would be true for fungal pathogens, but viruses are too small to be seen with a light microscope.

Attempt to grow the virus on specialist agar from a small amount of leaf which has been briefly dipped in dilute bleach; this would be true for many fungal and bacterial pathogens, but viruses don’t grow on agar.

Isolate the virus particles from the vector; it may be possible to isolate the virus from the vector, but the virus particles will still be too small to be seen.

Photo courtesy of Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org

The correct answer is:

The populations will decline simply because the food source will be reduced:

The BT chemical will kill the larvae which will reduce (or eliminate) their food source. The natural enemies will either leave or die.

The incorrect answers were:

It will have no effect as there are no natural enemies that are in the moth family; whilst there are no natural enemies in the moth family, the population will reduce as the food supply will vanish.

The spray will kill the predators as they may be eating the already poisoned larvae; the BT has a rapid knock-down action and the predators will not eat dead prey.

The predator population will increase because the toxic chemical will make the prey easier to catch; there is generally no problem with predators catching their prey and the BT will not slow the prey down (they are caterpillars after all). It will kill them rapidly and reduce the amount of food for natural enemies.

The correct answer is:

Smut fungus:

This is leaf infection of maize by Ustilago maydis (Mycosarcoma maydis). Infection causing the black sooty spores on the cobs are a much more familiar sight.

The incorrect answers were:

Crown gall caused by Agrobacterium tumefaciens; crown gall can produce galls of this size but the galls are never smooth they are usually brown and Agrobacterium does not infect grasses.

Gall-forming insect; this is reasonable suggestion but (difficult to tell from the photo) the leaf is distorted and there are no galls there.

Viral infection; the distortion seen here would not be so localised if it was caused by a virus.

Photo courtesy of Daren Mueller, Iowa State University, Bugwood.org

The correct answer is:

There are occasions when nets can be used to keep pathogens out of a crop:

This may be a bit of a trick question, but there are occasions when insect vectored viruses are kept out of a crop (such as seed potato production) with netting. It is the virus that is being excluded which happens to be inside an insect.

The incorrect answers were:

The netting has to be fine but it can be used to stop wind-blown fungal spores; no, the mesh would have to be so fine that light would not pass through.

Nets are never used to keep out pathogens; there are occasions, see above.

The netting has to be fine but it can be used to stop bacterial pathogens; no, the mesh would have to be so fine that light would not pass through.

Photo courtesy of Howard F. Schwartz, Colorado State University, Bugwood.org

The correct answer is:

Ultra Low Volume:

Application of less than 5L per Hectare (2 L per Acre) on field grown crops is generally considered to be ULV.  There are advantages and disadvantages to using this application technique; ULV sprayers are especially prone to wind drift due to extremely small droplet size.

The correct answer is:

The heat generated by the composting process will kill the pathogen spores:

If done correctly, the heat from the pile will kill the spores of pathogens. Larger piles are better as they have less surface to volume ratio.  Also, shallow compost areas are ineffective; compost should be kept heaped-up to better retain core heat.

The incorrect answers were:

Yes, the pathogens will survive and will infect any susceptible crop when compost is applied to the soil; this is not true if the composting process is done correctly.

Biocontrol will take place within the composting process and the fungal pathogens will rot along with the plant material; this is not true and the resting fungal spores are highly resistant to rotting.

Some pathogens will survive but most will not due to the length of time the composting process takes; composting should only take a few months and fungal resting spores will survive much longer than that.