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Answers to Exercise Questions​
     
     
   

Paul Vincelli
University of Kentucky

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An example data set is provided below.

Number of conidia at cytological stage indicated

Incubation
Period (hr)

Ungerminated

With germ
tube only

Immature
appressoriuma

Mature appressoriumb Infection
hypha
0 118 12 0 0 0
1 2 12 16 0 0
2 2 63 46 10 0
7-8 2 19 55 57 0
14-16 1 4 25 49 0
24 1 2 0 39 19

a stained dark blue
b stained light blue, has a brown tint from melanin accumulation in the cell wall of the appressorium.

 

The structured inquiry provided in Table 3 and at the end of the Discussion section provides for meaningful discussion that exercises higher-order thinking skills, particularly analytical thinking. Students generally give the discussion their full attention, knowing that they are each responsible for a portion of it. When several students are listed for a given question, I simply select one name on the spot, using some random selection process.

The following comments may be useful to new instructors with specialties in areas of plant pathology other than fungi.

Q1. At which cytological stage(s) do contact fungicides act?

The germ tube and immature appressorium are sensitive to contact fungicides. I suspect the mature appressorium is sensitive, as well, since it imbibes water to create the turgor pressure needed for penetration. Once penetration has occurred, the fungus is "home-free," safe from the action of contact fungicides, and infection and host colonization can continue uninterrupted. Sporulation on the leaf surface can be reduced somewhat by application of contact fungicides, but it cannot be completely prevented.

Q2. At which cytological stage(s) do systemic fungicides act?

It depends on the level of systemic uptake. Some systemic fungicides are incompletely absorbed by plants, so they may actually kill before penetration takes place. However, by virtue of their systemic nature, all act on the infection hypha. Thus, systemics can be used post-infection, within limits; typically systemics will eradicate an infection that is 24-48 hr old, although in a few cases, certain products can be applied as late as 96 hr post-infection. See Mill's infection periods for apple scab (Figure 8-24 in Agrios [1]), as an example of how one can determine the need for post-infection fungicide sprays.

Q3. At which cytological stage(s) do phytoalexins act?

As one would expect for cytoplasmic constituents, phytoalexins typically act once penetration occurs.

Q4. At which cytological stage(s) does cork formation act?

Cork formation is an induced defense response. While signaling between host and pathogen may occur prior to penetration, penetration has probably already occurred in most cases where cork is induced in response to infection. This is suggested by Figure 6-6 of Agrios (1), so students can reason this simply by examining the figure.

Q5. At which cytological stage(s) is a biocontrol agent most likely to interact with the pathogen?

This is a challenging question, since students really haven't learned much about biocontrol agents (BCA's) at this point in the semester in my course. However, I like to offer it as a point of discussion (and coach them if they need help). My own response is that the typical BCA would have only the period from spore deposition to infection to suppress or kill the pathogen (a time period of 14-16 hr period in this lab, and sometimes shorter in nature), which is not a long period of time. This sets up a nice opportunity for discussion of BCA's.

Q6. At which cytological stages(s) is the pathogen most sensitive to prolonged exposure to dry air?

The germ tube stage, of course, as explained in the introduction.

Q7. At which cytological stage(s), if any, will symptoms be present?

All of these stages, plus ongoing colonization of plant tissue, will have occurred by the time symptoms develop. This question helps students intuitively understand that a lot of activity in the disease cycle has taken place by the time symptoms occur.