with a microscope. Bacteria are very small and will be only
faintly visible at 400× magnification. With potato samples,
amyloplasts (organelles that contain starches) are visible
as clear, bean-shaped structures. The bacteria should be
visible as a diffuse mass that is similar to television static
Inquiry-Based Protocol Amendments Organized by
the Disease Triangle
Prior to embarking on additional experiments, promote student-driven inquiry by posing the following scenarios to the lab groups.
Ask the lab groups to brainstorm answers to the questions, and then
encourage each group to share their answers with the whole class to
generate a list of experimental variables. Due to time constraints, consider preparing your own variables prior to class that can be investigated immediately.
(1) Inoculate different hosts. There are several species of bacteria
that cause soft rot, and each species has a different host preference.
Since different potato varieties share common characteristics, will
a pathogen isolated from one variety of potato be able to infect
another variety of potato? Will a pathogen isolated from
a potato be able to infect other fruits and vegetables?
(a) Inoculate different potato varieties such as Yukon Gold,
Russet Burbank, Red Pontiac, or any other commercially
available varieties. Consider selecting smaller potato varieties to optimize space use in the classroom.
(b) Inoculate different susceptible plant products such as
carrots, green bell peppers, bananas, apples, tomatoes, and
cucurbits ( i.e., cucumbers, squashes, pumpkins; summarized in Bhat et al., 2010).
(2) Utilize different inoculation approaches. Sometimes potatoes
are damaged during shipment from the farm to the store. If a
truckload of potatoes is transported across the country, what
are some injuries that are likely to occur?
(a) Dip a whole potato in the inoculum solution.
(b) Prior to application of inoculum, damage the host tissue
by abrading, crushing, or superficial wounding to simulate damage that commonly occurs
(1) Alternate the temperature. In nature,
Pcc is most active at 10–20°C (50–68°F),
but soft rot often occurs at warmer
temperatures during shipment or storage in a warm pantry. What effect will
different temperatures have on the
development of disease?
(a) Alter the incubation temperature
(room temperature vs. warm temperature) or temperature extremes
(hot vs. cold).
(2) Modify water availability. If the potatoes were harvested following a rainstorm and were loaded into a truck wet,
Figure 2. Signs and symptoms of bacterial soft rot.
(A) Bacteria from the toothpick wound are accompanied by a
dark halo of diseased, softened tissue. The white organism is a
fungus, which is a common contaminant and not involved in
the disease. (B) A bisected potato lesion exhibiting tissue
collapse without any secondary darkening. (C) A bisected,
darkened potato lesion on the margin of the tuber. The black
lesion in the center of the potato is an abiotic disorder called
hollow heart, which is caused by incubation in moist
environments and is not related to soft rot. (D) A bisected
mock-inoculated potato, exhibiting cork formation at the
inoculation site. Note the small hollow heart lesion near the
center of the potato.
Figure 3. Micrographs of healthy and diseased potato. A small piece of tissue was
visualized with a microscope at different magnification without stain. (A) At 200×,
potato cell debris and amyloplasts (the dark, bean-shaped structures) are visible, but
bacteria are difficult to observe. (B) Infected tissue viewed at 400× shows scattered
amyloplasts surrounded by many bacilliform bacteria. (C) By contrast, healthy tissue
viewed at 400× shows scattered amyloplasts without associated bacteria.