Throughout the protocol, utilize only conventionally grown (
nonorganic) potatoes. Conventional potatoes are treated with a germination inhibitor, which will prevent potatoes from sprouting when
The following are needed for each lab group of two to four
• Potatoes (Russets are the most common; at least three for each
• Damp paper towels
• Sealable plastic bags
• Small beakers for the bacterial inoculum
• A 10% bleach solution containing two drops of dish soap per
• A large container for disinfesting the potatoes in 10% bleach
• Microscopy equipment (microscopes with 400× magnification,
slides, coverslips, metal probes)
• Optional: Latex or nitrile gloves (soft rot bacteria are not infectious to humans, but the odor could be a detriment with direct
contact with skin)
Performed by the teacher prior to the lab:
(1) A week prior to the experiments, initiate the cultures to acquire
inoculum. Cut a potato in half and incubate the halves in an
airtight plastic bag with 50 mL of tap water for five days at
room temperature. If infected, the potato will be soft to the
touch and the liquid will be cloudy with bacteria. Most potatoes will become infected with this approach, so consider incubating between three and five potatoes to collect sufficient
inocula for the class.
(2) The day before the lab begins, disinfest the remaining potatoes
by soaking in the 10% bleach solution for at least 30 minutes.
Ensure that each potato is completely submerged in the bleach
solution, and agitate the potatoes occasionally as they soak to
remove any bubbles. After incubation, remove the potatoes
from the bleach solution, rinse with fresh water, dry with paper
towels, and set aside to air dry overnight.
(3) Prior to the start of class:
(a) Mock inoculate (with water) at least three disinfested pota-
toes as negative (healthy) controls. Inoculate each potato
with a clean toothpick, wrap in a damp paper towel, and
incubate in the same manner as the students’ samples.
(b) Collect the bacterial inoculum by cutting a corner of the
culture bag and collecting the liquid in several containers
for the students. The bacteria can be stored in a refrigerator and used for at least six weeks.
Performed by the students:
(1) Discuss and outline the experiment (Table 3).
(2) For each lab group of two to four students, collect at least three
disinfested potatoes and ensure that they are free of obvious
disease symptoms incurred during shipment and storage.
(3) Dip a clean toothpick into a container of bacterial solution and
inoculate the bacteria into the potato by poking the toothpick
through the skin. Repeat the inoculation approach to ensure
that each potato is similarly inoculated ( i.e., a similar number
of toothpick wounds). The same toothpick may be used for
all the inoculations, but students must re-dip the toothpick in
the bacteria solution prior to each inoculation for consistency.
(4) After inoculating, wrap each potato in damp paper towels and
enclose in a sealable plastic bag.
(5) Incubate all potatoes on the benchtop for seven days. If infected,
the potatoes will become soft to the touch and produce a pungent odor.
(a) Record which inoculation approach (control or experimen-
tal) yielded the most disease symptoms (Figure 2). Since
each group inoculated three potatoes, each lab group’s sam-
ples should exhibit similar symptoms.
(b) Slice the potatoes through lesions and observe symptoms
within the potato (Figure 2B–D).
(7) After the plant tissue becomes infected, collect diseased tissue
and observe with a sample of healthy tissue as a comparison
Table 3. Experimental design scaffold (including
representative discussion points for students).
Question What conditions impact the
development of potato soft
(“If. . .then. . .because”)
If we inoculate a potato with
then __% of the potatoes
will be infected with bacteria
Materials (See text)
(What you changed on
purpose; this is the
Inoculated healthy potatoes
with soft rot bacteria
(How you measured what
Percentage of the potato
(either surface, interior, or
total potato) that exhibits
symptoms of soft rot.
(Also called constants; add
as many as needed)
disinfested potatoes, potato
(The normal condition; what
you compare your
experimental group and
independent variable to)