CRIME SCENE INVESTIGATION
with Powdery Mildew Fungi
PURPOSE: FORENSIC BACKGROUND INFORMATION:
This exercise can be used to stimulate the investiga- Powdery mildew is a plant disease caused by a fungus
tive nature of your students as they use forensic plant that forms a whitish-gray coating of fungal hyphae
pathology techniques with powdery mildew cleistothe- (thread-like structures) and spores on the surface of
cia (the sexual stage of powdery mildew fungi) to prove leaves, stems, fruits, buds and flowers (Figure 1). Dif-
their innocence in a mock murder investigation. ferent genera of powdery mildew fungi can be identified
most easily at
In order for students to the end of the
truly understand the nature plants growing
of science and technology, season when
they must model the process they reproduce
of scientific investigation sexually to
through inquiries. This les- form pinhead
Figure 1
son is suitable for grades 7 sized, black,
-12 and does not require the spherical struc-
purchase or maintenance tures called
of special cultures, since cleistothecia
cleistothecia produced by (Figure 2).
powdery mildew fungi are The distinctive
easy to find on plants in Figure 2 arm-like ap-
nature. Teachers will use pendages that
leaves infected with powdery radiate from
mildew fungi that they have the surface of
previously collected. the cleisto-
thecia and the
CRIME SCENARIO: presence of
An eyewitness thought he Figure 3 one or several Genera of Powdery Mildew Fungi
saw you and two others run- spore-contain- (Drawing courtesy of C.B. Kenaga,
E.B. Williams, and R.J. Green)
ning from a wooded park ing sacs called
where a murder took place. asci inside the
The police have brought cleistothecia help differentiate the genera (Figures 3, 4
you all in for questioning and key to genera of powdery mildew).
and during their investiga-
tion they notice that the leaf Additional background information and color images
pieces stuck to all of your are available in the K-12 Lab: Powdery Mildew Fungi:
shoelaces have the same Classification and Ecology.
powdery-like appearance
as the leaf pieces that were Disclaimer: Powdery mildew fungi have long been classified to genus
based on the number of asci contained in the cleistothecium and on the
found on the murder victims Figure 4 morphology of the hyphal appendages growing out of the wall of the
clothes. Although only cleistothecium. Recently, new research has resulted in the reclassifica-
circumstantial, the public is demanding the arrest of a tion of many of these genera according to the phylogeny (an estimate
of evolutionary relationships) inferred from the DNA sequence of the
suspect, and it appears that you are going to be arrested ribosomal ITS region and a number of morphological characteristics. In
for murder unless you can prove them wrong. You particular, the morphology of the conidia (asexual spores) and conidio-
must delve into the depths of forensic plant pathology phores (the structures that produce the conidia) is important in the new
and use diagnostic skills and tools to prove your inno- classifications. Despite these changes, this lab can still be used to teach
students about forensic evidence, the use of identification keys, and
cencethat is, if you ARE innocent! morphological variation among fungal species.
Lab exercise created by: Gail Ruhl, Purdue University, Botany and Plant Pathology Dept,
915 West State Street, West Lafayette, IN, 47907-2054, ruhlg@purdue.edu
INSTRUCTORS
CRIME SCENE INVESTIGATION
MATERIALS AND METHODS age envelopes from which you previously removed leaves (so
the Murder Victim sample will match only one of the Suspect
Collection Methods: samples) and remove several more leaves from that one enve-
In late summer or autumn collect leaves with mature (black) lope. Cut the leaves from that sample into smaller pieces and
cleistothecia on plants that show signs (pale, dusty-white coat- place them in the envelope labeled Leaf Material from Murder
ing on leaves) of powdery mildew. The cleistothecia should be Victims Shoes. Note: the number of total cut leaf pieces in this
visible to your eye without the aid of a magnifying glass. Asters, envelope must equal the number of groups in the class so that
azalea, cherry, filbert, grape, horse chestnut, lilac, oak, phlox, each group gets one piece of leaf material from the Murder
rose, zinnia, evening primrose (Oenothera spp., often called Victim. Record on your key the name of the known powdery
sundrops), viburnum, wheat, willow and many other shrubs, mildew genus that you placed in the envelope labeled Leaf
trees, flowers, weeds and garden plants are good sources of Material from Murder Victims Shoes.
cleistothecia.
Lab Set-up:
Leaves exhibiting abundant cleistothecia are good specimens Place all of the labeled envelopes (Leaf Material from Sus-
for teaching purposes. Infected leaves should be pressed flat pects Shoes and Leaf Material from the Murder Victims
between pieces of newspaper. Pressed, dried, leaf specimens can Shoes) on a designated table or lab bench.
be stored in envelopes for use over many years. Label the en- Place several black permanent markers or wax pencils on the
velope with the pressed leaves giving the name of the plant, the tables for students to label their petri dishes. For each lab group,
powdery mildew genus (you will need to identify the genus with set up a work station area consisting of :
the forensic key and diagrams), and location of collection, for dissecting microscope compound microscope
future reference. For your own purposes of cataloguing and ease clear tape two petri dishes
of choosing teaching material for labs, you may want to number forceps several microscope slides
your envelopes and keep a separate list of the specific genera of water in dropper bottles
powdery mildew found to be associated with each plant col- If supplies are limited, groups can easily share items.
lected. You will find that several different genera of powdery
mildew will infect the same host type. For example, Micros- Study questions:
phaera and Phyllactinia may both be found on oak. This is why Forensic Conclusions: Innocent or Guilty?
it is important to label your envelopes by powdery mildew genus
and not host plant. 1. Compare the forensic plant pathology observations you have
recorded for the two different samples. Are the appendages
Lab Preparation: observed on the two different samples the same or are they dif-
Set out four empty envelopes and your envelopes of pressed, ferent?
dried, leaf material infected with powdery mildew and covered
with cleistothecia. 2. If the appendages you observed on the cleistothecia found on
the plant material taken from your shoelaces (murder suspect)
Take three empty envelopes and label each of them Leaf Mate- are different from appendages on the cleistothecia found on
rial from Suspects Shoes. Label the three envelopes #1, #2,or the plant material taken from the murder victim, what does this
#3. Note: for a large class you may want to make duplicate enve- evidence suggest?
lopes of each number. (This exercise works best when students
work in groups of three or four.) 3. If the appendages on the cleistothecia found on the plant
material taken from your shoelaces (murder suspect) are the
Remove several leaves from three of your storage envelopes same as the appendages on the cleistothecia found on the plant
that contain the pressed, dried, leaf samples of three different material taken from the murder victim what does this evidence
known genera of powdery mildew. Be careful to keep the leaves suggest?
with each different powdery mildew genus separate from each
other. Cut the leaves, one powdery mildew genus at a time, into 4. Do you need an alibi as to where you were when the murder
smaller pieces. Clean the scissors before cutting the leaves of took place? Explain.
the next sample.
Place the cut leaf pieces of each different known powdery
mildew genus into one of the numbered envelopes (#1,#2 or
#3) labeled Leaf Material from Suspects Shoes. Note: the
number of cut leaf pieces must equal the number of groups in
the class so that each group gets one piece of Leaf Material
from Suspects Shoes. On a separate piece of paper (your key)
write down the envelope numbers and the name of the known
powdery mildew genera in each numbered envelope.
Full lesson plan with additional images, along with other
Take one empty envelope and label it Leaf Material from great lab exercises, available in the APS Education Center
Murder Victims Shoes. Choose one of the three known stor- at www.apsnet.org.
STUDENTS
FORENSIC TOOLS: You can quickly check to see if you have successfully trans-
CRIME SCENE INVESTIGATION
ferred cleistothecia to your microscope slide by placing it on a
You are a suspect in a murder investigation. You must attempt to white sheet of paper. If you can see black pinhead-sized specks,
prove your innocence and clear your name using dissecting and then you have cleistothecia. If you do not see tiny black specks
compound microscopes, clear tape, petri dishes, forceps, micro- then remove the tape from the slide and repeat steps #4 and #5.
scope slides, a water dropper and a forensic key with the special
characteristics of powdery mildew cleistothecia appendages. Now place the slide on the stage of the compound microscope
and examine the cleistothecia, first under
FORENSIC LAB PROCEDURES: 100x magnification, followed by 200x and
Collect and label forensic evidence. 400x (if available).
First, locate the numbered envelopes labeled
Leaf Material from Suspects Shoes. Identify forensic evidence.
As you examine the cleistothecia with a
Choose one of the numbered envelopes la- compound microscope, look at the append-
beled Leaf Material from Suspects Shoes ages on the cleistothecia. Carefully examine
and using a forceps, remove a piece of the both the base and tip ends of the appendages
powdery looking plant material from the on at least fifteen different cleistothecia to
numbered envelope and place it in a petri be sure you have seen the correct appendage
dish you have labeled Suspect.Record the type. This is necessary since tips break off
number of the envelope on the petri dish and easily, leaving only stringy looking arms
on your data sheet. that might lead you to a false identifica-
tion. Compare the appendages to the written
Locate the envelope labeled Leaf Mate- descriptions and/or the drawings in the keys
rial from the Murder Victims Shoes and included with this exercise. Decide what
using a different or clean forceps remove a kind of appendages they are, and record this
piece of the powdery looking plant material description along with a drawing of the ap-
from the envelope. Place the plant material pendage type on the Forensic Evidence Data
in a petri dish you have labeled Murder Table located on your Case Study Sheet.
Victim. Note: Cleanliness is very important
in this process of handling samples with After observing the appendages, determine
forceps. There are several infamous cases how many asci (spore-containing sacs) are
of sloppy handling of forensic evidence that in the cleistothecia by gently pressing down
have led to false convictions. on the clear tape with the end of the forceps
Take the labeled petri dishes with your piec- to break the cleistothecia open. It is help-
es of plant material to your work station. ful to look through the microscope at low
power (100x) while doing this in order to see
Examine forensic evidence. the asci pop out of the cleistothecia. Some
Put a small drop of water in the center of a cleistothecia may be immature and will not
microscope slide. Examine the leaf sample contain asci. Once you break open a ma-
labeled Suspect. The black speck-like ture cleistothecium, how many asci do you
cleistothecia should be visible to your eye see? One ascus or several asci? Record your
without the aid of magnification. Place the observations on the Case Study Sheet. This
powdery looking leaf material peppered information may be important in your effort
with black cleistothecia under a dissecting microscope, for to prove yourself innocent.
increased visibility with magnification. Tear off a piece of clear
tape that is approximately half the length of the microscope Repeat Sections B and C (Examination and Identification of
slide. While looking through the dissecting microscope, press Evidence) with the plant material in your petri dish labeled
the leaf piece firmly down with one hand. Using your other Murder Victim including the recording of results.
hand, hold onto one end of the tape and gently touch the rest of
the clear tape on top of the black, speck-like cleistothecia. They Record forensic evidence.
should stick to the tape. Repeat this process several times on Locate the Forensic Evidence Data Table on the Case Study
other areas on the leaf surface to gather more cleistothecia. Sheet. Record and draw the type of appendages (arms) found on
the surface of the cleistothecia (e.g. hooked tip; bulb-like base;
Place the tape (sticky side down) on top of the water droplet on antler-like dichotomous branching; or simple, string-like arms),
the microscope slide. Gently push down on the tape and smooth the number of asci (one or several) contained within each cleis-
the tape over the water droplet so that it sticks to the slide. If tothecium, and the name of the powdery mildew genus.
necessary, more water may be added by squeezing the water
dropper gently at the edge of the tape on the slide. The clear tape
is an excellent tool for gathering cleistothecia and other fungal
structures from a leaf surface.
CRIME SCENE INVESTIGATION
Forensic Evidence Data Table
Characteristics of Cleistothecia
Types of Number of asci
Appendages (sacs) per
cleistothecium Powdery Mildew
(one or several) Genus
Murder Suspect
(YOU) #______
Murder Victim
Forensic Key and Diagrams to the
Genera of Powdery Mildew Fungi
Appendages coiled or hooked at tip - Uncinula
Appendages simple and straight with bulb-like base - Phyllactinia
Appendages branching dichotomously (antler-like at tip)
Cleistothecium contains a single ascus - Podosphaera
Cleistothecium contains several asci - Microsphaera
Appendages simple or irregularly branched, often entangled
Cleistothecium contains a single ascus - Sphaerotheca
Cleistothecium contains several asci - Erysiphe
