Learning objectives
To learn about a specific method of identifying fungi, using a written and illustrated key. Powdery mildew fungi can be identified to genus by the morphology (appearance) of the sexual stage (chasmothecia).
To learn about scientific investigation using forensic techniques
Exercise description
Plant leaves infected with powdery mildew disease are collected after the sexual stage, the chasmothecia, are visible on the leaf surface. Chasmothecia from leaf material are examined with a compound microscope to identify the type of appendages present. The fungi are identified to genus using the appendage type and the number of asci (spore-containing sacs) inside each chasmothecium using both a written and an illustrated key. The students, using forensic plant pathology techniques, must attempt to prove themselves innocent in this CSI lab. Images for class use are in the Supplementary Information and References section.
Time frame
Infected leaves can be collected in late summer or autumn, pressed flat, and dried to preserve them until used. Fresh leaves also may be examined. Once you have a collection of powdery mildew infected leaves, the actual exercise takes little preparation and can be done at any time.
Study questions
Forensic Conclusions: Innocent or Guilty?
- Compare the forensic plant pathology observations you have recorded for the two different samples. Are the appendages observed on the two different samples the same or are they different?
- If the appendages you observed on the chasmothecia found on the plant material taken from your shoelaces (murder suspect) are different from appendages on the chasmothecia found on the plant material taken from the murder victim, what does this evidence suggest?
- If the appendages on the chasmothecia found on the plant material taken from your shoelaces (murder suspect) are the same as the appendages on the chasmothecia found on the plant material taken from the murder victim what does this evidence suggest?
- Do you need an alibi as to where you were when the murder took place? Explain.
Checking Students' Forensic Observations and Results
The Murder Suspect envelope # recorded by the student in the Forensic Observations section and the powdery mildew genus identified on the leaf material from the Murder Suspect should match the envelope #’s and corresponding genus identification on your key.
The powdery mildew genus the students identify on the Murder Victim should all be the same. It should be the ONE genus that you placed in the envelope labeled “Murder Victim” and recorded on your key.
Innocent or Guilty?
If the type of appendages on the leaf material stuck to the murder suspect’s shoelaces (the student/group) does not match the type of appendages on the leaf material found on the murder victim, this evidence would suggest that the murder suspect (student/group) is not guilty.
If the type of appendages on the leaf material stuck to the murder suspect’s shoelaces (the student/group) does match the type of appendages on the leaf material found on the murder victim, this evidence would suggest the possibility that the murder suspect (student/group) might have been in the same wooded park and indeed might be the murderer. An alibi would be requested as to where the suspect had walked with those shoes the past couple of days (weeks) and it would be necessary to determine whether there was vegetation in those areas that was infected with the same genus of powdery mildew (reasonable doubt).
You, as the teacher (moderator), could also ask what other means of testing might be available and lead the discussion into other identification techniques including DNA fingerprinting which can be used for identification of plant pathogenic fungi.
