This exercise can be used to stimulate the investigative nature of your students as they use forensic plant pathology techniques with powdery mildew cleistothecia (the sexual stage of powdery mildew fungi) to prove their innocence in a mock murder investigation.
In order for students to truly understand the nature of science and technology, they must model the process of scientific investigation through inquiries. This lesson is suitable for grades 7 -12 and does not require the purchase or maintenance of special cultures, since cleistothecia produced by powdery mildew fungi are easy to find on plants in nature. Teachers will use leaves infected with powdery mildew fungi that they have previously collected.
Note: This lab works best in regions that experience cold weather for part of the year, since colder temperatures at the end of the plant's growing season encourage the production of cleistothecia (sexual stage of the powdery mildew fungi) on leaves. This lab may not work as well in areas where warm weather prevails year-round, since this type of weather promotes primarily the conidial (asexual) stage of powdery mildew fungi, with sparse to no cleistothecia production.
An eyewitness thought he saw you and two others running from a wooded park where a murder took place. The police have brought you all in for questioning and during their investigation they notice that the leaf pieces stuck to all of your shoelaces have the same powdery-like appearance as the leaf pieces that were found on the murder victim's clothes. Although only circumstantial, the public is demanding the arrest of a suspect, and it appears that you are going to be arrested for murder unless you can prove them wrong. You must delve into the depths of forensic plant pathology and use diagnostic skills and tools to prove your innocence…that is, if you ARE innocent!
FORENSIC BACKGROUND INFORMATION:Powdery mildew is a plant disease caused by a fungus that forms a whitish-gray coating of fungal hyphae (thread-like structures) and spores on the surface of leaves, stems, fruits, buds and flowers (Figure 1). Different genera of powdery mildew fungi can be identified most easily at the end of the plant's growing season when they reproduce sexually to form pinhead sized, black, spherical structures called cleistothecia (Figure 2). The distinctive 'arm-like' appendages that radiate from the surface of the cleistothecia and the presence of one or several spore-containing sacs called asci inside the cleistothecia help differentiate the genera (Figures 3, 4 and key to genera of powdery mildew).
Additional background information and color images are available in the K-12 Lab: Powdery Mildew Fungi: Classification and Ecology.
Disclaimer: Powdery mildew fungi have long been classified to genus based on the number of asci contained in the cleistothecium and on the morphology of the hyphal appendages growing out of the wall of the cleistothecium. Recently, new research has resulted in the reclassification of many of these genera according to the phylogeny (an estimate of evolutionary relationships) inferred from the DNA sequence of the ribosomal ITS region and a number of morphological characteristics. In particular, the morphology of the conidia (asexual spores) and conidiophores (the structures that produce the conidia) is important in the new classifications. Despite these changes, this lab can still be used to teach students about forensic evidence, the use of identification keys, and morphological variation among fungal species.