Learning objectives:
To learn about a specific method of identifying fungi, using a written key and/or an illustrated key. Powdery mildew fungi can be identified to genus by the morphology (appearance) of the sexual stage. A key can also be produced as a class exercise. Sexual and asexual reproduction in fungi, host range diversity, and host-parasite relationships are also illustrated.
Exercise description:
Plant leaves infected with powdery mildew disease are collected, and the sexual stage (cleistothecia) is preserved. The cleistothecia are observed under the compound microscope and their appendages are compared with a written key and/or an illustrated key. The fungi can be identified to genus in this manner.
Time frame:
Infected leaves can be collected in late summer or autumn, pressed flat, and dried to preserve them until used. Dry leaves can be rehydrated when ready for use. Leaves also may be examined fresh. Once you have a collection of powdery mildew infected leaves, the actual exercise takes little preparation and can be done at any time. In a fall class, the teacher's pre-class setup time can be greatly reduced by having the students themselves search for and collect leaves exhibiting powdery mildew from different kinds of plants as part of the exercise. Hunting for the infected leaves is fun and will enhance the students' sense of discovery!
Study Questions:
1. To what order do powdery mildew fungi belong? What characteristics place them in this order and into different genera?
2. How do powdery mildew diseases damage plants?
3. What is the asexual (imperfect) stage of powdery mildew fungi?
4. Do you think there is any evolutionary significance to the appendages found on the cleistothecia of powdery mildew fungi? Do you find certain genera of powdery mildews only on certain plant species or families?
Study Question Answers:
1. To what order do powdery mildew fungi belong to? What characteristics place them into this order and into different genera?
The powdery mildew fungi belong to the order Erysiphales and to its single family Erysiphaceae. The Erysiphales cause a group of plant diseases commonly known as powdery mildews, so-called due to the enormous number of conidia produced on the surface of the host. The conidia and superficial mycelia appear as a white, powdery coating to the unaided eye. Powdery mildew fungi are obligate parasites (biotrophs) because they cannot be cultured on nutrient media, but require a living host plant. The sexual stage of fungi in this order is a closed fruiting body called a cleistothecium. Cleistothecia develop from mycelia growing on the plant surface. We separate the powdery mildew fungi into different genera based on the number of asci per cleistothecium (one or more than one) and the morphology of the armlike hyphal appendages on the cleistothecium.
2. How do powdery mildew diseases damage plants?
Since powdery mildew fungi are obligate parasites, they can grow and reproduce only in or on a living host. The powdery mildew fungus produces a specialized absorption hypha called a haustorium (pl. haustoria) within the cell of a host plant to obtain food from the plant cell. Not only does the host plant lose nutrients to the fungus, but photosynthesis also is disrupted. Powdery mildews also damage plants by causing premature death of infected plant tissues. Crop yields may be affected indirectly by the death of infected leaves and directly by the infection of susceptible fruits that either do not develop to maturity or are aesthetically unacceptable. Ornamental plants infected with powdery mildews may be unsightly and unsuitable for sale.
3. What is the asexual (imperfect) stage of powdery mildew fungi?
The asexual (imperfect) stage of powdery mildew fungi is the spore known as a conidium (pl. conidia). The conidia of the powdery mildew fungi are usually formed in chains at the ends of specialized hyphae called conidiophores. The white appearance of this fungus on plant leaves and flower parts is due to the masses of fungal hyphae, conidiophores and conidial chains.
4. Do you think there is any evolutionary significance to the appendages found on the cleistothecia of powdery mildew fungi? Do you find certain genera of powdery mildews only on certain plant species or families?
This question may be used to generate ideas and stimulate group or class discussions. Did students find the appendages to be host specific? If not, were there any similar epidermal characteristics on the infected leaves of the different host types which might relate to appendage types. Are there differences in appendages of cleistothecia found on dicots vs. monocots or on woody plants vs. herbaceous plants?
