Exercise: Powdery Mildew Fungi: Classification and Ecology

Purpose:

Powdery mildew fungi are ideal for use in a laboratory exercise on fungal classification, fungal spore types, host-parasite interaction during an ecology unit, or a study of host range. Students will discover how many different plant hosts they can find that are infected by the same genus of a powdery mildew fungus, or how many different genera of powdery mildew fungi can be found on the same plant host. This exercise demonstrates the diversity that exists within a fungal order. With a good collection of leaves infected with different powdery mildew fungi, students learn to use a written key and/or an illustrated key (or could even make their own key) to identify the powdery mildew fungus to genus. Since powdery mildew fungi reproduce by means of two spore types: asexual spores (conidia) and sexual spores (ascospores), discussions of the types of reproduction in fungi would be facilitated.

Background information:

Powdery mildews are one of the most common, conspicuous, widespread, and easily recognizable plant diseases. As a group, powdery mildew fungi infect many species of plants, including many trees and shrubs, numerous ornamentals, vegetables, cereals, grasses, and even weeds. However, individual species of powdery mildew fungi are usually very host specific. For example, the species of fungus causing powdery mildew on pumpkin is different from that causing the disease on roses.

Extensive losses in plant growth and crop yield occur annually due to powdery mildew. The primary sign of powdery mildew is grayish white, powdery blotches on leaves. Usually, powdery fungal growth appears first on the upper leaf surface (FIGURE 1). Eventually the entire leaf may become covered with mildew (FIGURE 2). Powdery mildew fungi also can infect other parts of the plant and may cause distortion and stunting of shoots, leaves (FIGURE 3), and flowers (FIGURE  4), and russeting on fruit (FIGURE  5). The type and extent of symptoms vary depending on the combination of powdery mildew fungal species and host plant species involved. Late in summer and fall, the sexual stage of many species of powdery mildew fungi, the cleistothecia, is visible as black or brown, pinhead-sized, spherical specks among the white to grayish mildew mycelium in the older infected areas on the leaves of many plants (FIGURE  6). (These are the leaves that should be collected and pressed flat between newspaper for storage and use in this laboratory exercise.)

Figure 1.  The powdery mildew fungus, Uncinula necator, growing on the surface of a grape leaf. (Courtesy J. Schlesselman). Click image to see a larger view.

Figure 2.  This leaf is so severely infected that it is completely covered with powdery mildew. (Courtesy APS, Compendium of Stone Fruit Diseases). Click image to see a larger view.

Figure 3.  Distortion and stunting of young grape leaves infected by powdery mildew. (Courtesy J. Schlesselman). Click image to see a larger view.

Figure 4. Flowers distorted by the apple powdery mildew fungus, Podosphaera leucotricha, with white fungal growth on the stems of flowers in the center of the blossom cluster. (Courtesy K. D. Hickey). Click image to see a larger view.

	Figure 5.    Net russeting of a Jonathan apple caused by powdery mildew. (Courtesy K.S. Yoder and K.D. Hickey). Click image to see a larger view.
	Figure 6.    The white, brown, and black spheres are the cleistothecia of Uncinula necator at various stages of maturity (Courtesy W. Gärtel). Click image to see a larger view.

The powdery mildew diseases of various crops and other plants are caused by many different species of fungi grouped into six main genera in the order Erysiphales, an order that includes a single family, the Erysiphaceae. The fungi causing powdery mildews are obligate parasites (or biotrophs), meaning that they cannot be cultured on nutrient media and require a living host to grow. These fungi reproduce by means of two spore types: asexual spores called conidia and sexual spores called ascospores.

The conidia are usually barrel-shaped or oval and are usually formed in chains (FIGURE 7) at the ends of specialized hyphae called conidiophores produced from the mycelium growing on the surface of a plant’s leaves, stems, flowers or buds. The combination of the mycelium, conidia and conidiophores gives the leaf surface a powdery appearance from which the name powdery mildew is derived. The fungi are spread when the conidia are carried by air currents to new plant surfaces. Under favorable conditions, the fungal spores germinate (FIGURE 8). Unlike most fungal parasites that invade plant tissues, most powdery mildew fungi grow superficially on top of the leaf surface (FIGURE 9). Fine, thread-like infection pegs penetrate the epidermal cells of the leaves and form haustoria (FIGURE 10). Haustoria are specialized hyphae for nutrient absorption from the plant cells. The plant is damaged by the loss of nutrients to the fungus, disruption of photosynthesis, and premature death of leaves or other infected plant tissues.

	Figure 7.    Conidia of Uncinula necator, formed in chains. (Courtesy W. Gärtel). Click image to see a larger view.
	Figure 8.    Germinating conidia of Sphaerotheca pannosa on the surface of a rose leaf. (Courtesy of D. L. Coyier). Click image to see a larger view.
	Figure 9.    Scanning electron micrograph of a colony of powdery mildew growing on the surface of a poinsettia leaf. (Courtesy G. Celio). Click image to see a larger view.
	Figure 10.    Powdery mildew fungi absorb nutrients from the host plant through haustoria. Most powdery mildew fungi have simple globose haustoria (a), but species in the genus Erysiphe have more complex ones with fingerlike lobes (b). [(a) Courtesy G. Schumann; (b) Courtesy Plant Pathology Department, Washington State University. Click image to see a larger view.

When environmental conditions or nutrition become unfavorable for growth (usually later in the growing season), the fungus shifts to the sexual stage and produces cleistothecia (sing. cleistothecium). The cleistothecia are closed, thick-walled, tiny, black, spherical structures (white to tan when young) that house sacs called asci (sing. ascus) (FIGURE 11 and FIGURE 12). The oval sexual spores, called ascospores, are produced within the asci. Fungi that produce ascospores in asci belong to the Ascomycetes, a fungal group that includes yeast, morels, and many plant parasites.

Figure 11.   A cleistothecium of Uncinula sp. broken open to show the transparent saclike asci containing oval ascospores. Notice that the tips of the appendages look like hooks. (Courtesy B. Kendrick). Click image to see a larger view.

Figure 12.   Cleistothecia of Microsphaera spp.: a) Light micrograph of a Microsphaera sp. cleistothecium broken open to show several transparent saclike asci containing oval ascospores. Notice how the tips of the appendages are dichotomously branched. (Courtesy R. Wick) b) Scanning electron micrograph (500X) of a cleistothecium of Microsphaera alni. In this close-up view you can easily see the dichotomous branching at the tips of the cleistothecial appendages. (Courtesy M.F. Brown and H.G. Broztman) Click image to see a larger view.

Cleistothecia have ‘arm-like’ appendages that radiate out from their outer surface. Inside each cleistothecium is a single ascus or many asci. Currently, the powdery mildew fungi are classified to genus based on the number of asci contained in the cleistothecium and on the morphology (physical appearance) of the hyphal appendages (arms) growing out of the wall of the cleistothecium (FIGURE 11, FIGURE 12, and Key to Genera of Powdery Mildew). Recently, Saenz and Taylor (1999) have proposed that powdery mildew genera be reclassified 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. If this occurs, the morphology of conidia and conidiophores will be used as the primary characters for classification