Collect leaves that show signs of powdery mildew. Recently infected leaves are best for observing hyphae and conidia. Conidia can be more easily observed by folding the leaf and looking for the conidia in profile with a dissecting microscope.
Collect leaves with mature (black) chasmothecia on plants with powdery mildew in late summer and autumn. The chasmothecia should be visible with your eyes, although a hand lens may help. Leaves with abundant chasmothecia may be pressed and dried for classroom use for many years. They can be stored in large envelopes or cardboard boxes for use throughout the year. Dried leaves tend to crumble. It might be easiest to remove chasmothecia by gently scraping the surface with a moistened, single-edge razor blade. If this is done over a white sheet of paper, it is easier to see if chasmothecia have been successfully transferred to the water drop on the slide. Dried leaves also can be softened by steaming for about 10 minutes on a screen over boiling water. Students may find it easier to remove chasmothecia from fresh leaves and can be encouraged to bring their own samples to fall classes.
To properly use the identification key in the lesson, it will be necessary to observe both conidia and chasmothecia on the same plant host, so fresh plant material should be obtained for each dried specimen if at all possible. Also, as an introductory exercise, it can be difficult to discern between ‘Conidia in chains’ (Blumeria, Golovinomyces and Podosphaera) and ‘Conidia in pseudochains’ (common for Erysiphe). When uncertain, it is suggested that students work the key in both directions. Mycologists frequently require additional information to discern among genera, including DNA sequence, host range databases, and morphology of the appressorium (infection cushion) and the conidia.
Moreover, the distinction between “partially endophytic” mycelium, found in Leveillula and Phyllactinia, and “epiphytic” mycelium, found in the Erysipheae and other powdery mildews, can be difficult to determine without the preparation of thin sections of infected leaf tissue. The presence of conidiophores emerging through leaf stomata is a common occurrence in Leveillula and indicates the presence of endophytic hyphae in that genus. In Phyllactinia, however, some epiphytic mycelium may be present (typically on lower leaf surfaces only), with conidiophores generally arising from the epiphytic rather than the endophytic hyphae.
Genera that may be found are: Phyllactinia on hazelnut and dogwood; Erysiphe (sect. Erysiphe) on sweet pea and phlox; Erysiphe (sect. Microsphaera) on azalea, oak, lilac, dogwood, and viburnum; Erysiphe (sect. Uncinula) on grape; Podosphaera (sect. Podosphaera) on apple and cherry; Podosphaera (sect. Sphaerotheca) on rose, strawberry, and cucurbits; Blumeria on wheat and turf grasses; Golovinomyces on cucurbits, zinnia, sunflower, phlox, and chrysanthemum; and Leveillula on tomato and pepper.
Reference: Braun, U., R.T.A. Cook, A.J. Inman, and H.-D. Shin. 2002. The Taxonomy of the Powdery Mildew Fungi. Pages 13-55 in: The Powdery Mildews: a Comprehensive Treatise, Berlanger, R.R., W.R. Bushnell, A.J. Dik, and T.L.W Carver (eds.). American Phytopathological Society, St Paul.
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