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Remarks (public):For a complete description including images see 
Remarks (internal):Notes: Blumer (46: 563) and Junell (1967) have re-described it recently. In 1907 the fungus appeared in epiphytotic proportions in Western Europe and in a few years spread over the entire region doing much damage to oak coppice and young plantations. For a number of years it did not form cleistothecia; finally when they appeared the pathogen was identified and was believed to have invaded Europe from America (39: 739). The disease can be controlled by dusting with sulphur or applying copper sprays in June and July (Westcott, 1950).
Description type:Non-original description 
Description:Microsphaera penicillata (Wallr.: Fr.) Lév., Annls Sci. nat. Bot., 3 ser. 15: 155 & 381, 1851.
= Microsphaera alni (Wallr.) Wint., sensu Salmon, Mem. Torrey bot. Club 9: 129, 1900, pro parte.
= Microsphaera alni (DC.) Wint., in Rabenhorst, Kryptopamenflora 1: 38, 1887, pro parte.
= Alphitomorpha penicillata Wallr., 1819.
= Alphitomorpha penicillata var. alni Wallr., 1819.
= Erysiphe penicillata (Wallr.) Fr., 1829.
Mycelium amphigenous but mostly on the lower surface, evanescent or persistent, sometimes forming definite patches on theleaf surface. Cleistothecia scattered to gregarious, globose-depressed, variable in size, 65-110 (-140) µm diam.; cells ofperidium large angular, 10-18 µm wide. Appendages 4-26, equal or up to 2 times the diameter of the cleistothecium, more orless rigid, generally colourless throughout, smooth and non-septate but occasionally amber brown, rough and 1-2 septatetowards the base, apex dichotomously branched 3 to 6 times, tips of ultimate branches regularly and distinctly recurved. Asci3-8, ovate to ovate-globose, with or without short stipe, 40-70 x 30-50 µm, 2-8-spored. Ascospores oval, 18-23 x 10-12 µm, hyaline.
Hosts: On species of Alnus, Betula, Syringa, Lathyrus, sweet peas and numerous other hosts (Salmon, 1900; Stevens, 1925; 39: 739; 41: 175)
Diseases: Powdery mildew of alder and lilac. Forms a white to grey floury appearance on the surface of broad leaves of many hard wood trees. It is more prevalent on sweet peas than Erysiphe pisi (CMI Descript. 155) in North America in spring when temperature and humidity are fluctuating. The foliage may be malformed, dropping prematurely or drying out and shrivelling. It is also prevalent on lilac in late summer and autumn, sometimes in dry seasons almost completely covering the foliage, but generally too late in the season to cause serious damage. Young leaves are more susceptible.
Geographical distribution: World-wide on alder and lilac and occasionally occurring on numerous other hosts. Distributed generally in North America and Europe, also reported from Chile, China, India and Japan (Salmon, 1900, 39: 739; 41: 175).
Physiological specialization: Not known.
Transmission: Spores wind borne.
Literature: Salmon, Mem. Torrey bot. Club 9: 129-159, 1900; Stevens, Plant Disease Fungi, pp. 139-141, 1925; Junell, Symb. bot. upsal. 19: 52-61, 1967; Westcott, Plant Disease Handbook, pp. 271-273, 1950.
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