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Title: First report of powdery mildew caused by Podosphaera leucotricha on Callery pear in North America

item MINNIS, DREW - Rutgers University
item Rossman, Amy
item CLEMENT, DAVID - University Of Maryland
item MALINOWSKI, M - University Of Maryland
item RANE, K - University Of Maryland

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/4/2009
Publication Date: 2/1/2010
Citation: Minnis, D., Rossman, A.Y., Clement, D., Malinowski, M.K., Rane, K.K. 2010. First report of powdery mildew caused by Podosphaera leucotricha on Callery pear in North America. Plant Disease. 94:279.

Interpretive Summary: Powdery mildew fungi are a very large and diverse group of parasites that attack crop and forest plants. Accurate knowledge of the distribution of powdery mildews is important for tracking the movement of these disease-causing fungi. In this research a powdery mildew fungus that infects pears, apples and other related plants was discovered for the first time in Maryland on leaves of the ornamental tree, Bradford pear. Although relatively common throughout the world, the fungus has been reported only once before on Bradford pear in Hungary. Knowledge of the distribution of plant pathogenic fungi is useful to agronomists and plant pathologists as well as plant regulatory and quarantine officials.

Technical Abstract: Podosphaera leucotricha (Ellis & Everh.) E.S. Salmon (Ascomycetes, Erysiphales) is the etiological agent of a powdery mildew disease that occurs on rosaceous plants, primarily Malus and Pyrus. This fungus is nearly circumglobal. In May 2009, leaves of Bradford pear (Pyrus calleryana Decne.), some distorted and displaying signs of powdery mildew, were collected in Columbia, Howard County, Maryland. Microscopic observation of the fungus revealed an Oidium anamorph with nipple-shaped appressoria; conidiophores erect, foot cells cylindric, straight, with terminal origin, 41–55 x 9.5–12.5 µm, with following cells present in variable numbers; conidia catenulate, broadly ellipsoid to rarely slightly ovoid, 22–27 x 11–17 µm, with fibrosin bodies. Chasmothecia were absent. Based on morphology and host, the fungus was identified as Podosphaera leucotricha using keys and descriptions. The specimen has been deposited in the U.S. National Fungus Collections as BPI 879141. Podosphaera leucotricha has been reported previously only once on Bradford pear, an important ornamental, in Hungary. Thus, this is the first report of Podosphaera leucotricha on this host in the U.S.A. Additionally, the powdery mildew fungus was heavily parasitized by Ampelomyces quisqualis Ces. sensu lato, a cosmopolitan coelomycetous mycoparasite of the Erysiphales that is well known on this species.