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Applied and Environmental Microbiology, July 2009, p. 4307-4314, Vol. 75, No. 13
0099-2240/09/$08.00+0     doi:10.1128/AEM.00119-09
Copyright © 2009, American Society for Microbiology. All Rights Reserved.

Zoosporic Tolerance to pH Stress and Its Implications for Phytophthora Species in Aquatic Ecosystems{triangledown}

Ping Kong,1 Gary W. Moorman,2 John D. Lea-Cox,3 David S. Ross,4 Patricia A. Richardson,1 and Chuanxue Hong1*

Virginia Polytechnic Institute and State University, Hampton Roads Agricultural Research and Extension Center, Department of Plant Pathology, Physiology and Weed Science, Virginia Beach, Virginia 23455,1 Pennsylvania State University, Department of Plant Pathology, University Park, Pennsylvania 16802,2 University of Maryland, Department of Plant Science and Landscape Architecture, College Park, Maryland 20742,3 University of Maryland, Department of Environmental Science and Technology, College Park, Maryland 207424

Received 18 January 2009/ Accepted 4 May 2009

Phytophthora species, a group of destructive plant pathogens, are commonly referred to as water molds, but little is known about their aquatic ecology. Here we show the effect of pH on zoospore survival of seven Phytophthora species commonly isolated from irrigation reservoirs and natural waterways and dissect zoospore survival strategy. Zoospores were incubated in a basal salt liquid medium at pH 3 to 11 for up to 7 days and then plated on a selective medium to determine their survival. The optimal pHs differed among Phytophthora species, with the optimal pH for P. citricola at pH 9, the optimal pH for P. tropicalis at pH 5, and the optimal pH for the five other species, P. citrophthora, P. insolita, P. irrigata, P. megasperma, and P. nicotianae, at pH 7. The greatest number of colonies was recovered from zoospores of all species plated immediately after being exposed to different levels of pH. At pH 5 to 11, the recovery rate decreased sharply (P ≤ 0.0472) after 1-day exposure for five of the seven species. In contrast, no change occurred (P ≥ 0.1125) in the recovery of any species even after a 7-day exposure at pH 3. Overall, P. megasperma and P. citricola survived longer at higher rates in a wider range of pHs than other species did. These results are generally applicable to field conditions as indicated by additional examination of P. citrophthora and P. megasperma in irrigation water at different levels of pH. These results challenge the notion that all Phytophthora species inhabit aquatic environments as water molds and have significant implications in the management of plant diseases resulting from waterborne microbial contamination.

* Corresponding author. Mailing address: Virginia Polytechnic Institute and State University, Hampton Roads Agricultural Research and Extension Center, 1444 Diamond Springs Road, Virginia Beach, VA 23455. Phone: (757) 363-3908. Fax: (757) 363-3950. E-mail: chhong2{at}vt.edu

{triangledown} Published ahead of print on 8 May 2009.

Applied and Environmental Microbiology, July 2009, p. 4307-4314, Vol. 75, No. 13
0099-2240/09/$08.00+0     doi:10.1128/AEM.00119-09
Copyright © 2009, American Society for Microbiology. All Rights Reserved.





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