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

Abstract

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.Phytophthora species, a group of oomycetes in the kingdom of Stramenopila and well-known plant pathogens, were first listed as "water molds" by Blackwell in 1944 (5), and this notion has since been generally accepted. These species are phylogenetically close to golden-brown algae, although morphologically and physiologically, they resemble fungi. Most algae are aquatic in nature. Phytophthora species produce flagellate zoospores as their primary dispersal structure (35)(36)(37)39). Zoospores can travel in aquatic environments actively on their own locomotion and passively through water movement (12,13,41).More than 20 species of Phytophthora, including P. ramorum, the sudden oak death pathogen, have been isolated from irrigation reservoirs and natural waterways (20-22, 30, 40, 43), and a number of previously unknown taxa also have been documented in aquatic environments (8,24). These pathogens pose a threat to agricultural sustainability and natural ecosystems, as agriculture increasingly depends on recycled water for irrigation in light of rapidly spreading global water scarcity (19,22). Recycling irrigation systems provide an efficient means of pathogen dissemination from a single point of infection to an entire farm and from a single farm to other farms sharing the same water resources (22,24).A search of science-based solutions to this crop health issue reveals a surprisi...

Protocolos de aislamiento y diagnóstico de Phytophthora Spp.: enfoque aplicado a la investigación

“…La mayoría de las especies de Phytophthora son favorecidas por alta humedad, temperaturas entre 15 y 35 °C, climas húmedos con lluvias abundantes y sustrato o suelo pobres en nutrientes. Cuando se presenten estas condiciones, es recomendable analizar muestras del sustrato o suelo para detectar al patógeno (Erwin y Ribeiro, 1996;Drenth y Sendall, 2001;Kong et al, 2009). …”

“…Most species of Phytophthora are favored by conditions of high humidity, temperatures between 15 and 35 °C, humid climates with abundant rains and nutrient-poor substrate or soil. When these conditions are present, it is advisable to analyze samples of the substrate or soil to detect the pathogen (Erwin and Ribeiro, 1996;Drenth and Sendall, 2001;Kong et al, 2009). It is suggested that the collections are made from moist soil, near apparently healthy roots and from those that are growing (Erwin and Ribeiro, 1996;Balci et al, 2007;Fichtner et al, 2007).…”

Comparative Analysis of Water Quality between the Runoff Entrance and Middle of Recycling Irrigation Reservoirs

“…DO differences were within 2 mg/L for most sampling dates, which may not change DO conditions in RIRs from being unfavorable for pathogen survival. Survival rates for zoospores of seven Phytophthora species were pH dependent [10]. Most pH differences were within 1.0 unit in RIRs and would likely to have little effect on pathogen survival.…”

“…Phosphorous becomes less available to the plants when pH is above 7.2 [3]. Water temperature (T) [4][5][6][7][8][9], pH [10], dissolved oxygen (DO) [11], EC [12] and oxidation-reduction potential (ORP) [13] also affect the survival and growth of plant pathogens in the same reservoirs.…”

Morphological, physiological and molecular characterization of Phytophthora alni isolates from Western Spain

“…Furthermore, pH level fluctuations in aquatic systems influence the dissemination of many Phytophthora species from a single point of infection to far distance points (Gibbs et al 1999;Hong and Moorman 2005;Kong et al 2009). In this view, effects of pH on the survival and growth of the pathogen are of great importance.…”