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 < or = 0.0472) after 1-day exposure for five of the seven species. In contrast, no change occurred (P > or = 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.

FIG. 1.

Comparative zoospore recovery of Phytophthora citrophthora (A) and P. megasperma (B) after incubating in Hoagland's solution (HS) and filtrate of irrigation reservoir water (FIRW) at different pH levels for up to 7 days. The values are the means plus standard errors (error bars) for three tests.

FIG. 2.

Micrographs illustrating the impact of pH on zoospores and germlings of Phytophthora citrophthora in 15% Hoagland's solution over a 2-week period. The effects of pH 3, 7 and 11 for different lengths of time (from 10 minutes [10 m] to 2 weeks) are shown. Abbreviations: l, lysed zoospore; g, germling; m, mycelium; e, encysted zoospore; c, cytoplasmic mass. Bar, 50 μm. Panels C, D, and K were taken at a magnification of ×100, and the other panels were taken at a magnification of ×400.

FIG. 3.

Relative percentages of encysted zoospores and germinated cysts of Phytophthora citrophthora in 15% Hoagland's solution at different pH levels and exposure times (from 10 minutes [10 m] to 72 h). Each bar shows the average plus standard error (error bar) of 18 counts in three replicate wells from two experiments.

FIG. 4.

Relative percentages of encysted zoospores and germinated cysts of Phytophthora citrophthora (A) and P. megasperma (B) in filtrate of irrigation reservoir water at different pH levels and exposure times (from 10 minutes [10 m] to 72 h). Each bar shows the average plus standard error (error bar) of 18 counts in three replicate wells from two experiments.