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During 1996 to 2001, samples submitted to clinics from commercial greenhouses involved 11 species and two unidentified isolates of Pythium from 110 plant samples, five potting soil tests, and five tests of irrigation water. Pythium irregulare was found in 45% of the plant samples, four of the five water samples, and three of the five potting soils. Pythium aphanidermatum accounted for 29% of all plant but 77% of the poinsettia samples. The Pelargonium samples received were infected with P. aphanidermatum, P. dissotocum, P. heterothallicum, group F, P. irregulare, P. myriotylum, and P. ultimum. The base pair sequence of the ITS1, 5.8S, and ITS2 regions of ribosomal DNA effectively differentiated the species encountered. The ras-related protein gene sequence did not differentiate P. aphanidermatum, P. arrhenomanes, and P. deliense from one another. One isolate each of P. cylindrosporum, P. dissotocum, P. heterothallicum, P. splendens, and P. ultimum exhibited resistance to the phenylamide fungicide mefenoxam, an isomer of metalaxyl, while 38% of the P. aphanidermatum and 37% of the P. irregulare isolates were resistant.

A new species of Phytophthora, previously referred to as taxon Dre II, is named Phytophthora hydropathica. It is heterothallic, but all isolates recovered to date are of the A1 compatibility type. Plerotic oospores are produced. Its sporangia are usually obpyriform and are nonpapillate and noncaducous. Isolates of P. hydropathica had nearly identical single-strand conformation polymorphism (SSCP)-based DNA fingerprints that are distinct from those of all existing species. Their closest relatives are P. parsiana and P. irrigata. This new species is able to grow at relatively high temperatures, with an optimum of 30°C and a maximum of 40°C. It was frequently isolated from irrigation water during warm summers. This species caused leaf necrosis and shoot blight of Rhododendron catawbiense and collar rot of Kalmia latifolia at two nurseries where irrigation reservoirs yielded P. hydropathica. Its potential impact on other horticultural crops is discussed.

Pythium isolates from commercial greenhouses in Pennsylvania were tested in vitro and in vivo for sensitivity to propamocarb. Isolates of Pythium aphanidermatum, P. irregulare, and P. ultimum consistently infected geranium seedlings that had been treated with propamocarb. P. ultimum and P. irregulare isolates exhibited dual fungicide resistance by overcoming propamocarb, mefenoxam, and a mixture of propamocarb and mefenoxam to infect seedlings. The sensitivity of isolates to propamocarb in vitro was not a good predictor of in vivo sensitivity. This is the first report of Pythium spp. resistance to propamocarb and dual resistance to propamocarb and mefenoxam.

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...