pini (P < 0.0001), and 0.6 for P. tropicalis (P = 0.0004), respectively. The only exceptions were observed in P. megasperma at 24 h, P. nicotianae Ruboxistaurin in vitro at 10 min and 24 h, as well as P. pini at 10 min. These results indicated that zoospore survival in runoff water containment basins is subjected to fluctuations of dissolved oxygen concentration in particular of hyperoxia conditions although there are slightly differences among the four species assessed in this study.

P. megasperma was least affected by elevated concentrations of dissolved oxygen as was by a range of pH in a previous study [7]. Differences in oxygen response were previously observed among oomycetes and fungi. By their oxygen response, these fungi and oomycetes can be grouped into three categories. First, mycelial growth is directly proportional to atmospheric oxygen level with the optimum at 21.0%. This pattern is exemplified by P. GW786034 mw nicotianae (syn. P. parasitica), P. citrophthora and T. basicola[17] and P. cactorum[15]. Second, mycelial growth has a clear optimal oxygen level typically well below 21.0%, which distinguishes this

group from those of the first pattern. Examples of this group included A. euteiches that had optimal growth at 5.0% [24]. Third, mycelial growth increases with increasing atmospheric oxygen only to a concentration, above which results in no further growth benefits. This pattern is illustrated by P. ultimum, of which mycelial growth was reduced at oxygen concentration of 1.3% but was the same for all oxygen levels from 4.0%

to 21.0% [25]. It is unclear how the elevated concentrations of dissolved oxygen affected zoospore survival of different species. In this study we did observe that zoospores of P. nicotianae, P. pini and P. tropicalis remained motile for more than 2 h after their release from sporangia while Mirabegron the most zoospores of P. megasperma had NCT-501 already encysted before they were added to the 500-ml volume at the various dissolved oxygen concentrations. It is reasonable to assume that motile zoospores are more vulnerable to environmental stresses including elevated concentrations of dissolved oxygen or hyperoxia than those encycled ones with cell wall. It is worth of noting that zoospores of P. nicotianae died instantly in a 9.5-L fish tank being bubbled with oxygen at 0.5 L min-1 for 20 min under a separate experiment [22]. The dissolved oxygen concentration in this fish tank was estimated to be over 27.3 mg L-1 according to the formula developed above. It also was previously reported that hyperoxia suppressed fungi and bacteria [29, 30]. Artificial oxygenation of irrigation water for pathogen mitigation may not be economically feasible. However, dissolved oxygen concentration in irrigation reservoirs can naturally rise up to 26.5 mg L-1 due to phytosynthetic activities [13]. Zoospores are the principal, if not sole, dispersal and infective propagules of Phytophthora and Pythium species in recycling irrigation systems [31–35].