Submitted to: Fungal Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/2/2016
Publication Date: 8/20/2016
Publication URL: https://handle.nal.usda.gov/10113/5454524
Citation: Preuett, J.A., Collins, D.J., Luster, D.G., Widmer, T.L. 2016. The effect of salinity on the growth, sporulation and infection of Phytophthora ramorum. Fungal Ecology. 23:123-130.

Interpretive Summary: The pathogen that causes Sudden Oak Death is a threat to forests in different geographical regions of the United States. However, very little is known about that habitat for which it can survive and infect plants. This study showed that this pathogen can survive, reproduce, and infect plant material in water environments that contain high salt content, including the Atlantic Ocean. This research is important in that it provides more information on which environments may be suitable for this pathogen and help in surveys to look for this pathogen. This will assist personnel in forestry and regulatory agencies (Forest Service and APHIS), who are impacted by this pathogen and can make strategic plans to help understand the spread and impact of this disease.

Technical Abstract: Phytophthora ramorum, a threat to Eastern U.S. forests, has been found in waterways outside the boundaries of infested ornamental nurseries. Very little is known about what factors are conducive to its survival and sporulation in water. This study examined the effect of salt on growth, sporulation, and infection. The effect of salt on the growth of six P. ramorum isolates was studied by amending V8 agar and V8 broth with different amounts of NaCl. Although growth of P. ramorum was negatively correlated with salt concentration, there was a level of tolerance even at the highest level tested (45 g/L). Sporangia formation in liquid broth was also affected by the increase of salt. No sporangia were observed in cultures that were amended with higher than 20 g/L of salt. Water collected from various sources with different salinity was used to better understand the environment where P. ramorum can survive and infect host material. Water samples were collected from natural bodies of water that had measured conductivity values of 5.6, 30.5, 32.3, and 35.3 mS. The water samples were added to cups containing P. ramorum-infested sand (1,000 chlamydospores/cm3). Rhododendron leaf disks were placed on the water surface for 1 week at 20 C and then plated on Phytophthora-selective medium (PARPH+V8). Very few leaf disks (= 3%) were infected at the three highest conductivity levels while 100% infection occurred at 5.6 mS. Similarly, Rhododendron leaf disks were floated on the surface of solutions containing different salt concentrations added to P. ramorum–infested sand at two chlamydospore levels (100 and 1,000/cm3) for 1 week and plated on PARPH+V8. The leaf disks were exposed to the salt levels of 0, 6, 14, 20, 35, and 45 g/L. The percentages of leaf disks infected exposed to 100 chlamydospores/cm3 were 61.1, 23.1, 3.3, 0, and 0%, respectively, while the percentages of infection at 1,000 chlamydospores/cm3 were 100, 70.0, 55.6, 2.2, and 0%, respectively. This research helps to better understand the effects of water quality on survival and infectivity of P. ramorum and expands the potential range to conduct surveys.