Ecology of Phytophthora ramorum and other Phytophthora species in watercourses

Lead project scientist: Kamyar Aram

Though plant pathogenic Phytophthora species have routinely been recovered from freshwater streams and agricultural irrigation-sources, their ecological role in these aquatic systems is yet uncertain. The fact that these water molds spread effectively through water sources is not surprising given their biology, but do the regular incidences of Phytophthora species in waterways indicate inoculum run-off from terrestrial or riparian sources or rather an establishment of a resident population in an aquatic system?

Phytophthora species have been associated with tree mortality along water courses and in estuaries.  For example, P. alni has caused extensive mortality of alder in European riparian systems and along the banks of lakes. P. lateralis has caused mass mortality of Port-Orford-Cedar along streams and rivers in Oregon and California.

P. gonapodyides is one of the most commonly isolated Phytophthora species from
streams and has been considered a saprobe and facultative pathogen, but there is little experimental evidence confirming its saprotrophic nature. Given the regular detection of these organisms in water, it is reasonable to hypothesize that P. ramorum may perennate in freshwater streams. Overall, the potential for any species of Phytophthora to persist in streams on plants or plant debris has not been well characterized.

The detection of P. ramorum by baiting is often complicated by the presence of other bait-infecting Phytophthoras. The characterization of Phytophthora species diversity in watercourses should shed light not only on the potential nature of P. ramorum’s occurrence in this environment, but also indicate the possible limitations of this methodology for detecting the pathogen. Bait detection of P. ramorum in streams may also be difficult when otherPhytophthora species are either more populous in the watercourse, or are better competitors for the bait materials.    

For Phytophthora, zoospores are the primary form of inoculum in water-saturated conditions, however, the nature of inoculum responsible for watercourse bait detections is unknown. Since zoospore motility is generally limited to a few hours after release from a sporangium, and zoospores encyst in response to turbulence, zoospore cysts may be the most likely propagules in watercourses. It is possible that other propagules also are responsible for the presence and infectivity of Phytophthora species in watercourses.

This work addresses three overarching questions aimed at understanding the ecology of P. ramorum and Phytophthora species in aquatic systems and its relevance to disease spread and baiting as an early detection tool. 

1. What is the source and nature of P. ramorum inoculum in watercourses?
2. What is the risk of re-introduction of the pathogen from aquatic sources to the terrestrial landscape?
3. What are the characteristics of zoospore biology and the infection process of P. ramorum on bait materials

Read more...

Fichtner, E., Aram, K. and Rizzo, D.M. 2009. Ecology of Phytophthora ramorum in watercourses: implications for the spread and management of Sudden Oak Death. [POSTER] Presented at the Fourth Sudden Oak Death Symposium, 15-18 June, Santa Cruz, California.