Pathways of movement for Phytophthora ramorum, the causal agent of Sudden Oak Death

Jennifer M. Davidson and Charles G. (Terry) Shaw

Phytophthora ramorum and the suite of diseases it causes, including Sudden Oak Death, are relatively new to science. As such, there are far more questions than answers regarding nearly all aspects of this pathogen's biology. However, since the discovery of P. ramorum 3 years ago, much has been learned about the transmission biology of this pathogen, especially in oak woodlands.

BACKGROUND

Phytophthora species have been studied for nearly 150 years, so extensive background knowledge exists on the transmission biology of this genus. In general, Phytophthora species that infect aerial parts of plants spread through a cycle of production of sexual, or more often, asexual spores, movement of spores, and infection of new host tissue. The new infection can then serve as another source of spores to begin the cycle again. Appropriate environmental conditions (temperature and relative humidity), as well as survival of the pathogen (either as spores or mycelia), are necessary for each step of the cycle. Transport of infected host tissue harboring a viable pathogen can also introduce Phytophthora to new areas where the pathogen may establish if it can complete the cycle described above.

The first step, production of spores, occurs on or within living plant tissue. P. ramorum has thus far only been found to infect aerial parts of plants, including leaves, green stems, and woody stems, but not roots. At 15 - 20 ºC, in culture and on foliar hosts such as bay and rhododendron, P. ramorum readily forms sporangia that are highly deciduous, a characteristic consistent with dispersal from aboveground plant parts. Both attached and detached sporangia of P. ramorum produce zoospores (asexual spores) in culture. In addition, P. ramorum prolifically produces chlamydospores (asexual spores) in culture and on some foliar hosts, indicating a possible mechanism for dormancy and survival in adverse conditions. Oospores (sexual spores) have not been observed in nature, perhaps because P. ramorum appears to require two mating types to produce sexual spores. Currently, the European population only has one mating type, and the North American population has the opposite mating type.

Once spores are produced by aboveground Phytophthora species, they are commonly dispersed through several mechanisms²,³. Often the spores are splashed or carried to the ground by rain or sprinkler drops. Spores landing in streams or irrigation runoff then potentially can travel long distances. Likewise, spores landing in the soil of a forest or nursery floor can be transported in soil sticking to shoes, equipment, or vehicles. For a few Phytophthora species, such as P. infestans, sporangia are dispersed in air without water drops (e.g., without rain, sprinklers). Finally, spores and hyphae may be spread through movement of infected plant material. As described below, spores of P. ramorum, like spores of the majority of aboveground Phytophthora species, are known to move via all of these mechanisms except dispersal in air without water drops.

A final step in the reproductive cycle involves successful infection of new host tissue. Reaching a susceptible host is essential. In addition, preliminary studies indicate that optimal infection of bay leaves by zoospores of P. ramorum occurs at 20° C. A film of water on the leaves for 12 hours appears to be necessary for infection by P. ramorum (D. Huberli, unpublished data).

PATHWAYS

Research to determine each component of the transmission cycle for P. ramorum will allow us to better evaluate possible pathways for pathogen spread. These pathways include transmission in natural ecosystems and spread through human activities such as commerce and recreation. Below, we present brief summaries characterizing pathways for both of these main areas. The concern is basic - movement of spores or infected plants may serve as a pathway for pathogen spread. What do we know about the mechanisms and potential for this movement to actually happen?

Pathways in natural ecosystems.

The life cycle of P. ramorum and mechanisms of spread have been studied in coast live oak and tanoak woodlands in northern California. Although all of the host species (currently, there are more than 20) have not been examined, dominant hosts such as coast live oak, tanoak, coast redwood, and bay are being tested for spore production. A major source of spores in these forests appears to be infected bay leaves¹. To date, sporulation has not been observed on intact bark of oak or tanoak trunks. Studies are underway to determine whether sporulation occurs on tanoak and redwood leaves and small branches.

Once spores are formed in these forests, rain carries them onto other plants, into streams, or onto soil¹. P. ramorum has been detected in streams throughout the rainy season, and in some cases, throughout the dry summer months (Davidson, P. Maloney, S. Tjosvold, unpublished data). However, it is unknown whether spores can move from stream water up to infect new plant tissue. Given that infection patterns in forests do not seem to follow stream courses, this may be a rare event. P. ramorum has also been detected in soil during the rainy, winter months. Hikers have been shown to carry the spores of P. ramorum on their shoes as they leave infested tanoak-redwood forests (S. Tjosvold, unpublished data). In addition, soil from car and mountain bike tires has tested positive for P. ramorum after these vehicles traveled on dirt roads through infested forests (S. Tjosvold, unpublished data). It is likely that woodland animals such as deer also carry spores in soil stuck to their feet. Laboratory experiments indicate that inoculum in soil can infect green leaf litter, which can then transmit P. ramorum to aerial plant parts via rainsplash (Davidson, unpublished data).

While these pathways provide evidence to explain the rapid spread of P. ramorum within a forested site, the mechanisms underlying long-distance jumps of P. ramorum-- for example over 250 km from Redway, California to Brookings, Oregon-- are still unidentified. Clearly, long-distance transport of infested soil via shoes, vehicles, or equipment may be one route of possible long-distance spread. Anecdotal evidence from northern California suggests that landscaping with infected horticultural plants at homes within woodlands adjacent to public wildlands may have introduced P. ramorum to new areas. Studies are underway to investigate the potential role of birds in moving spores over long-distance flight patterns. To date, wind dispersal of P. ramorum without rain has not been demonstrated.

Pathways of spread through human activity.

Nursery stock. Although there are numerous nursery hosts, information on transmission of P. ramorum is best known for rhododendron, typically one of the most susceptible plant genera to Phytophthora. P. ramorum has been isolated from rhododendrons in nurseries in the U.S. (California) and Europe. The California Department of Agriculture has traced pathogen transmission from one nursery to another on rhododendron stock. In Europe, P. ramorum was introduced to Majorca, Spain via a shipment of infected rhododendrons, and many of the infections found in nurseries in the United Kingdom can be traced to plant transport from other nurseries. P. ramorum sporulates prolifically from rhododendron leaves, producing both sporangia on the leaf and chlamydospores on or within the leaf (D. Rizzo, P. Tooley, unpublished data) (Figure 1a, b). Detached rhododendron leaves that were dried for up to 3 months still produced sporangia upon wetting (D. Rizzo, unpublished data). In laboratory inoculation experiments, infection spread from one rhododendron leaf to another via water drops that presumably carried sporangia or zoospores (D. Rizzo, unpublished data). P. ramorum has been isolated from irrigation water from an infested rhododendron nursery⁴; (S. Tjosvold, unpublished data). Fungicides commonly used to control other Phytophthora species on rhododendron may mask symptoms of P. ramorum, making visual diagnosis difficult. In addition, in infested nurseries, soil or mulch in the pots of rhododendron plants, other host plants, and even unsusceptible plants may contain spores of P. ramorum although the plants appear healthy.

Bark and wood products. P. ramorum has been isolated from external cankers on stems of oaks and tanoaks, including cankers on fallen trees. Sporulation has occurred from flooded chips of infected tanoak and the flooded, cut edges of coast live oak cankers, indicating that mulch or firewood may be infective (Davidson, E. Hansen, unpublished data) (Figure 2). However, sporulation has not been observed on the outside, intact bark of infected oak or tanoak logs. On oak, the pathogen has been recovered 3 cm into the wood (D. Rizzo, unpublished data). Therefore, debarking is not a sufficient treatment to eradicate the pathogen from oak wood. It is unclear, however, if the pathogen is infective directly from the surface of exposed wood. Heating and drying are recommended as a treatment to destroy P. ramorum in these hardwood species. However, studies are needed to determine if chlamydospores are produced in bark (phloem) and wood (xylem), and if so, whether these spores are destroyed by drying. Techniques should be developed to assess dormancy (viability) versus death of chlamydospores. Acorns do not appear to be infected in the field, although spores may land on the outside of acorns. To date, P. ramorum has not been found to infect the main trunk of Douglas-fir or coast redwood. Studies are underway to determine if redwood bark mulch contains spores that may have landed on the outside of the bark while the tree was standing in an infested forest. Soil on felled trees or logging equipment from infested forests may also contain spores.

Garlands, wreaths, and Christmas trees. Leaves and branches of hosts such as bay laurel, Douglas-fir, and redwood are used in wreaths and garlands. Douglas-fir is farmed for Christmas trees. Some of these forest products are grown or manufactured within infested counties in California and previously have been sold throughout the United States. As mentioned above, P. ramorum readily sporulates from bay leaves under moist, temperate conditions. In addition, chlamydospores are formed in and on bay leaves. Any treatment to kill P. ramorum in bay leaves would need to destroy chlamydospores. Again, it is crucial to be able to determine whether chlamydospores are dormant or dead. P. ramorum infects the small branches of Douglas-fir and small branches and needles of redwood. Studies are underway to examine sporulation on these two important economic hosts. Even without sporulation, fir wreaths and Christmas trees could serve as an infection pathway if hyphae were able to grow from infected branch tips and needles. Cones of redwood and Douglas-fir do not appear to be infected in the field, although spores may land on the outside of cones.

Greenwaste/compost. Greenwaste containing host material from infested areas may serve as a source of spores, especially from leaves of foliar hosts. Even with green material dried for several months, some plant tissue, such as rhododendron leaves, will still sporulate upon wetting. Although it has not been demonstrated, it is likely that spores could be dispersed from foliar hosts via rainsplash during transit in open containers, or that infected leaves could detach and blow away. Tests indicate that P. ramorum in greenwaste mulch is killed in compost after being held at 55° C for 2 weeks (M. Garbelotto, S. Swain, T. Harnik, K. Hayden, unpublished data).

Recreation and tourism. Spores of P. ramorum have been detected on the shoes of hikers and on the tires of mountain bikes and vehicles used on dirt roads or trails in an infested tanoak-redwood park in Santa Cruz County, California. Subsequent outings to other natural areas by these visitors may then spread the pathogen via infested soil. In this same study, a survey of those visitors with infected shoes showed that many people leaving the park were going to other parts of California, the United States, and Europe (S. Tjosvold, unpublished data). In infested park and recreation areas, educational signs coupled with a place to wash soil from shoes and equipment would help lower the probability of spread via park visitors. For popular destinations, taking these steps may be more realistic than attempting to close infested areas during the rainy season.

CONCLUSION

Pathways of spread in commerce mirror the pathways of spread in natural ecosystems, with the major difference being that, in trade pathways, more infected plant material is transported over greater distances, increasing the potential to introduce P. ramorum to new geographic areas. Background knowledge on the transmission biology of other Phytophthora species as well as data on the spread of P. ramorum in natural ecosystems can give us insight into the way P. ramorum may spread in trade pathways. Containment is important. Given the wide host range of P. ramorum, it is likely that this pathogen could establish itself in new geographic areas and cause the substantial forest destruction that we are now witnessing in California.

LITERATURE CITED

1. Davidson J. M., Rizzo D. M., Garbelotto M., Tjosvold S., Slaughter G. W. 2002. Phytophthora ramorum and Sudden Oak Death in California: II. Transmission and Survival. In Proceedings of the Fifth Symposium on Oak Woodlands: Oak Woodlands in California's Changing Landscape. 2001 October 22-25; San Diego, CA. General Technical Report PSW-GTR-184, ed. R. B. Standiford, D. McCreary, K. L. Purcell, pp. 741-9. Albany, CA: Pacific Southwest Research Station, USDA Forest Service, U.S. Department of Agriculture.
2. Erwin D. C., Ribeiro O. K. 1996. Phytophthora Diseases Worldwide. St. Paul, MN: APS Press. 562 pp.
3. Ristaino J. B., Gumpertz M. L. 2000. New frontiers in the study of dispersal and spatial analysis of epidemics caused by species in the genus Phytophthora. Annual Review of Phytopathology 38: 541-76.
4. Werres S., Marwitz R., Man in't Veld W. A., De Cock A. W. A. M., Bonants P. J. M., et al. 2001. Phytophthora ramorum sp. nov., a new pathogen on Rhododendron and Viburnum. Mycological Research 105: 1155-65.