The risk of spreading Phytophthora ramorum through forest operations: Sanitation and mitigation

Posted on behalf of Yana Valachovic and Brendan Twieg, University of California, Cooperative Extension,
Humboldt and Del Norte Counties


Theme: Sanitation and Mitigation
Topic: The risk of spreading Phytophthora ramorum through forest operations

Are humans responsible for spreading Phytophthora ramorum to currently uninfested forests?  Forested areas in northern California are regularly managed with heavy equipment and vehicles that have the potential to move considerable volumes of infested soil, debris, and water over long distances to locations where the pathogen is unlikely to move naturally. Epidemiology research suggests that P. ramorum may move a couple of miles per year on its own via windblown rain or overland in infested run-off, but vehicles and equipment can move infested materials tens or hundreds of miles within hours. Research conducted in southern Oregon suggests that vehicles on rocked roads within infested areas are not a vector (Hansen et al. 2013), but is that true where equipment is working off-road or on unsurfaced roads?

Current best management practices (available from the California Oak Mortality Task Force) on cleaning and sanitizing equipment and drafted water to prevent pathogen spread lack consistency and detail, partly because little research has been undertaken to quantify the risks of human-mediated P. ramorum spread (with the exception of ornamental plant movement). Measures successfully undertaken to control the spread of related soil-borne Phytophthoras among forested locations by vehicles and equipment are generally recommended for P. ramorum, but may not be appropriate due to important biological differences between these pathogens. To help guide us in prevention of P. ramorum spread via these pathways, a better fundamental understanding of the risks involved is needed. Some important fundamental questions are:

1. If infested soil is accidentally dropped or shed next to an epidemiologically important host, how likely is that host to become infected, and how does that likelihood relate to the amount of soil?
2. What are the most cost effective and practical field-appropriate cleaning measures that will affect an acceptable lowering of risk?
3. How do we define an acceptable risk?
4. Given that water from naturally infested watercourses have been shown by researchers to have some likelihood of infecting plants (Tjosvold et al. 2008), albeit low, is there an environmentally friendly and rapid chemical treatment that can eliminate the pathogen in water for firefighting and dust abatement?

Answers to these questions are needed to help inform current and future forest management strategies and policies.  

Literature cited

Hansen, E.; Peterson, E.; Hulbert, J. 2013. Roads are not significant pathways for SOD spread, in Oregon at least. pp. 84-85 In: Frankel, S.J.; Kliejunas J.T.; Palmieri, K.M.; Alexander, J.M. tech. coords. Proceedings of the Sudden Oak Death Fifth Science Symposium. Gen.Tech. Rep. PSW-GTR-243. Albany, CA: U.S. Dept. of Agriculture, Forest Service, Pacific Southwest Research Station. 169 p.

Tjosvold, S.A.; Chambers, D.L.; Koike, S.T.; Mori, S.R. 2008. Disease on nursery stock as affected by environmental factors and seasonal inoculum levels of Phytophthora ramorum in stream water used for irrigation. Plant Disease 92: 1566-1573.

I look forward to the discussion.

I have heard stories that a lot of the early spread of the disease, especially in Marin County around Mt. Tamalpais State Park, was due to hikers' boots carrying contaminated soil. Has this been confirmed and is it still a risk factor? In the east, one way the disease could be introduced is by hikers who have been in infected areas coming to some of our recreation areas, such as Red River Gorge National Geological Area. So, is this a pathway that we should be concerned about?

It is true that the pathogen can be detected on the bottom of people's shoes. And some researchers have speculated that this might help lead to localized spread.  The challenge is understanding the probability of this type of spread.

The pathogen moves primarily through wind and rain driven events.  However, it can spread with people moving infected plant material.  Infected plants have great potential for long-distance transport of the pathogen.

There are still lots of unknowns about effective cleaning of shoes, equipment, tires, etc.  Certainly the more organics (e.g bits of infected leaf material for example) that are lodged in a shoe or tire, the more likely the pathogen can be present.  The risks, costs and demonstrated benefits of cleaning and cleaning approaches still need to be worked out.  Are Clorox based products acceptable to everyone in every situation?  For many the answer is no, because of potential impairments to aquatic life or degradation of rubber.  To me, developing a better understanding of effective sanitation is imperative.

Tom, that's a really interesting story from Marin. I would love to hear more given that I work in the County and we talk about sanitation issues a lot in our outreach work. I personally haven't heard this theory about spread around Mt. Tam - can you fill me in?

Janice - I was in Marin County a few years ago and was told by a forester (I forget who) that P. ramorum was a soil-borne pathogen and could potentially be vectored by boots or bike tires. He was of the opinion that there should be formal procedures established at trail head for people to sanitize boots and tires.  Since the disease started around Mt. Tam, it made some sense to be concerned about boots used in this very popular hiking area that could carry contaminated soil elsewhere.  There is a lot more foot and bike traffic around Mt. Tam than heavy equipment traffic.

The National Park Service recommends the following in Muir Woods and other infected national parks:
1. Stay on established trails.
2. If you have been in an infected area remove mud and debris from shoes, vehicles, bikes, horses' hooves and pets' paws before going into uninfected areas.
3.Clean and disinfect boots, bike tires, equipment, and tools used at infected sites by removing mud and debris and spraying with Lysol or a 10% bleach solution.

If this a potential vector, it is of concern here in Kentucky because we have a significant number of west coast hikers and climbers who visit Red River Gorge and Mammoth Cave.  A similar situation exists at our cave parks like Mammoth Cave and Carter Caves, where hikers are required to sanitize boots before and after cave hiking to reduce the spread of white nose syndrome in bats.

So, if anybody has some insight into whether this is or isn't a legitimate concern, I would like to know.

Here is another quote from UC Extension that would seem to indicate that there is some concern:
To help prevent the spread of sudden oak death, people visiting invested areas should not remove leaves or branches and should clean dirt from their shoes before leaving. (December 2013 ANR News Blog).

Here in Kentucky, where black shank of tobacco caused by Phytophthora nicotianae was a major disease, boots are a known and important vector of disease spores.

Thanks for filling in some of those details, Tom. I wouldn't classify P. ramorum as "soil-borne" the way that other Phytophthoras (lateralis, for example) are but what Yana says about finding the pathogen in clumps of mud and organic debris on shoes and tires is true, and is a risk for spread. Local spread is going to be mostly in the canopy and correlated to weather events, but long-distance spread could definitely be tied to recreational users. That's why those more general outreach messages to the public from Muir Woods and UCCE suggest to clean shoes - we are trying to reach a well-traveled public that could potentially bring dirty materials back to their home forests.

I think what you're asking specifically though is what the true risk is for this spread. There has been some research on how long spores collected off of muddy boots remained viable. Most of the time, when the mud was left to dry, no spores were recovered after a day or so. When the muddy boots were wrapped in plastic and kept in the dark, spores were still viable 48 hours later (if I am recalling correctly). So there is a scenario where people hike through muddy Mt. Tam, throw their boots in a bag at the bottom of their suitcase, and fly off to hike somewhere else in the next day... and that's why we want people to clean their boots before they leave an infested forest.

Very helpful, Janice. Thanks!

For all kinds of reasons it is good to not travel with muddy shoes..... Just think of what the mud does to your suit case.

The conversation that I was trying to start here is about taking the general educational message about cleanliness to a level where people who work in the forest every day can effectively implement the standards. Implementing and enforcing cleanliness standards on all who come in contact with an infested forest is challenging. How do you create wash stations that don’t become infection stations for mountain bikers and hikers? How does someone who goes to check on their fish monitoring station efficiently and effectively clean their ATV before they go to the next monitoring location? Is the cleanliness standard a political decision or is a based on the biology of the pathogen? Just because you can detect on a shoe or piece of equipment does not mean that it will transmit if that shoe or piece of equipment moves to a new location. Your thoughts?

To further discussion on what Tom Kimmerer inquired about, I thought I would refer him to Cushman and Meentemeyer's 2008 article (Journal of Ecology, 96: 766–776); it is freely available on the web. I got two take-home messages from this paper that relate to these questions:

1) Recreating people definitely do spread soil containing the pathogen to non-infested areas. In this study, the non-infested areas were ones virtually without hosts, so they were looking simply at whether or not soil with detectable amounts of pathogen had been moved along trails. It had. Would it be enough to cause infection if dropped next to a host plant? Who knows?

2) A slightly higher (modeled at 5.4% higher) disease incidence was found in areas with public recreation access in Sonoma County than on private lands with no access (they controlled for environmental variables in the analysis).

These results don't necessarily suggest the boots/bike tires/horses spread pathway is of huge concern, but we've measured here that while a pair of boots might have ~75 to 200 ml of soil stuck to them, a tracked log loader can have over 500 liters. That's a big difference in the potential amount of P. ramorum inoculum being moved around in one fell swoop! Excuse the dramatic delivery.

Thanks, Brendan. That is useful. I want to emphasize that my concern is with the protection of forest resources in Kentucky specifically and eastern forests in general. I certainly understand that a tracked vehicle will carry more inoculum than a pair of boots, but that a contaminated pair of boots is much more likely to wind up in Kentucky than a dirty tracked vehicle. Certainly within the infected regions of California and Oregon, vehicles are an important vector, but not outside the region.

The higher disease incidence in recreation areas is a perfect illustration of why this is an important problem.

So here's another question: are hikers in the infected areas provided a) signage to indicate that they need to sanitize their boots and bicycles, and b) washing stations to remove soil? Signs and washing stations at trailheads could be quite useful. I know that the whole SOD community is severely budget limited, but it seems to me that we easterners have some financial skin in this, and maybe this needs to be brought to the attention of eastern forest managers.

Here's a little true story about boots and contaminants. During the Chernobyl disaster, a group of American forest ecologists visited the area at the invitation of the Russian government. They were not allowed to collect any samples, but they left the country without cleaning their dirty boots. It was analysis of those boots that allowed the US government to conclude that Chernobyl was not making weapons-grade nuclear material. This is only relevant to this discussion in that it demonstrates the ability of boots to transport foreign material.

I like your Chernobyl story, Tom. And you're certainly right that a pair of muddy boots is much more likely to get from California to Kentucky than a dirty tractor! I've just been thinking about this issue at more of a regional scale; despite that our county (Humboldt) is infested, most of its susceptible forest actually is not yet infested, and heavy equipment is likely to travel among infested and non-infested locations around here (in addition to recreationists, marijauna-grow soils, etc).

I'm not aware of any wash stations at trailheads here in Humboldt County, but there are signs at some of the State and Nat'l Park trailheads that suggest washing and sanitizing boots before leaving. However, especially given rapid SOD spread in the Humboldt Redwoods State Park area (which gets a lot of out-of-state and international visitors), we are aware that the signage needs to be better and more widespread. We're collaborating with the local parks to do get this done.

If anybody is aware of the existence of wash stations at any trailheads (maybe in Marin County?), a post with a little description of how they work would be appreciated. Ideally, we should all be Lysol-ing our boots, bike tires, etc. after a jaunt in any forest, but making this standard practice will take a huge amount of education and a boost in personal responsibility in the recreation community!

I agree, Brendan, the Humboldt Redwoods State Park area is exactly the kind of place that concerns me. In fact, I was there a few years ago (my oldest daughter went to Humboldt State University) hiking, flew home to Kentucky the next day, and was working in the woods a day after that. This was pre-SOD (it was around Mt. Tam then), but I certainly took no precautions.

I'm not sure that getting personal buy-in is that difficult if there is good public education among targeted groups. For example, the Red River Gorge Geological Area was getting trashed after it became extremely popular for sport climbing. The climbing community has since come together to educate their constituency, work on clean-up projects and do a lot to protect climbing routes from deteriorating.

I don't want to side-track this discussion too much away from the issue of regional dispersal as a result of forestry operations, but it seems that the vectors for regional dispersal (heavy equipment) and global dispersal (boots, fat tires) are quite different. The sanitation procedures may not be that different but the educational approach would have to be.

I'm posting this on behalf of Katie Palmieri who is having technical problems at the moment.


I know of only one private location that has a wash station for those entering and leaving the grounds. The location did not have SOD at the time they put in the wash stations (I haven’t spoken to them recently, so I can’t comment on the current status), so they were using it as a way to keep SOD from entering the area on shoes. The only reason this location could have a wash station though was because it was at the entrance gate (1 way in and 1 way out) and it was staffed constantly, allowing for regular water changes. So, really, unless a new wash station model is created, putting such stations in place requires a lot of maintenance and staffing to avoid creating a breeding ground for the pathogen.

The best we have to offer at this point is removal of organic material from shoes, equipment, tires, pets’ paws, etc. before leaving an area known to be infested. Removal should be done in an area where there won’t be additional exposure to spores, such as on a paved surface. Pressurized air is good for removing material from tools, and cars should have debris removed from tires and undercarriages at a local carwash before leaving the infested area. Shoes can have the added step of Lysol® if one wants to be extra thorough.

Really though, the ideal situation is for people to avoid areas that are infested altogether during wet seasons so as to minimize organic material adherence.