Ramorum Blight, Part 2

In Ramorum Blight, Part One we described the presence of the fungus-like pathogen Phytophthora ramorum (aka ramorum blight) at Bloedel Reserve — what it is and why it is a federally regulated plant disease. In Part Two, we’ll explore how the staff at Bloedel Reserve is working to eliminate this invasive disease.

Federal regulations currently require that the Bloedel staff take specific steps to control this disease on the grounds. These regulations mandate that plants testing positive for P. ramorum, along with any plants located within approximately two meters of the infected plants, be destroyed by burning them to ash. This destruction mandate, while necessary, could be potentially devastating to the Bloedel Reserve’s plant collections.

If the disease were to spread on the grounds, more plantings would have to be removed. This means that, although ramorum blight itself is almost inconsequential in its direct effects at Bloedel Reserve, the federally mandated removal of infected plants could result in significant damage to the grounds. In order to protect our collections and do our civic duty to help prevent the spread of this potentially devastating disease, the staff at the Bloedel Reserve is taking a number of steps to mitigate ramorum blight.

WSU Research Provides Valuable Advice

Early on, Washington State University’s research showed that P. ramorum does not go deeper into soil than about six inches. Proper steaming can help remove the organism from the infected soil where diseased plants have grown. Otherwise, the organism can potentially persist in the soil indefinitely. Bloedel Reserve steams the soil of infected areas, then covers them with several inches of well-rotted compost.

While steaming kills the disease, it also kills beneficial microorganisms that are essential to healthy soils. The added compost re-introduces these microbes to repopulate the soil. At the same time the compost is added, we also add a beneficial soil fungus called Trichoderma. Certain species of Trichoderma can suppress ramorum blight. WSU did studies at Bloedel Reserve with this beneficial fungus and determined that it acts as another protective measure by staying in the soil where it is introduced.

Physical Restrictions Also Help

Physical control measures also help slow the spread of ramorum blight at the Reserve. Trails in some areas have been re-graded so water will not flow across the trails but rather flow along the trail edges towards drains. This way water running off infected areas won’t get onto trails where people can spread the disease simply by walking through the water. The Bloedel staff also has come up with a way to keep people on the trails. Although most people know to stay on trails, there are always some guests who walk through garden beds to pick flowers or get a better photo. People stepping into an infected bed can spread ramorum blight by carrying it on their shoes and clothes.

The beautiful, rustic wattle fences that have graced the grounds since 2016 are Bloedel’s answer to that problem. Even though the fences are relatively short, they do a good job of deterring people from leaving the trails.

There are also “Stay on the trail” signs posted near the infected areas. They keep people from walking into beds when the wattle fences might not be enough of a deterrent.

Keeping It Clean

Sanitation is also key. Just like with human diseases, plant diseases can be spread through poor sanitation. Sanitation measures on the Reserve include:

• Cleaning and disinfecting garden tools at the end of every work day
• Disposing of garden debris from areas bordering known infected sites instead of putting it in compost
• Bagging debris from within known infected sites in order to burn it to ash at a later date
• Defining infected sites as “hot zones.” Staff wear rubber boots within these areas and disinfect tools and boots in a 10% bleach foot bath upon exiting the site.

Replanting and Restoring the Grounds

What happens to the sites where ramorum blight has been found? After the soil has been steamed and then top dressed with beneficial Trichoderma spores and well-rotted compost, it can be replanted. Because the treated site had ramorum blight, newly chosen plants must not be known hosts of the disease. The USDA maintains a list of more than 200 known host or associated host species. The Bloedel horticulture team uses that host list as a guide for what not to plant into treated sites. To be safe, we also avoid species and genera that are not on the list, but are closely related to plants that are. For example, not every species of Oregon Grape (Mahonia) is on the list. But other members of that particular genus are likely to be susceptible to ramorum blight because of their genetic relatedness. Ideally, any species in the same botanical family as host species on the list should be avoided. That USDA list can be found here.

Unfortunately because P. ramorum is microscopic and hard to detect, it is impossible to be sure that it is ever completely eradicated. Going forward, Bloedel Reserve will keep up its control measures to protect its historic and valuable collections. If you have questions about ramorum blight or just want to know more, please contact Bloedel Reserve staff member 20********@bl************.org“>Darren Strenge.