Ramorum Blight, Part 1

When Plants Get Sick

Just like people, plants can get sick too.  And just like COVID-19 and humans, there are high-impact diseases of plants. Bloedel Reserve has been dealing with one such disease since 2015. Commonly known as ramorum blight, it is caused by a fungus-like organism called Phytophthora ramorum.

Unequivocal identification of ramorum blight from visual symptoms alone is impossible. Symptoms include dark, water-soaked dead spots on the leaves. The spots often form a wedge shape, starting at the leaf tip or the leaf base and can be present on the upper and lower leaf surfaces. The problem is that many other species of the Phytophthora fungus produce the same symptoms. To confirm that P. ramorum is causing the symptoms, leaf samples must be sent to a properly-equipped plant diagnostic laboratory.

Ramorum in the Rhodies

Ramorum blight was first discovered at Bloedel Reserve in early 2015 when our observant Rhododendron Glen gardener found a sick Pieris near the waterfall overlook. The plant’s symptoms were bad enough to warrant submitting a leaf sample to the Washington State University (WSU) diagnostic laboratory. Oddly, the leaf lesions were atypical for Phytophthora, demonstrating that not every plant will react in exactly the same way to this disease.

A few days later, an email from WSU arrived. It came with a positive diagnosis, as well as an explanation of what was about to happen. Ramorum blight is of serious concern to the state and federal Departments of Agriculture. Because it is a federally regulated plant disease, any discovery of ramorum blight is followed by a visit from the United States Department of Agriculture (USDA).

More Dangers Lurking

Ironically, the impact at Bloedel Reserve from ramorum blight has been miniscule. The Reserve only lost a couple of Pieris specimens. Since the impact was so small, it would normally elicit only a small response from the staff. So why is the federal government so concerned about this disease?

As it happens, the organism P. ramorum that causes ramorum blight also causes another disease that many people may recognize: sudden oak death. This is the disease that was responsible for decimating millions of oak, live oak, and tanoak trees in California since it was first spotted in the mid-1990s. The disease continues to be a problem in California, Oregon, and elsewhere. Here in Washington state, our forests don’t appear to have quite the potential for the incredible destruction seen in California and southern Oregon. That said, native larches may be at risk if the disease takes hold in our forests.

An Unwelcome Traveler

Another reason the USDA considers ramorum blight to be a serious problem comes from the risk to and response from other nations. Other countries see what has happened in California and want to make sure that this disease does not arrive on their shores where it can cause damage to their natural resources. If ramorum blight spreads into our forests and wilderness areas, other countries may ban U.S. state or national exports. Bans like that could cost our economy millions of dollars.

A Partnership for All

That is why, although the direct effects of ramorum blight have been relatively minor at Bloedel Reserve, we are partnering with the USDA, WSDA, and WSU for the greater good of our country’s forests and wilderness. Look for Part 2 of this story to learn what Bloedel is doing to mitigate ramorum blight on its grounds.