Sudden Oak Death: An editorial about the current situation, future prospects and treatment

May 25, 2004 - 12:24

The current situation with sudden oak death (SOD), ramorum blight, reminds me of one about 10 years ago when Benlate was accused of causing every kind of plant damage possible. The result of that inappropriate reaction was the loss of all Dupont products in the ornamentals industry, billions of dollars going to attorneys and a few growers, and the weakening of our industry.

I was employed at the University of Florida at that time and was obligated to refrain from commenting on what I felt was the lack of scientific input into the situation. As the proud and harried president of a private corporation I am no longer bound by political goals. I have come to the conclusion that a little unemotional interpretation of the current SOD research information might be helpful, as the amount of SOD information available on the Web has been increasing by leaps and bounds on a weekly and sometimes daily basis.

When Monrovia Nursery in Azusa, Calif., inadvertently shipped infected camellias into many nurseries both inside and outside California's border every agency with any power to act did so. While some of these actions were reasonable and within the scope of scientific recommendations, others were clearly the result of hysteria and the inability to recognize the situation without the overtones of emotion.

Several years of concentrated efforts and quite an impressive sum of money have been dedicated to learn everything we can about the pathogen, Phytophthora ramorum. I am going to give my interpretation of some key questions about recent actions, and it will be based on this research in the hope that a few of the misinterpretations I hear and see in print will be laid to rest or at least challenged.

How did SOD happen?

It is too bad we don't have CSI (crime scene investigators) that can go back 15-20 years and trace the introduction and path that SOD took. At the ultimate risk of being wrong, wrong, wrong I present the following possible scenario.

It seems likely that a collector/grower of azaleas or rhododendrons saw a new plant they liked and brought it into the United States without following our established quarantine procedures. This does happen all of the time, unfortunately. Then the disease, looking like many other Phytophthora diseases in a nursery may have simply gone under the radar for many years. The nurseryman, in all likelihood, saw the symptoms on his plants but even if he sent a sample in for identification, it would have been diagnosed as Phytophthora on azalea (not an uncommon occurrence). Treatment with fungicides currently in use would have kept the disease from being particularly important, and no alarm bells would have sounded.

How did the pathogen escape?

Nurseries are often surrounded by native vegetation, since agricultural production in the middle of a city is not usually desirable or feasible. Some nurserymen actually enjoy gardening and landscaping around production areas. It is further possible that one or more infected plants were placed in close proximity to a Quercus or other susceptible native plant.

I think this is where nature became important. There was a long drought in California during the late 80s and early 90s. When trees are stressed by drought for long enough their natural resistance to pests like fungi and insects is weakened. This was followed by a year of horrendous rainfall (El nino, winter of 1996-1997). There is nothing a Phytophthora disease likes better than lots of water. The cat is out of the bag! Is this what really happened? We will never know for sure, but it is a possible scenario.

Is this the disease killing trees?

Yes. There is a long history of large trees being killed with other types of Phytophthora. In the 1960s John Mircetich (a PhD plant pathologist at USDA) published well-documented studies on another Phytophthora disease -- this time on cherry trees. We further find that other Phytophthoras cause crown rot on large walnuts, citrus and many other fruit trees. The root rot of avocado is caused by yet another Phytophthora, P. cinnamomi. This pathogen has been studied for decades and was the focus of many researchers at the University of California at Riverside. One of those researchers, Michael Coffey, has been working on Phytophthora since the early 1980s and certainly knows what he is doing when it comes to interpreting the current SOD outbreak.

Is the host range unusual?

No. There has been quite a bit of uproar over the ever-expanding list of hosts for P. ramorum. Some people have implied that this is a new and unusual ability for a Phytophthora species. Nothing could be further from the truth. Phytophthora cinnamomi has a host list of over 900 plants (including azalea and rhododendron). You may be familiar with Phytophthora parasitica (sometimes called P. nicotianae) that is known to attack many greenhouse crops including African violet, gloxinia, vinca, petunia, schefflera, zebra plant and spathiphyllum.

Is it unusual for Phytophthora to be soil-borne and cause disease of stems and leaves?

No. This is the standard for many Phytophthora species. Indeed, it is more unusual for a Phytophthora sp. to cause only a root disease than for one to attack the stem (or trunk) and leaves of a plant.

Can we completely control P. ramorum with fungicides?

Unlikely. I have been working with fungicides for 25 years, and I have never seen a product (even methyl bromide) that is 100 percent effective in killing anything. The desire for a guarantee is human enough but not scientifically sound. I am not suggesting we abandon treatment since we cannot attain 100 percent success. I should think using a combination of cultural, chemical and even regulatory efforts would be the only viable approach.

The best fungicide products have been researched for the foliar phase of SOD on azaleas and rhododendrons by University of California scientists Steve Tjosvold and Steve Koike. They reported the following as most effective: dimethomorph, fenamidone, mefenoxam and pyraclostrobin. Dimethomorph is registered now in California and elsewhere as Stature by SePRO Corporation. Fenamidone is under joint development by Bayer and Olympic Horticultural Products. Mefenoxam is Subdue Maxx (Syngenta Professional Products), and pyraclostrobin is Insignia (under development by BASF).

Nursery people producing the known hosts of P. ramorum might benefit from using a rotation of at least two of these products. The goal here is to keep the disease from having any chance to attack your plants, spread in your nursery or, heaven forbid, be spread to other growers. Certainly it is obvious this will be expensive. Don't pinch pennies here since this is your most important line of defense. Make the applications thorough and routine.

How can we stop this disease?

We can certainly stop it from damaging our container ornamentals in nurseries. Keeping it from escaping into the wild will not be accomplished by quarantine alone and may not be possible in the long run. A few short years ago we tried to stop daylily rust from spreading throughout the United States using quarantine steps. It did not work. I think scientists have a tendency to paint a bleaker picture than most people would like so I will temper this by saying that we never get anywhere by giving up before we start.

Learn everything you can about SOD. After you use the best methods available to prevent the disease in your nursery, call your industry representatives and your elected officials. Make sure they know what problems you face and that they understand the science behind the recommendations. After that, consider contributing to research funds like California Association of Nurseries and Garden Centers who have identified SOD as a special project. Finally, if you are a believer in anything, pray for weather that Phytophthora does not like.

To find out more about SOD, I suggest the following Web sites:

http://www.oda.state.or.us/plant/ppd/path/SOD

http://edis.ifas.ufl.edu

About The Author

A.R. Chase is pathologist and president of Chase Research Gardens, the industry's premier independent disease and chemical testing laboratory. She can be reached by phone at (530) 620-1624 or E-mail at mtaukum@aol.com

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