When I learned that the California black oak (Quercus kelloggii) in my backyard was among the species that sudden oak death (SOD) kills, I gazed dubiously out the window at the mulch under my tree and wondered if it was free of the pathogen that causes the disease. What about the new plants in my landscape? Could one of them carry SOD? And was it all right to wear my hiking boots in the garden? Fortunately, for an organism that was named only four years ago, Phytophthora ramorum has received lots of attention, and research adds daily to what we know. Yet my search for answers wasn’t easy. I didn’t realize when I began that a few questions would lead to so many others, and that answers would often be unavailable.
SOD was first noticed in Marin and Santa Cruz counties in 1995, when apparently healthy tanoaks (Lithocarpus densiflorus) became infected with a disease that seemed to progress rapidly and result in their death. Several species of true oaks (Quercus) were later also discovered to suffer high mortality rates; these were mainly California black oak, coast live oak (Q. agrifolia ), and, to a lesser extent, canyon live oak (Q. chrysolepis) and Shreve’s oak (Q. parvula var. shrevei). SOD has since been confirmed in fourteen California counties and an isolated pocket in southwestern Oregon. In the United States, outside of the Pacific Coast, the pathogen has not yet been found in the wild; however, any tree in the red oak group (recognized by their pointed leaf lobes) is vulnerable to a terminal form of the disease. As information accumulates, numerous other plants, both native and exotic, have been found to also carry the pathogen, in a less dramatically visible form, as foliar hosts.
The then-unknown species of Phytophthora that causes SOD was isolated in June of 2000 by Dr David Rizzo of UC Davis. Dr Matteo Garbelotto of UC Berkeley did the DNA sequencing, which showed that this was something new. In December of 2000, thanks to a tip from European colleague Clive Brasier, these principal SOD researchers realized that it was the same as an unnamed Phytophthora first noticed in 1993 in nursery stock in Europe (and recently observed in forests in the UK and the Netherlands). The disease caused by Phytophthora ramorum, as it was named in 2001, has hit hardest in Monterey, Santa Cruz, Marin, and Sonoma counties; in August of 2004, it was identified in oaks in San Francisco’s Golden Gate Park. It is especially common in the urban/wildland interface, where human activities are having an impact on its spread. Scientists sometimes compare the disease to the chestnut blight that ravaged the forests of the eastern states (SOD also appears to be of exotic origin), and, although the long-term ecological consequences are unknown, a look at the potential devastation emphasizes the need for concern. For confirmation, visit Sonoma County’s Jack London State Park, Marin County’s Bolinas Ridge Road between Bolinas and Pine Lake, or the canyons south of Monterey County’s Pfeiffer Big Sur State Park.
Phytothphora ramorum is related to the pathogen that caused the Irish potato famine. Canker hosts (oaks and tanoaks) exhibit symptoms such as bleeding, trunk cankers, and sudden browning of the canopy as death finally ensues. Foliar hosts manifest the disease as leaf lesions and, sometimes, twig infections, and can produce massive quantities of the spores that spread the disease. (An article by Dr Robert Raabe in Pacific Horticulture in April ’02 provides more details on SOD pathology.) In native forests, California bay (Umbellularia californica) is the most common host and a prolific producer of spores. In nurseries, the most common hosts are camellia, viburnum, and rhododendron (both native and exotic species). Over twenty of our most beloved and familiar native plants have been confirmed as susceptible to the pathogen. A total of sixty-one plants are now regulated (numbers are sure to climb); the list includes species, hybrids, varieties, and in two cases (rhododendron and camellia), entire genera. SOD has been isolated in thirty-five genera in nineteen families, including conifers, hardwoods, perennial herbs, monocots, and a fern. Oaks, it turns out, are an epidemiological dead-end, meaning that while they get the disease, they do not transmit it to other plants (P. ramorum does not make reproductive spores on oaks). Tanoak is the only species known to get both lethal trunk infections and to act as a foliar host. Much of the country’s forests are at risk. How can we contain this disease?
Containing SOD Spread
The principle mechanism of SOD spread is windblown rain; additional natural factors include transport down streams and, possibly, by animals. It is highly probable that soil transport by hikers, bicycles, and motor vehicles contributes to distributing the pathogen, especially during the rainy season. The natural spread of SOD is rather slow, but it may leapfrog to new areas when gardeners plant infected nursery stock. As of April, 2004, the federal government barred California nurseries from shipping regulated plants out of the state unless they have been inspected and declared free of the disease. Some states and countries apply even more stringent regulations.
The time lag between first suspecting a new host species, the studies needed for confirmation, and the implementation of regulations, means that a plant may be an innocent purchase today but could, in fact, be carrying SOD. Phytophthora ramorum has been detected at 140 nurseries in nineteen states (thirteen states are known to have received plants from a single California nursery). If you are in a county without SOD, and live in the urban/wildland interface, the best disease protection is to purchase locally grown plants from a nursery that does not carry stock from out of the county. Currently, there is no restriction on the movement of plants within the borders of regulated counties (those confirmed to have SOD in the wild). A regional grower could inadvertently sell infected plants and, since the distribution of the disease is spotty, introduce the pathogen to an area where it does not yet naturally occur. The risk, however, may be still higher at large retail nurseries that carry stock from many sources. As several researchers noted, encouraging nurseries to sell locally grown plants, and consumers to buy them, would slow the spread of any number of destructive plant pathogens and pests.
The pathogen can also be spread through compost and organic soil amendments. The high temperatures that commercial operations can achieve have been confirmed to produce a pathogen-free compost. Chipped garden mulch, on the other hand, does not reach the same high temperatures and, since the origin is variable, cannot be considered pathogen-free with complete confidence. In counties known to have SOD, if you have a heritage oak or garden in the urban/wildland interface, where the disease is especially prevalent, avoid commercial garden mulches of unknown origin.
There is also the problem of the safe disposal of infected yard waste. In most counties infested with SOD, there is nowhere for the private citizen to dispose of infected plant materials, and they must be kept on site. Marin and Santa Cruz counties have pilot disposal programs, but these are only available to certified arborists and landscape professionals.
One of the difficulties in detecting SOD is that the symptoms can be hard to recognize and to differentiate from other diseases that can cause similar symptoms. In addition, trees may have the disease for months before exhibiting any symptoms. The symptoms are also different on different plants. Although SOD may be strongly indicated, the presence of the pathogen can only be confirmed by lab tests. If you suspect that you have a tree with SOD, and are not in a known infested area, contact the local county Agricultural Commissioner, or Cooperative Extension agent.
To familiarize yourself with the symptoms of SOD, visit the website (www.suddenoakdeath.org) sponsored by the nonprofit California Oak Mortality Task Force (COMTF), a coalition of governmental agencies, research institutes, and private interests that coordinates efforts to combat the disease. The website has an abundance of excellent photographs of infected plants, in addition to a wealth of information on SOD for both the professional and private citizen. The updated Homeowner’s Guide to Sudden Oak Death, and the nursery guides for California and Oregon are particularly useful. An Oak Mapper feature uses state-of-the-art GIS technology that allows a variety of maps to be overlaid. Type in an address to see if diseased trees have been reported in the area or to report potential new locations of the disease.
Ultimately, there are no simple guidelines for gardeners. What you choose to plant and where you purchase plants and landscape products will depend on your individual situation. In an urban garden, far from any oak woodlands, the risk posed by infected foliar hosts or mulch is far less than that of a garden within an oak woodland. If you live in the urban/wildland interface, keep planting native species in your garden, but do not plant potential hosts anywhere near vulnerable trees. Avoid planting combinations of hosts, for example rhododendrons, camellias, toyons (Heteromeles arbutifolia), and oaks. If you live adjacent to an infested forest, do not plant red oaks, tanoaks, or madrone (Arbutus menziezii) unless you are willing to risk losing them (mature madrones survive, but saplings can die from SOD), and do not plant California bay near any oak forests.
To slow the spread of the disease in an area with SOD, avoid irrigating by overhead sprinklers, and use drip irrigation instead. Do not irrigate beneath oak trees at all, and regularly monitor their health. Know your local growers, and always check any plants you purchase carefully for signs of pests and diseases.
Unfortunately, there is no treatment available for the home gardener to use to prevent a tree with SOD from dying. For valuable landscape trees in vulnerable situations, there is a preventative treatment, Agri-Fos, that can only be applied by licensed applicators (trained arborists are listed on the COMTF website).
Do not move potential pathogen carriers, such as plants, soil, and firewood, from infested to healthy areas. It is not clear yet how long the pathogen can survive in soil (studies are underway), but in the meantime, always wash muddy boots, tires (and wheel wells and undercarriages), hooves and paws, before leaving an infested area. Most sources recommend that boot soles be treated with a mild disinfectant such as Lysol, a ten percent bleach solution, or a seventy-five percent ethanol solution. Sonoma State University’s Fairfield Osborn Preserve requires that hiking shoes be cleaned thoroughly of all soil particles during the rainy season, and then sprayed with a seventy-five percent solution of ethanol. Stay on designated trails in the woods, and, whenever possible, stay off of muddy trails in areas known to have SOD. Sterilize tools after pruning or cultivating around potential host plants. Choose carefully any wood mulch or compost products
The recent announcement that the genome of Phytophthora ramorum has been successfully mapped has generated a lot of excitement in the scientific community, as it is a major step toward developing improved methods for early, accurate, and inexpensive diagnosis and toward eventually finding a cure for SOD. In fact, notes Dr Garbelotto, one has only
to look at the short amount of time betweenthe discovery of the organism responsible for SOD and the enormous (and expensive) task of deciphering the genetics, to see that the disease is being taken seriously by the scientific community. Some of the questions that scientists hope to answer in the future are how long the pathogen survives in soil, potting media, and irrigation water, and how land-use practices and other factors affect the spread of the disease.
When planning additions to my landscape, I now refer to the list of host plants on the COMTF website. I monitor my oak tree for signs of disease and pay attention to the source of the landscape products and plants I purchase. Can I wear my hiking boots in the garden? Until studies conclusively show that it is completely safe, I will reserve my hiking boots for recreational use. As I walk along my favorite trail, I hope the search for a cure will be successful in time to save our magnificent woodlands.
I wish to acknowledge the contributions of Dr Matteo Garbelotto, UC Berkeley; Dr David Rizzo, UC Davis; Katie Palmieri, public information officer, California Oak Mortality Task Force; and the SOD research team at Sonoma State University (Drs Meentemeyer, Cushman, Rank, and Whitkus). All of these dedicated individuals gave generously of their time in the preparation of this article.