Phytophthora ramorum is the oomycete known to cause the disease sudden oak death (SOD). The disease kills oak and other species of trees and has had devastating effects on the oak populations in California and Oregon, as well as being present in Europe. Symptoms include bleeding cankers on the tree's trunk and dieback of the foliage, in many cases leading to the death of the tree.
P. ramorum also infects a great number of other plant species, significantly woody ornamentals such as Rhododendron, Viburnum, and Pieris, causing foliar symptoms known as ramorum dieback or ramorum blight. Such plants can act as a source of inoculum for new infections, with the pathogen producing spores that can be transmitted by rainsplash and rainwater.
P. ramorum was first reported in 1995, and the origins of the pathogen are still unclear, but most evidence suggests it was introduced as an exotic species to Europe and North America in separate events. Very few control mechanisms exist for the disease, and they rely upon early detection and proper disposal of infected plant material.
Presence The disease is known to exist in California's coastal region between Big Sur (in Monterey County) and Del Norte County. It is confirmed to exist in all coastal counties in this range, as well as in all immediately inland counties from Santa Clara County north to Trinity County. It has not been found east of the California Coast Ranges, however. It was reported in Curry County, Oregon, just north of the California stateline, in 2001. Sonoma County has been hit hardest, having more than twice the area of new mortality of any other county in California.
About the same time, a similar disease in continental Europe and the UK was also identified as Phytophthora ramorum.
Hosts and symptoms
In North America
It was first discovered in California in 1995 when large numbers of tanoaks (Notholithocarpus densiflorus) died mysteriously, and was described as a new species of Phytophthora in 2000. It has subsequently been found in many other areas, including some other U.S. states, Britain, and Germany, either accidentally introduced in nursery stock as a pathogen belonging to ornamental rhododendron, or already present undetected.
In tanoaks, the disease is recognized by wilting new shoots, older leaves becoming pale green, and after a period of two to three weeks, foliage turning brown while clinging to the branches. Dark brown sap stains the lower trunk's bark. Bark often splits and exudes gum, with visible discoloration. After the tree dies back, suckers try to sprout the next year, but their tips soon bend and die. Ambrosia beetles (Monarthrum scutellare) will most likely infest a dying tree during midsummer, producing piles of fine white dust near tiny holes. Later, bark beetles (Pseudopityophthorus pubipennis) produce fine, red boring dust. Small black domes, the fruiting bodies of the Hypoxylon fungus, often are present on the bark. Leaf death occurs more than a year after the initial infection and months after the tree has been girdled by beetles.
In coast live oaks and Californian black oaks, the first symptom is a burgundy-red to tar-black thick sap bleeding from the bark surface. These are often referred to as bleeding cankers.
In addition to oaks, many other forest species may be hosts for the disease; in fact, it was observed in the United States that nearly all woody plants in some Californian forests were susceptible to P. ramorum. Vulnerable species include rhododendron, madrone (Arbutus menziesii), evergreen huckleberry (Vaccinium ovatum), California bay laurel (Umbellularia californica), buckeye (Aesculus californica), bigleaf maple (Acer macrophyllum), toyon (Heteromeles arbutifolia), manzanita (Arctostaphylos spp.), coast redwood (Sequoia sempervirens), Douglas fir (Pseudotsuga menziesii), coffeeberry (Rhamnus californica), honeysuckle (Lonicera hispidula), and Shreve oak (Quercus parvula). P. ramorum more commonly causes a less severe disease known as ramorum dieback/leaf blight on these hosts. Characteristic symptoms are dark spots on foliage and in some hosts the dieback of the stems and twigs. The disease is capable of killing some hosts, such as rhododendron, but most survive. Disease progression on these species is not well documented. Redwoods symptoms show purple lesions on sprouts and needle discoloration and cankers on small branches, which can lead to sprout mortality.
The primary symptom of P. ramorum on Rhododendron are leaf spots that expand into lesions. The lesions pierce through the plant tissue so that the spots are the same on both the top and bottom of the leaf. The lesions are usually triangular in shape and extend along the midvein. They can appear anywhere water collects on the surface of the leaf. Leaf spots have diffused margins and can appear water soaked. In severe cases, the entire host plant can die.
In Europe
In Europe, ramorum blight was first observed on Rhododendron and Viburnum in the early 1990s, where it was initially found mainly on container-grown plants in nurseries. The principal symptoms were leaf and twig blight. By 2007, it had spread throughout nurseries and retail centers in 16 European countries, and had been detected in gardens, parks, and woodlands in at least eight countries. It has not caused significant harm to European oak species.
In 2009, the pathogen was found to be infecting and killing large numbers of Japanese larch trees (Larix kaempferi) in the United Kingdom at sites in the English counties of Somerset, Devon, and Cornwall. It was the first time in the world that Phytophthora ramorum had been found infecting this species. Since then, it has also been found extensively in larch plantations in Wales and in southwest Scotland, which led to harvesting of larch on hundreds of acres/hectares. The UK Forestry Commission noted that eradication of the disease would not be possible, and instead adopted a strategy of containing the disease to reduce its spread. Symptoms of the disease on larch trees include dieback of the tree's crown and branches, and a distinctive yellowing or ginger colour beneath the bark. In August 2010, the disease was found in Japanese larch trees in counties Waterford and Tipperary in Ireland. It had spread to Japanese larch plantations across the south of the country by February 2014. Coillte, who owned twenty forests where the disease was present, felled 16,000 trees in one of its forests, having already felled 150 hectares to contain the disease. In 2023 the disease was found to be infecting larch trees at Wyming Brook, Sheffield, with plans to fell over 1,000 trees to contain the spread of the infection.
The closely related Phytophthora kernoviae causes similar symptoms to P. ramorum, but infects the European beech (Fagus sylvatica).
Disease cycle The primary inoculum (sporangia) of P. ramorum. develop on the leaves of the primary host, which include tree species like the California bay laurel, a large hardwood tree that grows on the pacific west coast. These spores are then carried by rain and air currents to the leaves of the new bole canker host, which include broadleaf trees like tanoaks, where they begin to develop. The secondary inoculum infects the inner bark and sapwood, resulting in bleeding cankers on the bark of the new host, which are exacerbated by infected fallen leaves and rain splashing the understory of the canker host which can both serve as sources of inoculum. After the plant matter it infects dies and decomposes, P. ramorum is transferred to the soil by rainwater, where the final part of its cycle (soil phase) is poorly understood by scientists. However, it is suspected that chlamydospores play a role in the long-term survival of the pathogen, yet the triggers for germination are not known. Additionally, the spores of P. ramorum only seem to propagate successfully in a temperature range between , which is useful to know for protecting nurseries and identifying potential transfer windows for the disease.
Transmission P. ramorum produces both resting spores (chlamydospores) and zoospores, which have flagella enabling swimming. P. ramorum is spread by air; one of the major mechanisms of dispersal is rainwater splashing spores onto other susceptible plants, and into watercourses to be carried for greater distances. Chlamydospores can withstand harsh conditions and are able to overwinter. The pathogen will take advantage of wounding, but it is not necessary for infection to occur.Anon, Phytophthora ramorum and Phytophthora kernoviae: Key findings from research, DEFRA, Editor. 2005, DEFRA.
As mentioned above, P. ramorum does not kill every plant that can be used as a host, and these plants are most important in the epidemiology of the disease as they act as sources of inoculum. In California, California bay laurel (Umbellularia californica) seems to be the main source of inoculum. Green waste, such as leaf litter and tree stumps, are also capable of supporting P. ramorum as a saprotroph and acting as a source of inoculum. Because P. ramorum is able to infect many ornamental plants, it can be transmitted by ornamental plant movement.
In nursery settings, P. ramorum is dispersed mostly between adjacent, touching plants and exposure to infested surface water. Long-distance dispersal is enabled by shipments of infected nursery plants, which is likely the cause for introduction into Europe and North America.
Hikers, mountain bikers, horseback riders, and other people engaged in various outdoor activities may also unwittingly move the pathogen into areas where it was not previously present. Those travelling in an area known to be infested with SOD can help prevent the spread of the disease by cleaning their (and their animals') feet, tires, tools, camping equipment, etc. before returning home or entering another uninfected area, especially if they have been in muddy soil. Additionally, the movement of firewood could introduce sudden oak death to otherwise uninfected areas. Both homeowners and travelers are advised to buy and burn local firewood.
Transmission of P. ramorum may also occur through movement of propagules on the exterior of some animals, such as migratory birds, snails, and slugs.
The two mating types