Phytophthora ramorum causes sudden oak death, ramorum leaf blight, and ramorum dieback. Its primary threat is to forest ecosystems, along with regulatory costs and quarantines. Over 1 million trees have been lost in coastal California. P. ramorum was first observed in Germany and the Netherlands in 1993, and California in the mid 1990’s. It has been found in forests of California and Oregon; it has also been found in nurseries in California, Oregon, Washington, and British Columbia. A 2004 distribution of infected nursery stock resulted in detection and eradication in 21 states. In 2006, P. ramorum was detected in 62 nurseries in 11 states. As of 2011, 464 nurseries in 27 states have found the pathogen. There have been at least four global migrations, three in North America, and one in Europe. The economic impact of P. ramorum has been in the tens of millions of dollars in the United States. Losses have occurred in destroyed plant material, export markets, tourism effects, prevention costs, management and eradication costs, loss of wildlife habitat, increased fire risk, hydrologic changes, changes in forest structure, and lost aesthetic value.

P. ramorum was probably recently introduced to North America, due to its limited geographical distribution, relative to the wide distribution of host species. P. ramorum is spread by wind-blown rain, irrigation water, infested soil or potting mix, and movement by humans. Moist, cool, windy conditions help spread spores. The temperature range is 36-80°F, with optimum growth at 68°F. Temperature and humidity determine disease levels.

P. ramorum produces multiple reproductive structures. Sporangia are asexual structures that can germinate directly to infect hosts; they can also produce swimming zoospores, which swim through water films on leaves or in soil before infection. Infected tissue can give rise to new sporangia (and more cycles of infection), or tough chlamydospores to survive difficult environmental conditions (drought, winter, etc.) up to 650 days. Sexual reproduction can occur when different mating types are present in an area. This produces durable oospores, capable of surviving difficult conditions, as well as generating new genetic variability.

Hosts and Symptoms

P. ramorum has a large host range, with new hosts regularly identified. Newly emerging pathogens may expand their host range by migration to new areas with previously unknown susceptible hosts, introduction of new hosts to an infested area, or hybridization with related species. The host range of P. ramorum includes over 100 species, in over 40 genera. This includes many native and domesticated plants commonly found in Alaska, as well as in the nursery and greenhouse trade. There are ranges of susceptibility within species and sometimes cultivar.

Infection with P. ramorum can cause different symptoms in different hosts. In oaks and tanoaks, bleeding cankers can be formed, with red to black sap oozing on the bark surface; the host may be killed. Most of the hosts expected in Alaska (ex. rhododendron, lilac, and cranberry; see Table 1 below) display foliar lesions, leaf blight, twig dieback, or stem cankers. Foliar hosts are important for the survival and spread of P. ramorum. Some foliar hosts produce large numbers of spores, which are easily spread by rain splash or wind. Root infections are also possible, but may remain symptomless for many months.

In general, leaf symptoms are necrotic leaf lesions with irregular shaped borders. Typically, no fruiting bodies (spore-producing structures) are visible. Leaf infection can develop down petioles into twigs, but leaves often drop before lesions reach petioles. Since P. ramorum requires liquid water for zoospores to swim, lesions are often found where water accumulates: leaf tips, depressions, overlapping or cupped leaves, inner canopies, and leaves near or touching soil.

a hand holds two leaves with large brown spots

Phytophthora ramorum, Rhododendron sp. (upper leaf surface), Joseph OBrien, USDA Forest Service, Bugwood.org

a hand holds a leaf showing brown spots on underside

Phytophthora ramorum, Rhododendron sp. (bottom leaf surface), Joseph OBrien, USDA Forest Service, Bugwood.org

Phytophthora ramorum, Vaccinium sp., Joseph OBrien, USDA Forest Service, Bugwood.org

wet green leaves with subtle brown blotches

Phytophthora ramorum, lilac (upper leaf surface), Nina Shishkoff, USDA ARS

underside of wet green leaves with dark blotches

Phytophthora ramorum, lilac (bottom leaf surface), Nina Shishkoff, USDA ARS

**Table 1. Native and Domesticated Phytophthora ramorum Hosts in Alaska**
Common Name	*Genus species*	Symptoms
Japanese larch	Larix kaempferi	Leaf necrosis; dieback; canker with resin
Rhododendron	Rhododendron spp. (all species)	Leaf necrosis esp. leaf edges near petiole and tip; diffuse margins; premature drop; brown to black cankers on small branches
Lilac	Syringa vulgaris	Leaf lesions brown to black esp. leaf edges; death of leaf buds
High bush cranberry	Viburnum spp. (all species)	Leaf necrotic lesions at tip, edge, petiole; leaves may drop; stem cankers near base and small branches; distant leaves brown and stay attached
Kinnikinnik	Arctostaphylos uva-ursi	Leaf blight and twig dieback; stem and root rots
Holly	Ilex (multiple species)	Leaf blight
Ninebark	Physocarpus opulifolius	Leaf blight
Rose	Rosa (multiple species)	Necrotic leaf lesions, sometimes with chlorotic halo, spreading down petiole
Salmonberry	Rubus spectabilis	Leaves with dark marginal necrosis
Lingonberry, Low bush cranberry	Vaccinium vitis-idaea	Leaf lesions dark, margins diffuse
Veronica	Veronica spicata	Leaf necrosis
Fireweed	Chamerion angustifolium	Leaf lesions irregular shaped, may be vein-delimited
Northern willowherb	Epilobium ciliatum	Leaf lesions
Camellia	Camellia spp. (all species)	Water-soaked leaf lesions on tip or edge, diffuse margins or thick black zone lines, premature leaf drop
Andromeda	Pieris spp. (all species)	Similar to Rhododendron; branch tip dieback, premature leaf drop

Symptoms may resemble fungal infections, freeze damage, drought, fertilizer burn, chemical injury, sunburn, or root damage. Abiotic injuries often cause lesions with distinct margins, and may affect the entire plant. P. ramorum usually causes lesions with diffuse margins, and damages a few leaves or part of a plant. [see https://ppo.puyallup.wsu.edu/sod/education/mginfo/pr-symptoms/ ] Symptoms are not sufficient for diagnosis and require testing for determination.

A range of diagnostic tests are available, some are generic for any Phytophthora (i.e. species can’t be determined), these include onsite “dipstick” (lateral flow) devices, and Enzyme-Linked Immunosorbent Assay (ELISA). Species identity can be determined by growing a culture on special media, or conducting sensitive molecular testing, such as Polymerase Chain Reaction (PCR), or one of its variants.

P. ramorum is introduced to new areas in infected plant materials and infested soil. Windblown spores and infested water are responsible for more localized spread. Exclusion of P. ramorum requires large geographic barriers, due to windblown spores. Quarantines restrict movement of infected host material. A U.S. Federal Order in 2012 (DA-2012-14), addresses restrictions on importation of P. ramorum host plants for planting into the United States. This requires phytosanitary certificates or pest exclusion plans from the exporting country. Nursery inspections and sampling aid in containment. Eradication procedures are followed if the pathogen is detected, with trace-back and trace-forward to determine origin and destination.

Good sanitation practices (sterile media, disinfection of tools, treating irrigation water) and nursery/greenhouse Best Management Practices (BMPs) help with all pathogens. Some excellent references for BMPs are in the reference section. Of special note, it is recommended not to use systemic fungicides (mefenoxam, metalaxyl, phosphites or phosphonates) to manage Phytophthora spp. in a nursery or greenhouse. These chemicals slow growth, but do not kill Phytophthora. This masks symptoms and can contribute to spreading these pathogens.

While P. ramorum is found in the U.S., it has a limited distribution at present, and is not found in Alaska. This has cost millions of dollars and much effort to achieve. Introduction of the pathogen to Alaska would be very expensive to manage, and potentially devastating to our nurseries, greenhouses, and natural environment.

For a printable version of information on ramorum leaf blight view and download our factsheet here.