Sudden Oak Death - Hosts and Symptoms

Hosts and Symptoms

It was first discovered in California in 1995 when large numbers of tanoaks (Lithocarpus densiflorus) died mysteriously, and was described as a new species of Phytophthora in 2000. It has subsequently been found in many other areas including Britain, Germany, and some other U.S. states, either accidentally introduced in nursery stock, or already present undetected.

In tanoaks, the disease may be recognized by wilting new shoots, older leaves becoming pale green, and after a period of two to three weeks, foliage turns brown while clinging to the branches. Dark brown sap may stain the lower trunk's bark. Bark may split and exude gum, with visible discoloration. After the tree dies back, suckers will 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, may also be present on the bark. Leaf death may occur 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 USA that nearly all woody plants in some Californian forests were susceptible to P. ramorum. including 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's Oak (Quercus parvula v. shrevei). 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 but hikers have observed dead Douglas-firs with massive quantities of red frass surrounding their base. Redwoods exhibit needle discoloration and cankers on small branches, with purple lesions on sprouts that may lead to sprout mortality.

In late 2009 the disease was first found in Japanese Larch trees, in the English counties of Devon, Cornwall and Somerset. In August 2010 disease was found in Japanese Larch trees, in counties Waterford and Tipperary in Ireland.