Farm Forestry New Zealand
PESTS AND DISEASES OF FORESTRY IN NEW ZEALAND
Sudden oak death – new disease identified in United States
From Biosecurity Issue 29, August 2001.
A new disease of oaks (Quercus spp.) has recently been identified in the United States. While this disease could become established here, the immediate threat to New Zealand oaks is low. Known as sudden oak death, the disease causes crown dieback, stem bark lesions, basal cankers, and rapid mortality in tanoak (Lithocarpus densiflorus), coast live oak (Q. agrifolia), Californian black oak (Q. kelloggii) and Shreve oak (Q. parvula var. shrevie) found along the coastal belt in California.
Fungus implicated
An unknown species of the fungal genus Phytophthora has been isolated from bark lesions in trees showing disease symptoms and is believed to be the principle disease-causing agent. This pathogen has also been found to infect rhododendrons (Rhododendron spp.), huckleberry bushes (Vaccinium ovatum) and Pacific madrone (Arbutus menziesii) next to infested oaks in California, and rhododendrons and a Viburnum species (honeysuckle family) in Germany and the Netherlands. To date, sudden oak death has not been found in other oak species, although the susceptibility of other Quercus spp. has not been tested. The lack of any recorded impact of this disease on oaks in Europe suggests that European oaks may not be susceptible. Susceptible oaks not common in New Zealand Oaks are popular throughout New Zealand and can be found in parks, streets, private gardens, on farms as shade trees and occasionally in woodlots (Salmon, JT 1999). While there are almost 800 species of oaks in New Zealand, most species (including the oaks known to be susceptible to sudden oak death) are not widely planted. Examples of oak species commonly grown in New Zealand include the common or English oak (Quercus robur), the first oak to be planted in New Zealand in 1824 at Paihia, the sessile oak (Q. petraea), the red oak (Q. rubra), the pin oak (Q. palustris), and the turkey oak (Q. cerris).
Sorensen et.al. (2001) hypothesised that the Phytophthora implicated in sudden oak death co-evolved with Rhododendron species in the Oriental/Himalayan region. They believe the fungus was first transported to Europe and then to California (possibly via the Pacific Northwest), through the movement of Rhododendron material. The fungus was first noticed in Europe in 1993 and, subsequently, in California in 1995. There are no known control measures for sudden oak death.
Phytophthora is a cool temperate organism, with optimum growth at 20°C. Cultures of the fungus produce numerous sporangia. The pathogen is typically in phloem tissues of infected plants, but commonly extends to the outer portion of the xylem. To date, the pathogen has not been isolated from below-ground host tissues or from soil.
Mechanism of spread not understood
It is not yet clearly understood how sudden oak death spreads. One theory is that it spreads through the airborne movement of spores, as the sporangia are deciduous; however this has not been tested through research. Nursery stock, infested wood with attached bark, and cut flowers and branches of host plants are the most likely means of long-distance transport of this fungus. It is not known if the pathogen is soil-borne. Oospores and chlamydospores of other Phytophthora species are long-lived and can survive in soil and dead host material under adverse conditions.
MAF considers that, while this disease could become established here, the immediate threat to New Zealand oaks is low. MAF will continue to monitor the progression of the disease in California and Europe and, if appropriate, restrict the extent to which host material is imported into New Zealand from these areas.
Dr Mike Ormsby, National Adviser (Import Health Standards), MAF Forest Biosecurity, Ministry of Agriculture and Forestry
References
Sudden oak death Phytophthora sp. pest risk assessment (2001). USDA Forest Service. www.exoticforestpests.org
Sorensen, JT, Hrusa, F & Tyler, T (2001). Origin of the sudden oak death (SOD) pathogen and its potential impact: a working hypothesis. California Plant Pest & Disease Report, 19 (3-6), June-December 2000. pp 49-57.
Salmon, JT (1999). The Trees in New Zealand. Exotic Trees. The Broadleaves. Reed Books. pp 372.
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