..
MAJOR TREE PATHOGENS: SUDDEN OAK DEATH CAUSED BY PHYTOPHTHORA RAMORUM A new species of Phytophthora was discovered in Germany and the Netherlands from 1993 onwards, and was described formally in 2001 as Phytophthora ramorum. It causes damage to the branches of Rhododendron and, less often, Viburnum bushes, sometimes leading to death of these shrubs. This disease is now spreading on ornamental rhododendrons and viburnums in tree nurseries across Europe. Meanwhile, in the coastal fog belt of northern California and southern Oregon, a sudden death of oak trees was occurring, and this disease was also found to be caused by P. ramorum. It attacks several types of oak, but mainly the ‘live oaks’ and ‘tan oaks’ which are part of the natural vegetation of the coastal shrub/tree community. The fungus attacks the plants in two ways – either by producing lesions on the leaves and terminal shoots of a wide range of shrubs, leading to shoot dieback, or by producing cankers near the bases of the trees, where the cambium (which lies just beneath the bark and produces the annual rings of new wood) is progressively destroyed. The symptoms of this include cracking of the bark and seepage of dark, viscous sap from the bark near the base of the tree – a symptom termed gummosis (Fig. 1). In advanced stages of the disease the trees die suddenly and dramatically – hence the name, sudden oak death. However, although the death of the trees occurs suddenly it is preceded by a slow, progressive build-up of infection beneath the bark. This can often go unnoticed until the trees suddenly develop advanced symptoms.
Fig 1. Sudden oak death caused by Phytophthora ramorum. Top left: dead and dying trees of coast live oak (Quercus agrifolia) in the mixed oak community of the coastal fog belt of southwestern USA. Dead or dying trees are indicated. Top right: dark viscous sap is exuded from the bark at the base of a heavily infected coast live oak. Bottom left: removal of the bark reveals the presence of dark zone lines. Bottom centre: terminal die-back and wilting of the shoot tip of tan oak (Lithocarpus densiflorus) is one of the characteristic features of the disease. Bottom right: necrotic, spreading leaf spots are a further symptom of sudden oak death – in this case on leaves of Azalea or Rhododendron. [Images supplied by courtesy of Joseph O’Brien, USDA Forest Service, www.invasive.org; accessed 22 March 2004] There is a close parallel between P. infestans and P. ramorum, because both have been introduced into parts of the world where they did not occur before and both have caused widespread damage. Current evidence suggests that the European population of P. ramorum consists only of the A1 mating type, whereas the North American population is of the A2 mating type. This strongly indicates separate sources of origin of the two populations, but the natural geographical origin of the fungus remains unknown. Here there is a parallel with the spread of another aggressive Phytophthora species, P. cinnamomi, which attacks many plants across the world, but the geographical origin of this fungus is unknown. It is causing serious damage to the eucalypt vegetation of Australia. Whenever a fungus invades an area of natural vegetation it is likely that the fungus has been introduced from elsewhere, because plants in natural communities co-evolve with the resident pathogens. |
