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 dis-
ease 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
cankersnear 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. 14.20).
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.
There is a close parallel between P. infestansand
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.
ramorumconsists 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.FUNGI AS PLANT PATHOGENS 301
Fig. 14.20Sudden oak death caused by Phytophthora ramorum. (a) 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.
(b) Dark viscous sap is exuded from the bark at the base of a heavily infected coast live oak. (c) Removal of the bark
reveals the presence of dark zone lines. (d) Terminal die-back and wilting of the shoot tip of tan oak (Lithocarpus densiflorus)
is one of the characteristic features of the disease. (e) Necrotic, spreading leaf spots are a further symptom of sudden
oak death – in this case on leaves of Azaleaor Rhododendron. (Images courtesy of Joseph O’Brien, USDA Forest Service,
http://www.invasive.org; accessed 22 March 2004.)
(a)(c) (d) (e)(b)