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Phytophthora Diseases in Woody Plants
Everett M. Hansen

Oregon State University, Corvallis Oregon, USA 97331

Phytophthora is a genus of plant pathogenic water molds. They look and behave like true fungi in many ways, but are in fact related to certain algae, not fungi in the Kingdom Mycota. Species of Phytophthora are well known in many agricultural crops, limiting production unless special cultural or chemical controls are implemented. They are also present in many forest ecosystems, but apart from a few very aggressive species introduced beyond their areas of origin, they remain largely unknown. Their roles as agents of stress in forest trees are only now being discovered. In this presentation we review the known pathogenic behavior of Phytophthora on susceptible trees, and contrast this with the more subtle effects of the pathogen in forests where disease expression is localized or dependent on additional stress agents.

Phytophthora species are characterized by the production of motile zoospores in free water. These propagules typically initiate new infections, usually through the fine roots. Zoospores are attracted to roots by root exudates. Thus even at infection we can imagine that tree vigor influences disease. Zoospores germinate and penetrate the roots and the pathogen grows through the cortex and the phloem of woody roots, killing tissue as it advances.

Most Phytophthora species, on most host trees, kill infected fine roots and grow no further. Severity of disease depends on how many rootlets are killed, and on the tree's ability to regenerate new roots. In a few host-pathogen combinations, however, growth continues up the root to the bole. A basal canker may result, or the pathogen may colonize the entire circumference of the tree, leading quickly to death. I have worked extensively with P. lateralis on Chamaecyparis lawsoniana (Lawson's cypress or Port-Orford-cedar, POC). This aggressive species first appeared in western North America in about 1920. We do not know where it came from, but it has largely destroyed the horticultural use of POC, and continues to be destructive in the native forest.

Two host trees are known: C. lawsoniana in the family Cupressaceae, and the completely unrelated Taxus brevifolia, in the Taxaceae. The combination of unrelated hosts is interesting, and troubling. The very similar but as yet undescribed species that causes Sudden Oak Death in California attacks Quercus, Lithocarpus, and Rhododendron. Clearly konwledge of one host does not allow prediction of others.

P. lateralis has been spread along roads through the forests where its hosts grow. It is carried in mud and infested debris on vehicles. It also moves rapidly in water, and most of the cedar is quickly killed downstream from road crossings, if the pathogen is present. This disease has triggered an intensive disease management effort on the part of the federal forest management agencies in the United States.

The control program aims to stop the further spread of the pathogen by closing some roads permanently, and only allowing travel during the dry season on other roads. Vehicles are washed free of mud as they enter disease free areas. An ongoing program of roadside sanitation aims to reduce the amount of inoculum along roads, and thus reduce the chances for further spread. POC trees regenerate prolifically in disturbed soil on road edges, and of course, are very vulnerable to infection in this location. Small POC growing on road berms are systematically being removed.

The forests where POC grows are often very steep, and the pathogen seldom spreads uphill from the roads. An effort is being made to identify areas that are somewhat protected from infestation by their topography, and concentrate cedar production on these sites. These efforts at protection are essential, and are showing some signs of success. They do nothing to reestablish cedar on the sites where it has been killed by Phytophthora, however. A successful resistance program is underway to identify and test phenotypically resistant individuals, propagate them in seed orchards, and produce resistant seedlings for outplanting back on infested sites in the forest.

The destruction caused in forests by aggressive, exotic Phytophthora species, including P. cinnamomi in southern Europe, the SE United States, and Australasia, is relatively well known. We are only beginning to appreciate the impacts of less aggressive, apparently endemic Phytophthora species on forest health, however.

The exhaustive work of my German colleagues here at Freising has opened our eyes to new disease possibilities.

Phytophthora quercina, a newly described species, is widespread in European oak forests. Under some circumstances in Germany it is associated with rootlet mortality, progressive crown deterioration, and ultimately, death of mature oaks. Phytophthora mediated oak decline occurs on moisture retentive soils with higher pH, and often where soil nitrogen levels are elevated by anthropogenic inputs. In other places the pathogen is present, but not associated with crown deterioration. Other environmental phenomena, including drought and insect defoliation, are also correlated with oak decline. It is not difficult to imagine interactions between these biotic and abiotic stressors affecting water transport and root energy reserves that might lead to dieback and even death. Proving the connection has been difficult, but an international project of the European Union has made significant progress.

While P. quercina is a demonstrated pathogen, there are other Phytophthora species present in forest soils. In Lorraine and Alsace, we found seven species of Pytophthora in oak forests, in addition to P. quercina. Thomas Jung and colleagues report a similar number in Germany. Most are only weakly pathogenic when tested on seedlings and are not associated with obvious tree disease, even where they are abundant. Presumably they cause at least scattered mortality of fine roots. Can they contribute to more lasting damage in interaction with alternating cycles of drought and inundation, or with defoliation? For now we know only that they are present. It will take careful and creative experimentation to elucidate their roles in the ecology of these forests.