2.3.2
PHYTOPHTHORA IN A FRENCH OAK FOREST

E HANSEN1 and C DELATOUR2

1Botany and Plant Pathology, Oregon State University, Corvallis, Oregon, USA; 2Pathologie forestiere, INRA, Nancy, France

Background and objectives
Phytophthora is known as a genus of plant pathogens, dangerous in agriculture and forestry alike. The destruction caused by P. cinnamomi after introduction to native plant communities in Australia and elsewhere is well known and there are recent reports implicating Phytophthora species in forest declines in Europe. However, in other ecosystems P. cinnamomi is widespread but does not kill trees and there are scattered reports suggesting that Phytophthora species may be natural components of healthy forest floras. Unfortunately, little is known about the behaviour of Phytophthora species in forests of the world. In support of a larger European Union project investigating Phytophthora and oak decline, we are exploring the species present in an intensively managed French oak forest, their local distribution and seasonal abundance, and possible pathogenic behaviour.

Materials and methods
The Foret d' Amance, on the Lorraine Plateau near Nancy in north-east France, covers about 1200 ha and is composed primarily of Quercus petraea and Q. robur, although stands of Fagus and various conifers are also present. The topography is gentle and most streams and drainage channels flow only during periods of heavy rain. Loamy soils derived from alluvial deposits occur in most places. The area has been managed for forest products for hundreds of years and periodic cutting continues. Despite extensive disturbances from harvest, insect defoliation, roads and war, Amance forest is healthy. Decline of individual trees and scattered mortality is caused by Collybia fusipes and other pathogens and by storms but, overall, growth is good and symptoms of decline are absent. Phytophthoraspecies were recovered primarily by baiting with Chamaecyparis foliage baits. Soil samples were collected, flooded and baited, and baits were floated in streams and standing water in the forest. In addition, forest litter (primarily fallen leaves) was collected and baited. Roots were sampled and plated directly or baited. Baits were plated on Phytophthora-selective media with pimaricin and hymexazol, and cultures were identified on corn meal agar with -sitosterol and on potato dextrose agar.

Results and conclusions
Results to date indicate a diverse, and abundant Phytophthora community in this forest, which persists without causing visible symptoms of disease. Phytophthora species are apparently resident in all of the streams that originate on the forest and in most places where water accumulates after heavy rain. Thirty-six collections were made from 13 separate streams and all were positive; 18 out of 27 puddles yielded Phytophthora. They were readily baited directly from the water and from submerged leaf litter and, occasionally, from fallen leaves on ground not subject to flooding, but never from the soil immediately beneath those leaf samples. Phytophthora was not recovered from soil samples, except those collected near the water-line in stream courses or puddles and from a small forest nursery. Phytophthora is apparently abundant in the water of this forest. Nearly every bait had been colonized in sampling conducted between October and January. Species identification has not yet been confirmed, but at least four species have been isolated on two or more occasions; these include two heterothallic and two homothallic species. A seemingly sterile heterothallic species with ovoid/pyriform nonpapillate sporangia predominates in most collections.

Sampling and experiments continue to test the hypothesis that some or all of these Phytophthora species maintain their populations saprophytically in leaf litter in water. We have continued to monitor the seasonal dynamics of the populations and to test roots for infection.