1Dept. Forest Sciences, University of BC, Vancouver, B.C. Canada V6T 1Z4; 2Pacific Forestry Centre, Victoria, B.C. Canada V8Z 1M5

Background and Objectives
Armillaria ostoyae is a native pathogen found in all the major ecological forest zones of the southern half of British Columbia. It is most widespread and damaging in the warm moist forests of the southern Interior of the province. The challenge to forest pathologists is to devise silvicultural regimes that will minimize the damage done by Armillaria. This involves at least three different types of consideration. (1) Goals: Forest management objectives are changing rapidly. Maintenance of animal and plant habitat, of watershed values, and more generally of a ‘natural’ forest now takes priority over maximizing timber yields in many places in B.C. Since gap-forming root diseases such as Armillaria are agents of diversity playing a major role in habitat creation and succession in natural forests, total elimination of Armillaria may not be desirable. (2) Assessment: Survey methods to assess the amount of Armillaria either before harvest or in young stands leave much to be desired. The major difficulty is that most Armillaria infection cannot be detected by above-ground symptoms. Furthermore, the relationship between infection and mortality early in a rotation and eventual timber yields is complex, and models developed to elucidate such relationships suffer from the lack of sufficient data. (3) Intervention: The effects of various regular and alternative silvicultural practices (choice of silvicultural system and species; intermediate stand entries; stump removal; etc.) need elucidation. Such effects are likely to be ecosystem specific. In this paper we present a general model of Armillaria development and argue that management goals, assessment protocols, and silvicultural interventions are best formulated in terms of that model.

Results and conclusions
The widely accepted model of Armillaria infection revolves around the two poles of inoculum potential and tree vigour. These can be thought of as describing the ability of the pathogen and host respectively to marshal energy and mineral nutrient resources at the point of invasion or spread. We propose that the balance between these two typically results in either of two outcomes for whole stands or ecological zones, namely active spread leading to continued tree mortality or quiescence. In the active state, the pathogen continually invades new host tissues, and this serves to maintain high inoculum potential and leads to further spread. In the quiescent phase, infections are effectively isolated by the host reactions of periderm formation, resinosis, callus formation and compartmentalization, but they do remain alive. In productive coastal forests of B.C., tree vigour is high, and quiescence is the common state, the disease being active for only a brief period in young plantations. In the dry Interior Douglas-fir forests tree vigour is low and the active state dominates. In the moist Interior forests, either state can occur. Here a proper goal of management is quiescence rather than elimination of the pathogen, and the conditions that result in a change from a quiescent to an active state and visa versa need to be understood in order to manage Armillaria effectively. In most of the older and undisturbed native forest in the moist Interior of B.C., Armillaria is in a quiescent state. Harvesting or partial cutting leads to rapid stump invasion, and the resulting increase in inoculum potential allows the pathogen to spread either to new regeneration or to remaining mature trees. As the inoculum in stumps declines with time, and the vigour of new plantations increases, further spread may slow and, without renewal of inoculum potential by such as activities as thinning, the pathosystem may enter a quiescent state.

In this paper we will present evidence from a number of studies to show that goals of management and methods of assessment are best formulated in terms of an active/quiescence dichotomy, and that attaining or maintaining quiescence is a major goal for silvicultural interventions in the moist forests of the Interior of B.C.

ICPP98 Paper Number 3.7.7S


1Van Arsdel Tree Service, Inc., Tijeras, NM 87059, USA; 2Department of Forest Resources, Clemson University, Clemson, SC 29634, USA

Background and objectives
Urban and woodlot trees, including post oak, water oak, live oak, winged elm and hackberry, growing near Bryan/College Station, TX frequently exhibit abnormal chlorotic leaves followed by rapid death of the tree. Consistent isolations of Pythiaceous fungi (especially Phytophthora species) from these trees indicate that these fungi are contributing to the foliage discoloration, retarded growth, and ultimate death of these trees. These symptoms are distinctly different from those caused by the oak wilt and oak decline fungi. These symptoms are the same symptoms frequently described in publications aboot dying oaks in Texas from 1934 to 1949. This root rot condition is notable in the Post Oak Region where it is associated with oak decline (Cephalosporium), alkali, and salt problems, and where oak wilt (Ceratocystis

Materials and methods
Small roots, and adjacent soil samples, were collected from symptomatic trees and incubated in holes placed in fresh pear (usually Kiefter) or apple (usually Granny Smith) fruits [1]. Fruit tissues developing firm, brown rot were cultured on PCH 121 medium and identity as Pythiaceous fungi confirmed. The local physical environment was studied to detect potentially stressful growth conditions. Direct chemical control with soil injections of metalaxyl and ethazol in the root zones of symptomatic trees was attempled. Additional chemical applications of sulfur, ammonium, magnesium, and zinc sulfates were added. Physical site modifications, such as removal of fill soil and the installation of French drains, were tested. The modification of irrigation cycles was also tested to increase the amount of air in the root zone.

Results and conclusions
Roots and soil of all symptomatic trees, as well as 30 percent of non-symptomatic trees, readily yielded isolates of Phytophthora species, including P. cinnamomi, and Pythium species. Symptomatic trees were usually located in areas where there was frequent standing water or where there was exposure to saline or alkali water. In addition, trees with earth fill over the root zone were likely to be symptomatic as were trees injured during construction. The first visible symptom was a generalized slight yellowing of the leaves. This persisted for as little as a few months or for as much as several years. This chlorotic stage was replaced by a transition period in which the color rapidly changed to an increased yellowing or a mottling of yellow and green colors. After several weeks the leaves rapidly turned orange and then brown, but remained attached as the tree died. Soil injections with ethazol provided symptom remission on affected trees, with the beneficial effects lasting at least 9 months, especially where there was less than 15 cm of soil fill. Soil injections with metalaxyl gave symptom remission in a higher percentage of cases, but the effects seemed to be shorter-lived (36 months) than with ethazol. The greening of leaves and symptom remission resulted from fungicide treatment, but the application of magnesium and zinc sulfates and nitrogen turned the leaves green within a day ortwo and provided an indication of chemical uptake. Physical site modification, such as removal of soil fill extended the time that the control was effective. Changes in drainage were also generally effective and economical, as well as changing the irrigation practices of the home owner to less frequent and deeper watering.

1. Campbell WA, 1948. Plant Disease Reporter 33,134-5.
2. Shew HD, Benson DM, 1982. Phytopathology 72,1029-32.