Occurrence of Armillaria spp. 

 in Forests of the 

 Northern Rocky Mountains 



Geral I. McDonald 

 Neil E. Martin 

 Alan E. Harvey 



INTRODUCTION 



Root disease caused by various species of Armillaria is 

 integral to most forested ecosystems worldwide (Wargo 

 and Shaw 1985). As early as 1930, the fungus was recog- 

 nized as a root pathogen in the Western United States 

 (Hubert 1931, 1950; Johnson 1976). Armillaria commonly 

 occurs as rootlike rhizomorphs growing on plant debris or 

 is epiphytically attached to root systems of dead, diseased, 

 or healthy host plants (Garrett 1960; Kile 1980; Leach 

 1939; Raabe and Trujillo 1963; Redfern 1973). Isolates ob- 

 tained from such rhizomorphs, as well as isolates obtained 

 from mycelial fans, rotten wood, and sporophores, can 

 belong to species or specific clones whose apparent patho- 

 genicity varies (Kile 1983; Rishbeth 1982; Wargo and 

 Shaw 1985). 



Severity of this root disease tends to increase as man- 

 agement intensifies. Partial cutting (Filip 1977; Filip and 

 Goheen 1982; Redfern 1978), excessive grazing (Bega 

 1979), and fire control (Shaw and others 1976) all appear 

 linked to increased activity of Armillaria spp. In Queens- 

 land, Australia, Armillaria spp. were found in nearly all 

 stumps after clearcutting of a first-rotation introduced 

 pine forest (Anon. 1982). Chemical and mechanical killing 

 of hardwoods is linked to increased Armillaria activity 

 (Pronos and Patton 1977; Swift 1972) in remaining con- 

 ifers. Conifers planted to replace clearcut conifer or hard- 

 wood forests have experienced significant Armillaria- 

 related mortality (Redfern 1978; Shaw and Roth 1978). 

 Armillaria spp. frequently interact with insects and other 

 pathogens (Dunbar and Stephens 1975; Madziara- 

 Borusiewicz and Strzelecka 1977; Singh and Raske 1983; 

 Wargo 1977, 1981) to compound losses. Slash disposal 

 methods may also affect Armillaria spp.; woody debris in- 

 corporated into the soil can be a significant source of new 

 infections (Garrett 1960; Raabe and Trujillo 1963). Even 

 method and quality of planting (Rykowski 1981) can 

 influence damage caused by this organism (Singh and 

 Richardson 1973). 



Aside from total removal of woody plant parts followed 

 by clean cultivation, no technique has proved effective in 

 controlling this pathogen (Pawsey 1973; Shaw and Roth 

 1978). Other promising management techniques are the 

 planting of tolerant or resistant species or varieties; ap- 

 plication of chemicals or fire; site amendments (including 

 extraction of wood from the soil) prior to planting; and 

 judicious selection of cutting times based on overall risk 

 prediction. Effective application of such techniques re- 

 quires much new knowledge of the biology, genetic archi- 



tecture, physiology, and ecology of hosts and the causal 

 agent. We particularly need to know variation in patho- 

 genicity and virulence of the pathogen and variation in 

 resistance of the hosts. 



Many studies have reported that in terms of geographic 

 distribution and host range, Armillaria spp. are ubiqui- 

 tous (Ehrlich 1939; Hobbs and Partridge 1979; Hubert 

 1950; Swift 1972). The primary objective of such studies 

 has been to determine the degree of its damage rather 

 than the distribution of the fungus. Typical examples are 

 Carey and others (1984), Williams and Marsden (1982), 

 and James and others (1984), wherein sample points were 

 selected by location of symptomatic trees or root disease 

 centers. 



What do we need to know about Armillarial How ex- 

 tensive is the distribution of various species of Armillaria 

 and to what extent are these species pathogenic or sapro- 

 phytic? Useful information could also include host, patho- 

 genic or saprophytic condition, and taxonomic affiliation of 

 each fungal sample. Knowing probability of occurrence 

 (such as a proportion of 0.04-ha plots) of pathogenic and 

 nonpathogenic entities in relation to other ecological 

 criteria could serve as the basis for classifying forest lands 

 according to risk of specific host species, given various 

 management actions. Control measures could be similarly 

 categorized. 



In 1983, we began a study of population level genetic 

 and ecological interactions between Armillaria and its 

 conifer and hardwood hosts. The two primary objectives of 

 this research were (1) to predict Armillaria risk by habi- 

 tat type, host species, and stand management history; and 

 (2) to develop ways for assessing genetic and physiologic 

 responses between host and fungus populations in North- 

 ern Rocky Mountain forests. This paper reports on the oc- 

 currence of vegetative parts of Armillaria spp. by climax 

 series. 



MATERIALS AND METHODS 



Plots 0.04 ha in size were randomly selected from for- 

 ested lands of the Northern Rocky Mountains as follows: 

 A 1.27-cm grid was placed over l,270-cm:l,609-m maps of 

 15 Northern Rocky Mountain National Forests. Random 

 coordinates were drawn 15 times for each National Forest 

 to locate 15 sections (l,609-m2 blocks) on each. Aerial 

 photo indexes were used to obtain 12,000:1, 16,000:1, or 

 24,000:1 photos of the selected sections. Each photo was 

 matched in turn with drainage patterns and other features 

 common to the photos and maps for locating the target 



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