Armillaria novae-zelandiae and other basidiomycete wood decay fungi in New Zealand Pinus radiata thinning stumps By I. A. Hood 1,5 , J. Oliva 2 , M. O. Kimberley 1 , N. Arhipova 2,3 and R. Bakys 2,4 1 Scion (New Zealand Forest Research Institute), Private Bag 3020, Rotorua 3046, New Zealand; 2 Department of Forest Mycology and Plant Pathology, Swedish University of Agricultural Sciences, Uppsala, Sweden; 3 Latvian Forest Research Institute (Silava), Salaspils, Latvia; 4 State Forest Service, Kaunas, Lithuania; 5 E-mail: ian.hood@scionresearch.com (for correspondence) Summary Two studies were undertaken in a young New Zealand Pinus radiata stand to find a biological control agent for armillaria root disease caused by Armillaria novae-zelandiae. Fresh thinning stumps were inoculated with saprophytic basidiomycete species as wooden dowel cul- tures or as homogenized aqueous mycelial suspensions, while all stumps were also inoculated with dowel cultures of A. novae-zelandiae. After a period of between one and two years, no differences were found among test fungi in the percentage of Armillaria species isolated from stumps. However, inoculation of Stereum sanguinolentum as homogenate, and of Phlebiopsis gigantea, Sistotrema brinkmannii, Hypholo- ma acutum and Rigidoporus concrescens as dowel cultures, significantly increased their isolation incidence in comparison with stumps not treated by these species. Phlebiopsis gigantea, Resinicium bicolor and R. concrescens were also isolated with greater frequency nearer to their dowel inoculation points. Results suggest that P. gigantea and S. sanguinolentum should be included in further testing. However, a future trial is likely to be more effective if evaluation is directed towards the portion of the stump below the soil surface, particularly the root system. 1. Introduction Armillaria root disease is caused mainly by the pathogen Armillaria novae-zelandiae (G. Stev.) Herink in contemporary plan- tations of Pinus radiata D. Don in New Zealand (Hood et al. 2002). Although no longer a major problem, infection neverthe- less remains widespread, if mostly hidden, in many production forests (Self et al. 1998; van der Kamp and Hood 2002; Hood and Kimberley 2009), and localized outbreaks of mortality in young stands still provoke periodic concern among plantation managers. Research is being undertaken to find control methods that could be included in an economic, integrated procedure for use on sites prone to disease. This would provide an alternative to the effective but costly recommendation of removing infested stumps before establishing the next crop (Shaw and Calderon 1977; van der Pas 1981; van der Pas and Hood 1984; Self and MacKenzie 1995). Methods reported to reduce the incidence or effect of disease include the planting of vig- orous, healthy stock (Hood et al. 2006), control of dothistroma needle blight caused by Dothistroma septosporum (Dorog.) M. Morelet (Shaw and Toes 1977) and treatment of seedling root systems with species of Trichoderma before planting (Cutler and Hill 1994; cf. Hohmann et al. 2011). Stand thinning was not effective and actually increased disease incidence during the first half of the rotation, apparently by providing fresh inoculum substrate in the form of new stumps (Oliva et al. 2009). This treatment was found to have no adverse effect over the full period but neither was it advantageous (Hood and Kimberley 2009), and although thinning forms an essential part of stand silviculture, it is not seen as a disease man- agement possibility. Equally unsuccessful was a study to identify clones of P. radiata resistant to A. novae-zelandiae for planting on infested sites in New Zealand (Hood et al. 2009). The introduced P. radiata has been exposed to indigenous A. novae-zelandiae for a little over a century in a plantation system offering little opportunity for resistance to develop through natural selection (cf. Cruickshank et al. 2010). Although finding a host cultivar that is resistant or tolerant to the pathogen in New Zealand remains theoretically feasible, it may prove to be a challenging undertaking. A more likely approach may be to reduce the inoculum potential using a suitable biological control agent to prevent the pathogen from accessing the stump wood food base. Although A. novae-zelandiae invades stumps by means of basidiosp- ores (Hood et al. 2008; Power et al. 2008), it also colonizes vegetatively by mycelial growth through the root system or from existing root collar infection prior to felling. It is nevertheless conceivable that a rapidly growing wood decay basidio- mycete fungus may still lessen or even prevent invasion by decreasing the amount of wood substrate available to Armillar- ia species through aggressive spatial competition within the stump (Rishbeth 1976). Efforts to find candidate agents for stump treatment have been attempted in field experiments, with some success reported (Rishbeth 1976; Fedorov and Bob- ko 1988a,b; Pearce et al. 1995; Łakomy 1998; Chapman and Xiao 2000; Prospero et al. 2006; see also Sierota 1984). Labo- ratory studies challenging Armillaria species with cultures of various basidiomycetes have also shown promise (e.g. Kuntze 1954; Orło s 1957; Pearce 1990; Yang and Hood 1992). This is apart from many attempts to control Armillaria species using non-basidiomycete fungi, such as species of Trichoderma, which though often effective differ from decay fungi in being unable to take full nutritional advantage of the wood substrate cell wall molecules for sustained performance (Rish- beth 1976; e.g. Garrett 1958; Sokolov 1964; Munnecke et al. 1981; Reeves et al. 1990; Cutler and Hill 1994; Filip and Yang-Erve 1997; Kwa sna et al. 2004). Received: 25.9.2014; accepted: 24.12.2014; editor: O. Holdenrieder For. Path. doi: 10.1111/efp.12171 © 2015 Blackwell Verlag GmbH http://wileyonlinelibrary.com/