Efficacy and environmental fate of Chondrostereum purpureum used as a biological control for red alder (Alnus rubra)

Abstract

A field trial to assess the ability of Chondrostereum purpureum to limit the resprouting of cut red alder (Alnus rubra) was established in British Columbia, Canada. Overall, 92% of stumps inoculated with C. purpureum died in the first year and 100% were dead by the second year. C. purpureum was thus as effective as the herbicide treatments Carbopaste and liquid glyphosate (12% Vision) and reduced resprouts to zero. Naturally occurring C. purpureum spores colonized about 40% of untreated cut stumps and about 20% of the stumps treated with chemical herbicides. The peak production of C. purpureum basidiocarps occurred about 22 months after the trial was established, with C. purpureum on 66% of treated stumps. The following year, the basidiocarps persisted on 23% of treated stumps. About 600 young red alder trees were cut to act as spore traps 2 years after trial establishment, when the most C. purpureum basidiocarps were present on treated stumps in the trial. These spore-trap trees were subsequently sampled, and PCR markers were used to verify and characterize 43 dikaryotic C. purpureum individuals, none of which were identified as the released isolate 2139. There was little difference in average band-sharing with the applied isolate among pre-trial (37% average) and spore-trap (38% average) populations. There was therefore no evidence that the C. purpureum strain applied inundatively to stumps in this trial was detectable on nontarget trees in the local area or had any measurable impact on the local C. purpureum population.

Introduction

Environmental concerns have called for the reduction of chemical herbicide use in forests and given support to the concept of integrated vegetation management. The use of chemical herbicides, however, is at present the preferred approach for vegetation management in reforestation sites and in utility company rights-of-way. Mechanical treatment alone is ineffective for many target tree species as they can resprout vigorously without further treatment. There is therefore a need for effective alternative treatments such as indigenous plant pathogenic fungi formulated for use as biocontrol agents for weeds, known as mycoherbicides (Templeton et al., 1979). While biocontrol of forest weeds using indigenous mycoherbicides is designed to minimize risk, it is a new approach that should be carefully evaluated.

The basidiomycete Chondrostereum purpureum (Pers.) Pouzar is found worldwide in temperate zones. It is a facultative saprophyte with broad-spectrum pathogenicity towards many hardwood species. A primary invader of woody angiosperms, C. purpureum usually enters its host through a fresh wound, cut stump, or stem lesion (Brooks and Moore, 1926, Rayner and Boddy, 1988, Spiers and Hopcroft, 1988). The fungus grows through xylem tissues of the host plant, causing cambial necrosis, decay, sapwood staining, and sometimes death of the host (Rayner, 1977, Wall, 1986, Wall, 1991). Infection by C. purpureum can also cause foliar discoloration or silvering, known as silver-leaf of orchard fruit trees (Bishop, 1979, Grosclaude et al., 1973, Setliff and Wade, 1973).

In Canada, it has been demonstrated that infection of hardwood stems by C. purpureum reduces the number and viability of resprouts (Wall, 1990). The C. purpureum biocontrol strategy involves covering the entire surface of freshly cut target stumps with mycelial fragments in a protective nutrient formulation. The effectiveness of C. purpureum as a mycoherbicide has been evaluated for the control of Acer rubrum L. (red maple), A. saccharum Marsh. (sugar maple), A. macrophyllum Pursh (bigleaf maple), Alnus rubra Bong. (red alder), Betula alleghaniensis Britton (yellow birch), B. papyrifera Marsh. (paper birch), Fagus grandifolia Ehrb. (beech), Populus tremuloides Michx. (trembling aspen), P. grandidentata Michx. (bigtooth aspen), Prunus pensylvanica Linn. f. (pin cherry), and P. serotina Ehrh. (black cherry) (de Jong et al., 1990, Dumas et al., 1997, Harper et al., 1999, Jobidon, 1998, Pitt et al., 1999, Wall, 1986, Wall, 1990, Wall, 1991, Wall, 1994, Wall, 1997, Wall et al., 1992).

Current understanding of the ecology and pathogenicity of C. purpureum is steadily growing. The fungus exists in all biogeoclimatic zones in North America, which precludes the risks associated with the introduction of a foreign species for biological control. A number of studies have examined the population structure of C. purpureum at regional to intercontinental scales, using different methods for differentiation of subgroups, including mating interactions (Rayner and Boddy, 1986), SDS–protein profiles (Ekramoddoullah et al., 1993), isozyme analyses (Shamoun and Wall, 1996), ribosomal DNA and mitochondrial DNA profiles (Ramsfield et al., 1996, Ramsfield et al., 1999), random amplification of polymorphic DNA (RAPD; Gosselin et al., 1996, Gosselin et al., 1999), and morphology, pathogenicity, and RAPD (Spiers et al., 2000). The associated risk of moving isolates over large geographic areas and across biogeoclimatic zones was assessed with the conclusion that the C. purpureum population is panmictic and has no geographic or host specialization (Gosselin et al., 1999, Ramsfield et al., 1996, Ramsfield et al., 1999).

The wide host range of C. purpureum, recorded on over 100 hosts, is an advantage when considering a candidate biocontrol agent, but infection of nontarget plants due to biocontrol application of C. purpureum would be undesirable. Studies have indicated a low risk of direct nontarget infection associated with the application of C. purpureum mycelium (Becker et al., 1999a, Becker et al., 1999b), but secondary infection from spores originating from the biocontrol-treated stumps is of concern (de Jong, 1992, de Jong et al., 1990, Gosselin et al., 1999). Basidiocarps are usually produced by the fungus on infected stumps and trees and may persist for up to 2 years (Wall, 1997). When environmental conditions are favorable, basidiospores are released that may infect freshly wounded deciduous trees. In Canada and in other countries, C. purpureum is commonly found on windblown trees, logged areas, woodpiles, and pruned areas, and these may be important inoculum sources for the phytopathogen. A similar increase in spore load with a similar environmental impact is hypothesized to be associated with the use of C. purpureum as a stump treatment. An extensive study was performed in 1993 and 1994 of the natural occurrence of C. purpureum on Vancouver Island in relation to its use as a biocontrol agent, which concluded that the added fructification due to biocontrol use was the same order of magnitude or less than naturally occurring levels (de Jong et al., 1996). A fate assessment of C. purpureum, concerned with the release and dispersal of spores, was performed by modeling the dispersal part of the epidemiological process to simulate the dissemination of the disease (de Jong, 1992, de Jong et al., 1990, de Jong et al., 1991).

Many factors influence or limit the spread of C. purpureum spores into the environment: weather conditions that influence the release, dispersal, deposition, germination, and survival of spores as well as the stress level and susceptibility of the hosts (Grosclaude et al., 1973, Wall, 1991). The requirement of a fresh wound for infection allows C. purpureum to be applied specifically to host individuals. If the C. purpureum formulation is applied to only part of a stump, resprouting often occurs in the unaffected region of the stump. This dependence on a freshly wounded host is hypothesized to be the most important constraint limiting infection by C. purpureum. The application of C. purpureum mycelium to freshly cut stumps largely removes this constraint and hence efficacy of biocontrol is then dependent upon other factors affecting colonization and competitive ability such as lateral rate of growth and production of phytotoxins.

The following experimental trial was established to gain information on the biological impact, in terms of efficacy and environmental fate, of C. purpureum, applied to A. rubra under nearly operational conditions, as a formulation that could be developed as a commercial product. Spore traps were established to assess the impact of the field trial on nontarget host plants and the local C. purpureum population. It was hypothesized that the amount of C. purpureum infection due to spores from the biocontrol trial would increase on nontarget hosts around the treated area for a relatively short period of time, then return to background levels. This field trial was established to examine the dynamics of this mycoherbicide pathosystem and to evaluate (1) the potential of an application of C. purpureum mycelium to inhibit stump sprouting in red alder compared to manual brushing and chemical herbicide treatments, (2) the persistence of C. purpureum sporocarps, (3) the succession of other fungi on stumps, and (4) the incidence of infection of nontarget red alder stumps by spores from treated stumps.