Elsevier

Forest Ecology and Management

Volume 434, 28 February 2019, Pages 76-86
Forest Ecology and Management

Frequency of stump infections by Heterobasidion annosum s.l. and benefits from urea treatments vary with tree species and season in European Alpine forests

https://doi.org/10.1016/j.foreco.2018.12.011Get rights and content

Highlights

  • Picea abies stumps are more infected by Heterobasidion than stumps of other species.

  • Stump infection frequency increases throughout the year, peaking in the autumn.

  • Manual operational stump treatment with urea is effective regardless of the species.

  • Infections of treated stumps are predicted to increase slightly in the autumn.

  • Treatment benefits depend on species and raise throughout the year, peaking in autumn.

Abstract

The destructive forest pathogen Heterobasidion annosum sensu lato (s.l.) may be controlled either by timing logging operations in periods of the year characterised by a low risk of airborne stump infection or by treating stump surfaces immediately after felling. 1616 stumps of four of the most widely-distributed European host tree species, either untreated or manually and operationally treated with urea, were sampled from 29 pure and mixed forest stands of the western Italian Alps and analysed for the presence of the pathogen. The aims of the study were to (i) compare the overall and seasonal frequency of stump infection of different tree species; (ii) assess the efficacy of treatments on different tree species; (iii) explore if the efficacy of treatment is affected by the season of application, and (iv) estimate the benefits resulting from treatments at different time periods. Overall, the frequency of infection of untreated stumps was significantly higher (P < 0.05) in Norway spruce (Picea abies) than in European larch (Larix decidua), silver fir (Abies alba) and Scots pine (Pinus sylvestris). In general, the frequency of stump infection of each tree species increased throughout the year, reaching a maximum in autumn, possibly in relation to the higher availability of airborne inoculum in that season. Manual operational stump treatment with urea reduced the frequency of infected stumps compared to controls both overall and for every tree species, although the reduction was significant only in the overall analysis and in the analysis performed separately for Norway spruce and larch. Binary logistic regression models point to the general efficacy of treatments regardless of the season of application, although the frequency of infection of treated stumps is predicted to increase slightly in autumn. Despite the efficacy of treatments being slightly reduced in autumn, the predicted reduction of stump infection probability due to treatments, hence the benefits provided by treatments, is expected to vary in magnitude depending on the species but to increase similarly according to a roughly sigmoid curve throughout the year, approaching a maximum in autumn. The information provided by this paper may help forest managers to design profitable and/or sustainable tactics to control the disease.

Introduction

Root and butt rots caused by the fungal species complex Heterobasidion annosum (Fr.) Bref. sensu lato (s.l.) are among the most destructive diseases of conifers worldwide (Garbelotto and Gonthier, 2013). In Europe, the species complex comprises three native species, each characterised by a distinct host preference: H. annosum sensu stricto (s.s.) on pines (Pinus spp.) and other conifers and even broadleaves, H. abietinum Niemelä & Korhonen on firs (Abies spp.), including the widely distributed silver fir (A. alba Miller), and H. parviporum Niemelä & Korhonen on Norway spruce (Picea abies (L.) Karsten) (reviewed by Garbelotto and Gonthier, 2013). In addition, the North American H. irregulare Garbel. & Otrosina is also present as an alien invasive species in coastal pine and oak stands of central Italy (Gonthier et al., 2007, Gonthier et al., 2012b, Gonthier et al., 2014).

Regardless of the Heterobasidion species involved, infections occur by means of meiospores through freshly cut stumps or wounds (primary infection). Once established, H. annosum s.l. may infect uninjured neighbouring trees by growth of the mycelium through root contacts or grafts (secondary infection) (reviewed by Garbelotto and Gonthier, 2013). Due to its peculiar infection biology and its ability to exploit the presence of recently created stumps, the fungus is particularly destructive in intensively managed forests, especially in monoculture plantations established on former agricultural land (Korhonen et al., 1998). H. annosum s.l. is deemed to play a subordinate role in more natural forest ecosystems (Shaw et al., 1994). However, high levels of disease incidence have also been reported in mixed and naturally regenerated forest stands. For instance, disease incidence as high as 71% was reported in Norway spruce and European larch (Larix decidua Miller) subalpine mixed forest stands of the western Italian Alps (Gonthier et al., 2012a). In this area, where all three European native species of H. annosum s.l. are present (Gonthier et al., 2001), disease incidence has been reported to differ significantly from stand to stand depending on the size of the trees and on the host species, Norway spruce being the most severely affected host (44.2% of infected trees), followed by silver fir (17.6%), larch (12.0%) and Scots pine (P. sylvestris L.) (5.6%) (Gonthier et al., 2012a). While susceptibility to the disease has been reported to vary among tree species as a consequence of secondary infections (Vollbrecht et al., 1995, Greig et al., 2001, Gonthier et al., 2012a), the susceptibility of stumps of different species to primary infection has not yet been definitively determined, as all efforts carried out so far by inoculating the pathogen have given rise to variable and sometimes conflicting results probably reflecting differences in methodology, the availability of natural airborne inoculum and interactions with environmental factors at the time of inoculation (reviewed by Redfern and Stenlid, 1998). In addition, by relying on natural airborne infections, the comparative analysis of infection frequency would require stumps of different species either to co-occur in the same mixed forest stands or, at least, to be present in separate stands yet located in the same area consistently infested by the pathogen, a rather uncommon condition to be met. It should be noted that good knowledge of the susceptibility of stumps to airborne infections might allow for tactics to be implemented to control the disease in mixed stands.

Root and butt rots caused by H. annosum s.l. may be controlled in several ways (reviewed by Gonthier and Thor, 2013). In general, satisfactory levels of disease control may be achieved either by timing logging operations (e.g. cutting or thinning) in periods of the year characterised by a low risk of stump infection due to limited spore loads or by protecting stump surfaces immediately after felling with chemical or biological treatments (Gonthier and Thor, 2013). For instance, based on the seasonal availability of airborne inoculum or on the probability of stump infections, winter thinnings and winter or spring thinnings are recommended in northern European countries and in the Alps, respectively, to minimise the risk of stump infection (Brandtberg et al., 1996, Gonthier et al., 2005). Several products have been developed to treat pine or spruce stumps, including the biological control agent Phlebiopsis gigantea (Fr.) Jülich, sodium tetraborate decahydrate (borax), and urea (Holdenrieder and Greig, 1998, Pratt et al., 1998, Gonthier and Thor, 2013). When tested on Norway spruce stumps in the Italian Alps, all of the above treatments performed equally well (Nicolotti and Gonthier, 2005). Nevertheless, 30% urea solution was recommended for subsequent large scale operational treatments (Nicolotti and Gonthier, 2005) due to its long history of stump treatment in Europe (reviewed by Pratt et al., 1998) and its effects on non-target organisms inhabiting stumps that are moderate and only transient (Varese et al., 2003, Vasiliauskas et al., 2004). While the efficacy of urea on Norway spruce stumps is supported by a large body of literature (Brandtberg et al., 1996, Pratt et al., 1998 and references therein; Nicolotti and Gonthier, 2005, Thor and Stenlid, 2005) and is well-documented even in long-term experiments (Oliva et al., 2008), its treatment efficacy on stumps of other tree species has been poorly investigated, except for pine species, Sitka spruce (Picea sitchensis (Bong.) Carrière), hybrid larch (Larix × eurolepis) and Cilician fir (Abies cilicica (Antoine & Kotschy) Carrière) (Pratt et al., 1998, Lehtijärvi et al., 2011, Wang et al., 2012).

Another poorly understood aspect is whether treatment efficacy may be affected by the season of application. It has previously been shown that the performance of borates on artificially inoculated Sitka spruce stumps depended on the inoculum density of H. annosum s.l. (Pratt, 2000). As the availability of natural airborne inoculum has been reported to vary significantly throughout the year in the Alps (Gonthier et al., 2005), treatments performed in different seasons may differ in their efficacy levels. However, if and to what extent this may happen still requires elucidation.

An efficient and standardised application method is always crucial for the final outcome when performing treatments against H. annosum s.l., and this is particularly true for manual stump treatment programmes (Gonthier and Thor, 2013). While experimental efficacy trials are generally performed by researchers with the maximum standardisation and the highest stringency, the question of whether operational stump treatments, i.e. treatments performed with commercial products directly by workers, may be effective in reducing H. annosum s.l. primary infections has been determined only for mechanised stump treatments (Thor and Stenlid, 2005), but not yet for manual stump treatments.

This study was carried out in pure and mixed forest stands of the Aosta Valley Region, in the western Italian Alps, an area reported as consistently infested by H. annosum s.l. (Nicolotti and Gonthier, 2005, Gonthier et al., 2012a) and where manual operational stump treatments with 30% urea solution had been performed. The specific goals were: (i) to compare the overall and seasonal frequency of primary infection of stumps of different tree species native to the Alpine range; (ii) to assess the efficacy of operational stump treatment with urea on different tree species; (iii) to explore if and to what extent the treatment efficacy is affected by the season of application, and (iv) to determine, on a seasonal basis, the predicted reduction of stump infection probability due to the treatment so as to obtain an estimate of the benefits resulting from treatments at different time periods and depending on the species.

Section snippets

Materials and methods

The study was conducted over the period 2004–2007 during thinning operations in 29 naturally regenerated pure and mixed forest stands in the Aosta Valley, in the western Italian Alps. The main characteristics of forest stands and the periods of thinning and treatment are reported in Table 1. The treatments were performed manually, on different dates from early spring to late autumn depending on the stand (Table 1), with garden sprayers by forestry workers of the Aosta Valley Autonomous Region

Results

A total of 1616 stumps were sampled; of these, 804 were treated stumps and 812 were control stumps. Regardless of their treatment status, 737 were of larch, 558 of Norway spruce, 203 of Scots pine, and 118 of silver fir. Isolates of H. annosum s.l. were obtained from wood discs exposed in 26 out of the 29 forest stands (Table 2). Based on the outcomes of the diagnostic PCR-based assay performed on those isolates, H. parviporum was present either alone or in co-occurrence with other species in

Discussion

In this study, conducted in the context of ordinarily programmed thinning of forest stands in the western Italian Alps, I have shown that, regardless of the host species, the frequency of stump infection by H. annosum s.l. increased throughout the host growing season, and mostly significantly in the last time period, with Norway spruce displaying significantly higher rates of infection compared to the other investigated tree species native to the Alpine range. Furthermore, I have shown that

Conclusions

The co-occurrence of four of the most important and widespread European coniferous tree species in pure or mixed forests relatively close to each other as well as of the three native H. annosum s.l. species make the Alpine area ideal for comparative analyses of stumps’ susceptibility to pathogen airborne infections as well as of the performances of stump treatments. This paper provides evidence that stumps of Norway spruce are significantly more frequently infected than stumps of the other tree

Acknowledgments

This work is dedicated to the memory of Giovanni Nicolotti, a mentor and a dear friend. I am grateful to the staff of Regione Autonoma Valle d’Aosta/Région Autonome Vallée d’Aoste for helping with treatments and field samplings; a special thank to Jean-Claude Haudemand and Giancarlo Zorzetto. I am also very grateful to Ilenia Marafante and Guglielmo Lione who provided help with the laboratory work and statistical analyses, respectively.

Funding

This work was supported by the Regione Autonoma Valle d’Aosta/Région Autonome Vallée d’Aoste, Assessorato Agricoltura, Risorse Naturali e Protezione Civile/Assessorat de l’Agriculture, des Ressources Naturelles et de la Protection Civile, Italy.

Competing interests statement

I declare no competing interests.

Data availability statement

All data used in this study have been made available in the article text, and Mendeley Data repository.

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