Infection rate of pine twisting rust (Melampsora pinitorqua) in Scots pine (Pinus sylvestris) regenerations with retained aspens (Populus tremula) : evaluation of the importance of large aspen trees compared to aspen sprouts

Rostsvampen knäckesjuka (Melampsora pinitorqua) är en allvarlig skadegörare i tallungskogar i Sverige. Svampen värdväxlar mellan inhemsk tall (Pinus sylvestris) och asp (Populus tremula). De tydligaste symptomen är böjda eller avbrutna skott i de övre grenvarven i tallarnas krona. Detta kan resultera i reducerad tillväxt och försämrad timmerkvalitet. Väl medvetna om riskerna med knäckesjuka har skogsägare i Sverige under lång tid kämpat med att utrota aspen både i tallföryngringar och i plantskolor. Nuförtiden har attityden gentemot asp förändrats i och med att naturvård implementerats i skogsskötseln i högre grad. Detta har resulterat i en ökning av antalet aspar som lämnas kvar i naturvårdssyfte på föryngringsytorna. Syftet med studien var att utvärdera betydelsen av kvarlämnade naturvärdesträd av asp, jämfört med självföryngrat aspsly, som spridningskälla för knäckesjuka i unga tallbestånd. Huvudhypotesen var att löv från aspsly utgör en större smittokälla än löv från vuxna aspar. Fältstudien utfördes i Västerbottens län. Elva tallungskogar mellan 5-15 år valdes slumpmässigt ut bland bestånd infekterade av knäckesjuka samt innehållande naturvärdesaspar. Förekomst och omfattning av årets angrepp av knäckesjuka registrerades för varje träd inom 18 GPS märkta provytor i varje bestånd. Även antalet rotskott av asp räknades på provytorna. Samtliga stora aspar i bestånden registrerades med GPS för analys av avstånd mellan aspar och provytorna. Resultatet påvisade en högst signifikant korrelation mellan antalet rotskott av asp och infektionsgraden på de unga tallarna. Ingen av de två variablerna avstånd till närmsta stora asp eller antalet stora aspar inom 100 m från provytorna var signifikant korrelerade med infektionsgraden på tallarna. Aspträdens roll i spridningen av knäckesjuka verkade först och främst vara att skjuta rotskott. Det var en signifikant positiv korrelation mellan tallens toppskottslängd och angreppsgraden av knäckesjuka. Välväxande tallar föreföll mer mottagliga för angrepp än mindre produktiva tallar. GIS-kartor producerades för att visualisera den rumsliga distributionen av knäckesjukeangrepp i relation till täckningsgraden av aspsly och förekomsten av stora aspar. Risken med att lämna asp som naturvärdesträd i tallföryngringar kan anses låg så länge som preventiva skogsskötselåtgärder verkar för att hålla nere mängden aspsly i produktiva högriskområden. Abstract (eng) Pine twisting rust (Melampsora pinitorqua) is a serious fungal pathogen that in Sweden infects young Scots pine (Pinus sylvestris) stands and has European aspen (Populus tremula) as alternate host. Pine twisting rust (PTR) infects the pine's annual shoots. Main symptoms are bent or broken shoots in the upper crown of young Scots pines which can result in reduced growth and timber quality. Well aware of the risk of PTR Swedish foresters has fought to eliminate aspen, both in Scots pine regenerations and in nurseries. Today, the attitude towards aspen trees has changed with the increased implementation of environmental consideration in Swedish forest management. This has resulted in an increased number of mature aspens retained for conservation purposes in pine regenerations. The aim of this study was to evaluate the importance of mature aspen trees compared to young aspen sprouts as causal agents of PTR in young Scots pine stands. The main hypothesis was that leaves of aspen sprouts carry more infection compared to leaves on mature aspen trees. The field study took place in the county of Västerbotten in northern Sweden and eleven 5-15-year-old Scots pine stands were randomly chosen for the study based the occurrence of PTR and large aspen trees. The occurrence and severity of current-year PTR infection on each pine was registered within 18 GPS-positioned sample plots per stand. Also the number of aspen sprouts was counted. Furthermore, all large aspen trees growing in the stand were GPS-positioned. The result showed a highly significant correlation between the number of aspen sprouts and infection rate on Scots pine saplings. None of the two variables concerning the distance- or the numbers of large aspen were significantly correlated to PTR infection rate. It appeared that the adult aspen's role in the spread of PTR was primarily by producing aspen sprouts. There was a significant positive correlation between leader length and PTR infection rate and hence, productive pines with vigorously growing shoots appeared more susceptible to the pathogen than less productive trees. GIS-maps were created to visualize the spatial distribution of PTR infection in relation to coverage of aspen sprouts and occurrence of large aspens. The risk of leaving aspen trees as green tree retention could be considered as low as long as preventive management practices are used in order to keep the number of aspen sprouts low in fertile, high risk areas.Pine twisting rust (Melampsora pinitorqua) is a serious fungal pathogen that in Sweden infects young Scots pine (Pinus sylvestris) stands and has European aspen (Populus tremula) as alternate host. Pine twisting rust (PTR) infects the pine’s annual shoots. Main symptoms are bent or broken shoots in the upper crown of young Scots pines which can result in reduced growth and timber quality. Well aware of the risk of PTR Swedish foresters has fought to eliminate aspen, both in Scots pine regenerations and in nurseries. Today, the attitude towards aspen trees has changed with the increased implementation of environmental consideration in Swedish forest management. This has resulted in an increased number of mature aspens retained for conservation purposes in pine regenerations. The aim of this study was to evaluate the importance of mature aspen trees compared to young aspen sprouts as causal agents of PTR in young Scots pine stands. The main hypothesis was that leaves of aspen sprouts carry more infection compared to leaves on mature aspen trees. The field study took place in the county of Västerbotten in northern Sweden and eleven 5-15-year-old Scots pine stands were randomly chosen for the study based the occurrence of PTR and large aspen trees. The occurrence and severity of current-year PTR infection on each pine was registered within 18 GPS-positioned sample plots per stand. Also the number of aspen sprouts was counted. Furthermore, all large aspen trees growing in the stand were GPS-positioned. The result showed a highly significant correlation between the number of aspen sprouts and infection rate on Scots pine saplings. None of the two variables concerning the distance- or the numbers of large aspen were significantly correlated to PTR infection rate. It appeared that the adult aspen’s role in the spread of PTR was primarily by producing aspen sprouts. There was a significant positive correlation between leader length and PTR infection rate and hence, productive pines with vigorously growing shoots appeared more susceptible to the pathogen than less productive trees. GIS-maps were created to visualize the spatial distribution of PTR infection in relation to coverage of aspen sprouts and occurrence of large aspens. The risk of leaving aspen trees as green tree retention could be considered as low as long as preventive management practices are used in order to keep the number of aspen sprouts low in fertile, high risk areas