Science > Research > Fungal parasites of lodgepole pine dwarf mistletoe in British Columbia

Fungal parasites of lodgepole pine dwarf mistletoe in British Columbia

T.D. Ramsfield ¹, S.F. Shamoun ², B.J. van der Kamp ¹

¹ Department of Forest Sciences, Faculty of Forestry, University of British Columbia, Vancouver, BC  V6T 1Z4

² Canadian Forest Service, Pacific Forestry Centre, Victoria, BC  V8Z 1M5

[ Abstract | Introduction | Objective | Survey | Caliciopsis arceuthobii ]
[ Colletotrichum gloeosporioides | Future Research | Acknowledgments ]
[ Other Publications ]

Abstract

Lodgepole pine dwarf mistletoe (Arceuthobium americanum) is a serious pest in British Columbia (BC) forests. In BC the trend toward reduced cut block sizes and partial cutting systems results in infection of the regenerating conifers by seed dispersal from infected adjacent and overstory trees. Biological control using native fungal parasites is currently being studied as an alternative method of control. During the summer of 1998 diseased dwarf mistletoe plants were collected from the interior of BC. Fungi reported to infect lodgepole pine dwarf mistletoe include Caliciopsis arceuthobii, Cylindrocarpon gillii and Colletotrichum gloeosporioides and all except C. gillii were found to infect A. americanum in BC. Colletotrichum gloeosporioides was distributed throughout the province and was isolated from infected shoots and berries. Future studies will be focused on determining the effects of C. gloeosporioides on the aerial shoots and endophytic system of lodgepole pine dwarf mistletoe and inoculation of healthy lodgepole pine dwarf mistletoe to investigate its potential as a biological control agent.

Introduction

Lodgepole pine dwarf mistletoe (Arceuthobium americanum) (Figure 1) parasitizes lodgepole pine trees throughout the interior of British Columbia, resulting in suppressed growth, decreased wood quality, reduced seed crop production and possible infection points for fungi. Losses attributed to dwarf mistletoes in British Columbia are estimated to be 3.7 million m3 per year.

Management of dwarf mistletoes has been attempted through chemical, biological and genetic means, however success has been limited. Silvicultural control is the most effective treatment because dwarf mistletoes are obligate parasites, very host specific, have relatively slow rates of spread and infections are easily identified by the presence of witches' brooms. Large clearcuts reduce dwarf mistletoe spread into the regenerating stand because the edge to interior ratio is maximized. As a result of the British Columbia Forest Practices Code however, there is a move to reduced cut block size and partial cutting systems. This will result in increased dwarf mistletoe infection from adjacent stands and infected overstory trees.

Inundative biological control using endemic fungal parasites of lodgepole pine dwarf mistletoe is being studied as an alternative method of control. Mass culturing and application of the selected fungal parasite may result in mistletoe mortality and a reduction in seed production, reducing dwarf mistletoe in the stand. For biological control to be effective, it must result in a significant reduction in seed production for a period of time that is long enough for the regenerating conifers to grow above the mistletoe infections that were treated and therefore escape infection.

Figure 1

1. Witches' brooms on infected lodgepole pine.
2. Female infection.
3. Male Infection.
4. Range of lodgepole pine dwarf mistletoe in BC.

* from Dwarf Mistletoe Guidebook, Forest Practices Code of British Columbia, July 1995, BC Ministry of Forests

Objective

The objective of this research is to develop an inundative biological control strategy for lodgepole pine dwarf mistletoe. This will be conducted by surveying the province of British Columbia and identifying fungi that are parasites of lodgepole pine dwarf mistletoe, observing the symptoms they cause and assessing their efficacy as biological control agents through field application.

Survey

During the summer of 1998, fungi were collected throughout the province and two of the major fungal parasites of lodgepole pine dwarf mistletoe that are reported in the literature, Caliciopsis arceuthobii and Colletotrichum gloeosporioides, were found to be present in British Columbia (Figure 2).

Figure 2. Distribution of fungal parasites surveyed in the summer of 1998.

Caliciopsis arceuthobii

Caliciopsis arceuthobii (syn. Wallrothiella arceuthobii) attacks the female flowers of A. americanum. When the flowers are pollinated, they are inoculated with fungal ascospores by the pollinating insect. The fungus grows within the developing seed and the seed tissue is replaced with fungal tissue, preventing seed development. The fungus is recognized by black perithecia present on the female flowers (Figure 3). This fungus is very difficult to culture and grows very slowly.

Figure 3. Perithecia of Caliciopsis arceuthobii on female flower of A. americanum.

Colletotrichum gloeosporioides

Colletotrichum gloeosporioides attacks both male and female shoots and has been reported to infect the mistletoe endophytic system. The fungus causes dark brown / black lesions at the shoot nodes, which coalesce over time and cause blistering of the epidermis to reveal black spore masses (Figure 4, 6). Under moist conditions, conidia are exuded from acervuli (Figure 5). All portions of the plant can be infected, including the fruit (Figure 6), and lesions are produced 2 to 3 weeks after infection. The fungus can be easily cultured, grows rapidly and conidia are produced in culture (Figure 6).

Figure 4. Colletotrichum gloeosporioides lesion on A. americanum.

Figure 5. Colletotrichum gloeosporioides conidia exuded from acervuli under moist conditions.

Figure 6.Colletotrichum gloeosporioides

A. Lesions on shoot. B. Acervuli formation on shoot under moist conditions.  C. Acervuli formation on fruit under moist conditions.  D. Conidia  E. Appresoria formed in culture.  F.  Culture on malt extract agar.

Future Research

Future research will focus on Colletotrichum gloeosporioides as a biological control agent for lodgepole pine dwarf mistletoe. This fungus was chosen because it is distributed throughout the range of lodgepole pine dwarf mistletoe in British Columbia, is easily cultured, conidia are produced in culture, both male and female plants are infected, infection of the mistletoe is rapid and it has been reported to infect the endophytic system. Infection of the endophytic system may increase the period of efficacy by causing shoot regeneration and seed production to be slowed compared to fungi that attack only the aerial shoots.

Studies to be initiated in the summer of 1999 will focus on the effects of Colletotrichum on the aerial shoots and endophytic system by inoculating dwarf mistletoe infections in the field and observing the results. Destructive sampling of some of the inoculated infections will be conducted to determine the extent and mode of penetration of Colletotrichum into the endophytic system. Other inoculated infections will be observed to determine how the dwarf mistletoe responds to infection by Colletotrichum and to answer questions regarding new shoot formation and seed production.

It is hoped that this study will provide a tool for foresters to reduce the effects of lodgepole pine dwarf mistletoe in regenerating stands.

Acknowledgments

This work is being supported by MycoLogic Inc. and the Science Council of British Columbia.

Other Publications by T.D. Ramsfield

Other Publications by S.F. Shamoun

Other publications by B.J. van der Kamp

Other Publications on dwarf mistletoe

[ Abstract | Introduction | Objective | Survey | Caliciopsis arceuthobii ]
[ Colletotrichum gloeosporioides | Future Research | Acknowledgments ]
[ Other Publications ]