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Seed fungus

This seed-borne pathogen was first reported in Ontario bareroot nurseries in 1964 on fall-sown pine seeds which had failed to germinate. It was next identified as an important pathogen in Britain on Sitka spruce seeds imported from western North America. Based on its asexual spore stage, the fungus was named Geniculodendron pyriforme. Subsequently, however,Caloscypha fulgens (Figure 13) was found to be the fungus' sexual state, and taxonomically this name has priority.

Hosts and damage

In British Columbia, the pathogen has been found in about 25% of all stored spruce seed (Sitka, Engelmann, white, and Engelmann x white hybrid) and some Douglas-fir and grand fir seedlots. The fungus is pathogenic to seeds of these (Figure 14) and several other conifers. Although exact losses have not been determined for local nurseries, the fungus causes significant pre-emergence losses in Ontario and British bareroot nurseries. In British Columbia, experiments have shown that losses of container-sown Sitka spruce seeds increase with prolonged, cool, post-sowing temperatures. However, losses in container nurseries should be less severe because intercavity spread by the fungus would be difficult.

Since diseased seeds do not germinate, the disease has no above-ground symptoms. Thus, a cursory diagnosis might not differentiate this problem from poor seed viability or pre-emergence damping-off. The contents of C. fulgens-killed seeds, however, are firm and mummified, whereas seeds destroyed by damping-off have rotten contents.

Life history (Figure 15)

Caloscypha fulgens inhabits forest duff, and consequently, cones picked from the ground - especially from squirrel caches - may contain diseased seeds. Incidence of diseased seeds depends upon several factors, especially the length of time cones are on the ground. Because the pathogen is known to spread during stratification (Figure 16) and in seedbeds, or within individual container cavities, low levels of diseased seeds are also important. Seeds of some species likely escape fungus infection because their cones are collected directly from trees or, as in the case of lodgepole pine, the cones are tightly closed. Neither sexual nor asexual spores play any apparent role in inoculation; rather, they probably serve only in fungus dissemination.

Within infested seedlots, 1-35% of the seeds are infected and it is from these seeds that the fungus spreads during stratification. The problem can intensify further if moist, stratified seeds are cold-stored at the nursery prior to sowing. Additional spreading and killing can occur following seed sowing, particularly during prolonged periods of cool, wet weather.

Management

Sowing non-infested seeds provides the most effective means of prevention, but this is not always practical. Within infested seedlots, fungus spread is confined to the post-storage period because very low storage temperature and moisture prevent C. fulgens growth. Optimum temperature for growth of the pathogen is 20C; thus, stratifying seeds at the lowest temperature possible (e.g., 1 or 2C) helps to hinder pathogen growth. Shortening the stratification period also reduces, but does not eliminate, disease risk because even at 1 or 2C, appreciable fungus growth occurs after 3-4 weeks. Compared to stratification in sand or other media, fungus spread is less when seeds are naked-stratified. Since the fungus inhabits forest duff, such material should not be added to seedbeds (e.g., to provide mycorrhizal inoculum) without first ensuring that it is free of the pathogen.

In Britain, adding a fungicide to the water used to soak seeds prior to chilling has reduced pathogen spread during stratification. In Ontario and Britain, applying a fungicide to seeds before sowing has also been beneficial. Because the pathogen attacks only dormant seeds, infested seedlots should not be sowed until soil or growing medium temperatures are high enough for rapid germination. This applies to bareroot and container operations, especially when seedlots do not receive a pre-sowing fungicide dusting, or when container cavities are multiple sown to compensate for low seed viability.

Selected References

Salt, G.A. 1974. Etiology and morphology of Geniculodendron pyriforme gen. et sp. nov., a pathogen of conifer seeds. Trans. Brit. Mycol. Soc. 63: 339-351.

Sutherland, J.R. 1979. The pathogenic fungus Caloscypha fulgens in stored conifer seeds in British Columbia and relation of its incidence to ground and squirrel-cache collected cones. Can. J. For. Res. 9: 129-132.

Thomson A.J., J.R. Sutherland, T.A.D. Woods, and S.M. Moncrieff. 1983. Evaluation of seed disease effects in container-sown Sitka spruce. For. Sci. 29: 59-65.

Look Alikes

Other Fungi

Insects

Environmental

Penicillium
Sirococcus blight

Several seed insects

Heat
Chemical burn

Summary

Seed Fungus

Principal, locally grown hosts

Host age and season when damage appears

 

Nursery type and location

 

 

 

     

Bareroot

 

Container

 
 

Age

Season

Coastal

Interior

Coastal

Interior

Grand fir, all spruces, Douglas-fir

Seed

Spring to early summer

Yes

Yes

Yes

Yes


Figures

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Click on this image to see a larger version Figure 13. Perfect (sexual) state of the seed fungus.

 

 

 


  Click on this image to see a larger versionFigure 14. Whitish, crust-like mycelium of the seed fungus on Engelmann spruce seeds.

 

 

 


Click on this image to see a larger version Figure 15. Life history of the seed fungus.

 

 

 

 

 

 

 

 Click on this image to see a larger versionFigure 16. Mycelium of the seed fungus growing on stratified Sitka spruce seeds.