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Melampsora Foliage Rusts

Only two foliage rusts occur in British Columbia forest nurseries: conifer-aspen rust (Melampsora medusae) and conifer-cottonwood rust (M. occidentalis). They derive their common names from the fact that they require at least one coniferous and one Populus spp. Host to complete their life cycle. Since susceptible Populus are rarely grown in local forest nurseries, emphasis in this handbook is on how these diseases affect conifers. Both rusts occur predominantly in bareroot nurseries. Other foliage rusts, such as fir-willow rust (M. abieti-capraearum) and larch-willow rust (M. paradoxa), could be bothersome, especially if bareroot production of their coniferous hosts is increased.

Hosts and damage

Coniferous hosts for M. medusae are Douglas-fir, western larch, tamarack, and ponderosa and lodgepole pines; the Populus host is trembling aspen. Douglas-fir, black cottonwood, and balsam poplar are hosts for M. occidentalis. The rusts occur throughout the province wherever their hosts are present.

Both rusts produce yellow-orange, spore-producing pustules on foliage of their hosts (Figures 47-49). On coniferous needles or stems, these appear in late spring through mid-August and on Populus leaves in early summer to late fall. On the latter, the rust is most abundant on the underside of the leaf; corresponding chlorotic spots are present on the upper leaf surface. On conifers, symptoms are confined to the current year's foliage and often to the primary needles of rising 1+0 seedlings. Affected needles are usually killed and shed in the fall. Shoots of severely affected seedlings are killed. Disease intensity on individual seedlings and within nurseries is greatest near diseased Populus hosts. The scarcity of such trees near local nurseries probably accounts for the low level of Melampsora damage.

Life history (Figure 50)

Melampsora medusae and M. occidentalis both require their Populus and coniferous hosts to complete their life cycles. The rusts overwinter as teliospores on dead Populus leaves on the ground. These spores germinate in the spring, producing wind-borne basidiospores, which results in infection of coniferous foliage. About 2 weeks later (late spring), masses of yellow-orange aeciospores are produced on needles of the coniferous host. They serve as inoculum for infection of live Populus leaves during the summer. Approximately another 2 weeks later, urediniospores (in yellow-orange pustules) are produced on the Populus leaves. These spores serve as inoculum for rust spread and intensification on Populus throughout the summer. In late summer, teliospores (the overwintering spores) are again produced on Populus leaves, completing the rust's life cycle.

Management

Elimination of Populus hosts in the immediate vicinity of conifer nurseries usually gives adequate disease control. When Populus cannot be eliminated, and where feasible, the fallen leaves can be raked and destroyed to eliminate the overwintering spores - i.e., the inoculum for coniferous seedlings. If neither of these procedures is practical, germinants and new growth of 2+0 seedlings can be protected with fungicidal sprays.

Selected References

Hunt, R.S. 1978. Melampsora foliage rusts in British Columbia. Environ. Can., Pac. For. Res. Cent., For. Pest Leafl. 49, Victoria, B.C.

Ziller, W.G. 1974. The tree rusts of western Canada. Dept. Environ. Can. For. Serv., Publ. No. 1329. Ottawa, Ont.

Look Alikes

Other Fungi

Insects

Environmental

 

 

Nutrient Precipitates

Summary

Melampsora Foliage Rusts

Principal, locally grown hosts

Host age and season when damage appears

 

Nursery type and location

 

 

 

     

Bareroot

 

Container

 
 

Age

Season

Coastal

Interior

Coastal

Interior

Douglas-fir, all hard pines, all larches

1+0
2+0

Early to mid-summer

Yes

Yes

Yes

Yes


Figures

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Click on this image to see a larger version     Figure 47. Aecia of Melampsora occidentalis on Douglas-fir needles.

 

 

 

 

 


Click on this image to see a larger version     Figure 48. Aecia of Melampsora medusae on lodgepole pine seedling.

 

 

 

 

 


Click on this image to see a larger version     Figure 49. Uredinia of Melampsora occidentalis on black cottonwood leaves.

 

 

 

 


Click on this image to see a larger version     Figure 50. Life history of Melampsora foliage rusts.