See:
Poplar (Populus spp.) - Leaf Rust
Fir, Douglas (Pseudotsuga menziesii) - Rust
Cause Several species of Melampsora can affect different Populus species in the Pacific Northwest. M. medusae (formerly M. albertensis) affects aspen. Melampsora rust fungi overwinter as telia on fallen leaves. In spring, the telia produce basidiospores, which infect the current-season needles of conifer hosts. Most commonly the coniferous aecial host is Douglas-fir, but species of Larix (larch) also host poplar-infecting Melampsora. Yellow aecia develop on the needles, and the wind disperses aeciospores to nearby trees. Uredinia then develop on the aspen host. Humid or moist conditions favor urediniospore production. Wind can disperse urediniospores long distances. Under optimal conditions, Melampsora species may undergo multiple cycles on the uredinial host until leaf abscission. In late summer, telia begin to form in place of uredinia. Repeated early leaf loss due to rust can reduce growth and yield during the following growing season.
Symptoms In midsummer, small bright-yellow to orange pustules (uredinia) develop on the top, bottom, or both surfaces of leaves, depending on the species of Melampsora. An angular yellow area may be on the side of the leaf opposite the pustule. Heavy infections may cause defoliation and may lead to a premature, fall budbreak or predispose trees to other diseases and to cold damage. In autumn, black angular areas (telia) develop in infected tissue.
Cultural control
- Keep susceptible conifers 500 yards away from aspen stock.
Chemical control
Effective if applied shortly after rust appears.
- Armada 50 WDG at 3 to 9 oz/100 gal water. Do not use a silicone-based surfactant. Not for nursery or greenhouse use. Group 3 + 11 fungicide. 12-hr reentry.
- Bonide Fung-onil Multi-purpose Fungicide at 2.25 teaspoons/gal water. Group M5 fungicide. H
- Daconil Weather Stik at 1.38 pints/100 gal water. Group M5 fungicide. See special eye irritation provisions on the label. 12-hr reentry.
- Eagle 20 EW at 6 to 12 oz/100 gal water for nurseries or landscapes. Group 3 fungicides. 24-hr reentry.
- Propiconazole-based products. Group 3 fungicides.
- Banner MAXX at 16 fl oz/100 gal water. 12-hr reentry.
- Bonide Infuse Systemic Disease Control at 0.5 Tbsp/gal water. H
- Rally 40 WSP at 4 to 6 oz/A. Nursery or forest-area use only. Group 3 fungicides. 24-hr reentry.
References Newcombe, G., Stirling, B., and Bradshaw, Jr., H.D. 2001. Abundant pathogenic variation in the new hybrid rust population of Melampsora x columbiana on hybrid poplar. Phytopathology 91:981-985.