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Poplar (Populus spp.) - Leaf Rust
Cause The fungus Melampsora occidentalis is the most common cause of needle rust in the Pacific Northwest. The alternate host is black cottonwood and other poplars. The aecial stage is found on the current-year needles. Urediniospores and teliospores are on the alternate host, poplar.
Symptoms Yellow pustules in late spring or early summer on the lower needle surface. The area above the pustule, on the upper surface, becomes light green. In summer, golden, yellow, or orange powdery pustules (uredinia) appear on the cottonwood leaf undersurface. In late summer and autumn, small slightly raised areas or crusts appear. At first orange-yellow, they change to dark brown or black. These are telia, the stage that overwinters on fallen leaves, infecting the alternate host (Douglas-fir) the next spring. Under severe disease pressure, reddish brown sunken cankers form on the fir stems of current-season growth. These cankers can girdle the stems, resulting in dieback.
Cultural control Remove black cottonwood and other susceptible poplars from the vicinity of Christmas tree plantations.
Chemical control One (1) application when new shoots are 1 to 1.5 inches long.
- 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.
- Eagle 20 EW at 12 to 18 fl oz/100 gal water plus a spray adjuvant. The same product, Rally 40 WSP, also is registered for nursery use only. Group 3 fungicide. 24-hr reentry.
- Heritage at 3.2 to 8 oz/100 gal water plus a non-silicone-based wetter sticker. Registered for conifers including Christmas trees. Group 11 fungicide. 4-hr reentry.
- Myclobutanil 20 EW T&O at 6 to 12 fl oz/100 gal water plus spreading agent. May observe a PGR effect. Registered for landscape, greenhouse, and nursery ornamentals. Group 3 fungicide. 24-hr reentry.
- Propiconazole-based fungicides. For nurseries and landscape plantings. Group 3 fungicides.
- Banner MAXX at 16 fl oz/100 gal water. 12-hr reentry.
- Infuse Systemic Disease Control at 2 Tbsp/gal water. H
- ProStar 70 WG at 3 to 6 oz/100 gal water. Registered for evergreen trees grown in greenhouse, or outdoor container, or field nursery. Group 7 fungicide. 12-hr reentry.
- Spectracide Immunox Multi-Purpose Fungicide Spray Concentrate for Gardens at 3 fl oz/gal water. Group 3 fungicide. H
- Terraguard SC at 2 to 8 fl oz/100 gal water is labeled for Christmas tree/conifer plantations. Group 3 fungicide. 12-hr reentry.
- Ziram 76 DF at 2 lb/100 gal water. Registered for conifers including Christmas trees. Group M3 fungicide. 48-hr reentry.
Reference Ziller, W.G. 1974. The Tree Rusts of Western Canada. Canadian Forestry Service Publication 1329.