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Cause Melampsora spp., fungi that overwinter on dead leaves and young infected twigs. Several species have been reported from all states including M. abieti-capraearum (with true fir, Abies spp., as the alternate host); M. epitea f. sp. tsugae (with hemlock as the alternate host); M. paradoxa (with larch as the alternate host); and M. ribesii-purpureae (with currant and gooseberry as the alternate hosts). Molecular evidence suggests that many more species could be described based on many factors including Salix sp. infected. The urediniospores can reinfect willow any time conditions are favorable. It can take two weeks to go from urediniospore infection to formation of new urediniospores. Telia are produced in late summer and fall. In the spring teliospores produce basidiospores that infect the alternate hosts. Spores produced on the alternate hosts will also infect willow but not the alternate hosts.
Symptoms These species cause indistinguishable yellowish leaf spots on willow. The rusty spots (uredinia) appear on leaves, later turning brown or black (telia). Seriously infected leaves fall to the ground. The disease is more noticeable in August and September.
Symptoms on alternate hosts vary. True fir and hemlock develop orange-yellow, elongate aecia on current-year needles. Larch needles generally are lightly infected but are prematurely cast. Yellow leaf spots occur on currants and gooseberries.
Cultural control
- Collect and destroy infected leaves when they fall.
- If possible, remove the alternate host from the vicinity.
Chemical control
- 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 6 to 12 fl oz/100 gal water. Group 3 fungicide. 24-hr reentry.
- Myclobutanil 20 EW T&O at 6 to 12 fl oz/100 gal water plus spreading agent. May observe a PGR effect. Group 3 fungicide. 24-hr reentry.
- Spectracide Immunox Multi-Purpose Fungicide Spray Concentrate for Gardens at 1 fl oz/gal water. Group 3 fungicide. H
Reference Bennett, C., Aime, M. C., and Newcombe, G. 2011. Molecular and pathogenic variation within Melampsora on Salix in western North America reveals numerous cryptic species. Mycologia, 103:1004-1018.
Pei, M.H., and McCracken, A.R. 2005. Rust Diseases of Willow and Poplar. CABI publishing. pp264.