See:
Willow (Salix spp.) - Marssonina Leaf and Twig Spot
Cause Several different fungi can cause leaf spots on willow including Cylindrosporium salicinum, Drepanopeziza salicis (formerly Pseudopeziza salicis), Marssonina spp., Rhytisma salicinum (tar spot), and Septogloeum salicis-fendlerianae. These were all considered common on willow in Washington. The leaf spots produced by these various fungi are difficult to distinguish from one another without the aid of a microscope. These fungi survive in old infected leaf debris and conditions that keep foliage wet for extended periods generally favor the fungi.
Symptoms
Tar spot - Black, tar-like thickened spots are produced on the upper leaf surfaces. The surface of the spot is smooth and shiny early in the season, bur becomes dull black and ridged in the fall as fruiting bodies are developed within the spot. These fruiting bodies mature the next spring on the fallen leaves on the ground.
Cultural control
- Rake and destroy fallen leaves.
- Avoid overhead irrigation or any practice that keeps plants wet for extended periods of time.
Chemical control These may be useful when used during early spring growth before rainy weather.
- 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.
- Protect DF at 1 to 2 lb/100 gal water plus 2 to 4 oz spreader-sticker. Group M3 fungicides. 24-hr reentry.
- Spectracide Immunox Multi-Purpose Fungicide Spray Concentrate for Gardens at 1 fl oz/gal water. Group 3 fungicide. H
Reference Shaw, C.G., and Harris, M.R. 1960. Important diseases and decays of trees native to Washington. Ag Extension Service, WSU. Extension Bulletin 540.