Cause The fungi Cladosporium sp., Coniothyrium sp., Phyllosticta magnoliae, and Septoria sp. have been found by the OSU Plant Clinic in leaf spots of magnolia. Phyllosticta and Cladosporium were found most frequently. The bacterium Pseudomonas syringae pv. syringae also is often encountered as a leaf-spotting organism. Many of the leaf-spotting fungi survive on infected leaves and plants. Cool, wet, rainy spring conditions common west of the Cascade Range are favorable for numerous leaf-spotting organisms.
Symptoms Phyllosticta leaf spot first appears as tiny purple to black spots on the upper surfaces of leaves. As the spots increase in size, the center turns off-white while the border remains purple to almost black. Spots may also be surrounded by a faint yellow halo. Small, black fruiting bodies (pycnidia) appear in the center of larger leaf spots.
- Collect cuttings for propagation from pathogen-free trees.
- Avoid overhead irrigation that keeps foliage wet for extended periods.
- Space plants for good air circulation especially at budbreak.
- Remove and destroy fallen leaves.
Chemical control Important during early shoot growth for fungal leaf spots.
- Banner MAXX at 5 to 8 fl oz/100 gal water. Group 3 fungicide. 12-hr reentry.
- Bonide Fung-onil Multi-purpose Fungicide at 2.25 teaspoons/gal water. H
- Concert at 22 to 35 fl oz/100 gal water. May cause injury to buds, blooms or tender new growth. Landscape use only. Group 3 + M5 fungicide. 12-hr reentry.
- Daconil Weather Stik at 1.4 pints/100 gal water. Group M5 fungicide. 12-hr reentry.
- Heritage at 1 to 4 oz/100 gal water plus a non-silicone-based wetter sticker. Group 11 fungicide. 4-hr reentry.
- Junction at 1.5 to 3.5 lb/A. May be useful if the cause of the leaf spot is unknown. Group M1 + M3 fungicide. 48-hr reentry.
- Mancozeb-based products can be used as mixing partners and provide some protection. Group M3 fungicides. 24-hr reentry.
- Fore 80 WP at 1.5 lb/100 gal water plus a spreader-sticker.
- Protect DF at 1 to 2 lb/100 gal water plus 2 to 4 oz spreader-sticker.
- Monterey Liqui-Cop at 3 Tbsp/gal water. H
- Spectro 90 WDG at 1 to 2 lb/100 gal water. Group 1 + M5 fungicide. 12-hr reentry.
- Tourney 50 WDG at 1 to 4 oz/100 gal water. Group 3 fungicide. 12-hr reentry.
Reference Hagan, A.K. Magnolia diseases. In: R.K. Jones, and D.M. Benson, (eds.) 2001. Diseases of Woody Ornamentals and Trees in Nurseries. St. Paul, MN: APS Press.