Cause The fungus Plagiostoma fraxini (asexual: Discula fraxinea) has been identified in the Pacific Northwest. The pathogen overwinters on blighted twigs and petioles. Spore spread and infection occurs during wet weather in spring. More infections and larger symptoms can occur if wet weather persists. New infections and lesion growth are prevented when the weather turns hot and dry. Young, developing shoots and leaves are most susceptible. Acervuli with viable spores can be found on dead petioles for up to a year.
Symptoms Small water-soaked spots on young tissues coalesce rapidly in spring. Young lesions may be greenish brown to dark brown and may girdle the leaf rachis. Can be similar to frost injury. The end result is discrete brown or tan blotches on distorted leaves. Elliptical to round acervuli can be found with a hand lens in necrotic tissue. Shoots also may be blighted. Severe infections can cause some defoliation or dieback in lower limbs. Most ash are susceptible, but green ash is relatively resistant.
- Rake and destroy fallen leaves.
- Prune out dead twigs and branches if practical.
- Begin foliar sprays at budbreak and continue at regular intervals during wet 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.
- Bonide Fung-onil Multi-purpose Fungicide at 2.25 teaspoons/gal water. Group M5 fungicide. H
- Cleary's 3336 EG at 12 to 16 oz/100 gal water. Group 1 fungicide. 12-hr reentry.
- Daconil Weather Stik at 1.38 pints/100 gal water. Group M5 fungicide. 12-hr reentry.
- Infuse Systemic Disease Control at 3 Tbsp/gal water. Group 3 fungicide. H
- Mancozeb-based products. 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.
- OHP 6672 4.5 F at 10.75 to 20 fl oz/100 gal water. Group 1 fungicide. 12-hr reentry.
- Spectro 90 WDG at 1 to 2 lb/100 gal water. Group 1 + M5 fungicide. 12-hr reentry.
- Tourney EZ at 1 to 4 oz/100 gal water. Group 3 fungicide. 12-hr reentry.
- Trigo at 3 to 9 oz/100 gal water. Group 3 + 11 fungicide. 12-hr reentry.
- A few chemicals are registered for tree injection. Do not inject trees less than 2 inches in diameter or that are suffering from various stresses.
- Fungisol for fall application only. Washington and Idaho only.
- Tebuject 16 capsules used at a 6-inch spacing at the base of the tree.
Note: Only mancozeb products have been registered for use in British Columbia. Some registered products offer only suppression of this disease and thus are not recommended for use. These products include Phospho-Jet.
Reference Rossman, A.Y., Castlebury, L.A., and Putnam, M.L. 2004. First report of ash anthracnose caused by Discula fraxinea in Oregon. Plant Disease 88:222.