Cause Erysiphe euonymicola (formerly Oidium euonymi-japonici), a fungus that occurs only on Euonymus japonica and is found wherever the host grows. The green cultivar Patens is very susceptible. In the past, the fungus was rarely observed on variegated cultivars but now is much more common. The fungus overwinters as mycelia and spores on infected leaves and stems. Airborne spores are spread to newly emerging leaves in the spring. Infection is favored by temperatures of 70°F to 80°F and relative humidity between 85% and 100%. Free water on leaf surfaces will often suppress spore germination.
Symptoms The disease first appears on leaves and tender stems as small, scattered white patches. These are generally first seen on the new growth. A thick felt-like mat forms on both sides of leaves, causing some yellowing and defoliation.
- Water forcibly sprayed on both sides of leaves under fast-drying conditions.
- Remove and destroy heavily infected plant parts.
- Improve air circulation and reduce humidity around plants.
Chemical control Fungicides will do best when used before symptoms develop. Few materials have good eradicant activity. Use at 7- to 14-day intervals; using shorter intervals when environmental conditions favor disease development. Alternate or tank-mix products from different groups that have different modes of action.
- 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.
- Banner MAXX at 5 to 8 fl oz/100 gal water may be effective. Group 3 fungicide. 12-hr reentry.
- Bayleton 50 T&O at 5.5 oz/275 to 550 gal water. Landscape only, not for use on plants for sale. Group 3 fungicide. 12-hr reentry.
- Bicarbonate-based products. Might be used to supplement a normal program when powdery mildew is first observed. Do not mix with acidifying agents. Thorough coverage is essential. Addition of a surfactant will be helpful. O
- Armicarb "O" (85% potassium bicarbonate) at 2.5 to 5 lb/100 water. 4-hr reentry.
- MilStop (85% potassium bicarbonate) at 2.5 to 5 lb/A. Oregon and Washington only. 1-hr reentry.
- Monterey Bi-Carb Old Fashioned Fungicide is registered for home use in all states. H
- Compass O 50 WDG at 1 to 2 oz/100 gal water. Do not use organosilicate additives. Group 11 fungicide. 12-hr reentry.
- Cygnus 50 WG at 1.6 to 3.2 oz/ 100 gal water plus a non-organosilicone spreader-sticker. Best used before symptoms develop. Group 11 fungicide. 12-hr reentry.
- Eagle 20 EW at 6 to 12 fl oz/100 gal water. Group 3 fungicide. 24-hr reentry.
- Heritage at 1 to 4 oz/100 gal water plus a non-silicone-based wetter sticker. 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. Group 3 fungicide. 24-hr reentry.
- Ortho RosePride Disease Control at 1 Tbsp/gal water. H
- Pageant at 6 to 12 oz/100 gal water. Do not use more than two (2) consecutive applications before switching to a different fungicide group. Do not use on Euonymus vegetus as injury may occur. Group 7 + 11 fungicide. 12-hr reentry.
- Rubigan AS at 6 to 10 oz/100 gal water. Group 3 fungicide. 12-hr reentry.
- Spectracide Immunox at 1 fl oz/gal water. H
- Strike Plus 50 WDG at 3 to 9 oz/100 gal water. Group 3 + 11 fungicide. 12-hr reentry.
- Terraguard SC at 4 to 8 fl oz/100 gal water. Although a 16 oz/100 gal water rate can be used for initial applications to existing infections, use only in conjunction with a regular scouting program that detects initial symptom development. Oregon Only. Group 3 fungicide. 12-hr reentry.
- Tourney 50 WDG at 1 to 4 oz/100 gal water. Group 3 fungicide. 12-hr reentry.
- Trinity at 4 to 12 fl oz/100 gal water. Group 3 fungicide. 12-hr reentry.
Note Do not use Insignia on Euonymus vegetus as injury may occur.
References Horst, R.K. 1990. Westcott's Plant Disease Handbook, 5th ed. Reinhold, NY: VanNostrand.
Ziv, O., and Hagiladi, A. 1993. Controlling powdery mildew in euonymus with polymer coatings and bicarbonate solutions. HortScience 28:124-126.