Cause The fungus Colletotrichum fuscum has been reported from Oregon and is favored by warm wet weather. It overwinters in plant debris.
Symptoms Small, light, or purplish brown spots develop on leaves. Spots remain small, about 0.12 inch, with a purple margin and can be circular or angular. Numerous fruiting bodies (acervuli) of the fungus may be seen at the spots' centers. If seedlings are attacked, they may wilt and die.
Cultural control
- Rotate plantings from year to year.
- Treat seed with hot water at 131°F for 15 min.
- Avoid dense plant spacing.
- Optimal media pH is 5.8 to 6.2.
- Remove and destroy infected plants or plant parts if practical.
- Remove and destroy plant debris at the end of the growing season.
- Do not water from overhead, or adjust irrigation so plants are not wet for extended periods.
Chemical control Use in the spring on new growth just prior to wet weather.
- Heritage at 1 to 8 oz/100 gal water plus a non-silicone-based wetter sticker. Group 11 fungicide. 4-hr reentry.
- Insignia SC at 6 to 12 fl oz/100 gal water. Do not use with organosilicate-based adjuvants. Use preventively only. Group 11 fungicide. 12-hr reentry.
Reference Horst, R.K. 2013. Westcott's Plant Disease Handbook, 8th ed. Netherlands: Springer Science+Business Media.