Cause A fungus of undetermined identity has been observed inciting powdery mildew on comfrey (Symphytum officinale) in Oregon. The fungus Golovinomyces cynoglossi (syn. Erysiphe cynoglossi) has been reported on Symphytum asperum, S. bulbosum, S. caucasicum, S. cordatum, S. officinale, and other Symphytum species in Europe while Peronospora symphyti has been report on S. asperum in the UK, S. cordatum in eastern Europe, and S. officinale in Europe. Golovinomyces cynoglossi has been reported on other members in this plant family (Wood forget-me-not, Myosotis sylvatica; Navel Seed, Omphalodes cappadocica) in Washington, so it is possible that the powdery mildew fungus occurring on comfrey in Oregon is G. cynoglossi.
Symptoms White powdery patches develop on leaves. They start off as small, discrete patches and progess to large patches of white mycelium and conidia that may cover the entire leaf surface, and are prominent on the upper leaf surface.
Cultural control
- Space plants for good air circulation.
Chemical control Fungicides should be applied prior to disease spread or ideally, disease onset.
- Bicarbonates are registered to control powdery mildew and can be very effective. Thorough coverage is essential.
- Bi-Carb Old Fashioned Fungicide at 4 teaspoons/2 gal water on 7- to 10-day intervals. May be applied up to the day before harvest. HO
- Carb-O-Nator at 2.5 to 5 lb/100 gal of water on 5- to 14-day intervals, depending on disease pressure. Preharvest interval is zero (0) days for all listed crops. 4-hr reentry. O
- Kaligreen at 2.5 to 3 lb/A on 7- to 10-day intervals. May be applied up to the day before harvest. 4-hr reentry. O
- MilStop SP at 2 to 5 lb/A on 7- to 14-day intervals. Can be applied up to and including the day of harvest. 1-hr reentry. O
- Quadris Flowable (Group 11) at 6 to 15.5 fl oz/A is lableled for other diseases in herb crops and can be applied on 7-day intervals for powdery mildew control in Oregon. Do not make more than one (1) application of a Group 11 fungicide before alternating to a labeled fungicide with a different mode of action. Preharvest interval is 0 days. 4-hr reentry.
References Glawe, D. A. 2004. First report of powdery mildew of Myosotis sylvatica (wood forget-me-not) caused by Golovinomyces cynoglossi (Erysiphe cynoglossi) in Washington State. Online. Plant Health Progress doi:10.1094/PHP-2004-1124-01-HN.
Glawe, D. A., and Ammirati, J. F. 2006. First report of powdery mildew of Omphalodes cappadocica caused by Golovinomyces cynoglossi (Erysiphe cynoglossi) in North America. Online. Plant Health Progress doi:10.1094/PHP-2006-1127-03-BR.