Cause A fungus, Golovinomyces biocellatus (syn = Erysiphe biocellata, formerly Erysiphe cichoracearum; anamorph Oidium erysiphoides), which overwinters on mint, stubble, and many wild hosts. It is seldom serious enough on peppermint to warrant control measures; however, it is very destructive on Scotch spearmint. Interactions with some herbicides increase susceptibility to this disease.
Symptoms Gray, powdery fungus growth on leaves, then leaves yellow and drop.
Cultural control
- Plowing to cover all stubble reduces disease incidence and severity.
Chemical control
- Badge SC (Group M1) at 0.75 to 1.5 pints/A on 10-day intervals. Preharvest interval is 0 days. 24-hr reentry for greenhouse use; 48-hr reentry for all other applications.
- Bicarbonates are registered to control powdery mildew. 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. H 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
- Demethylation-inhibiting (DMI) Fungicides (Group 3) are labeled for use.
- Rally 40WSP at 4 to 5 oz/A on 14- to 21-day intervals. Do not apply within 30 days of harvest. 24-hr reentry.
- Tilt at 4 fl oz/A. Apply when plants are 2 to 4 inches high or when conditions become favorable for disease development. Make a second application 14 days later. Do not apply within 7 days for harvest. 12-hr reentry.
- JMS Stylet Oil at 1 to 2 gal/A. Do not spray if temperature is below 50°F, above 90°F or when plants are wet or under heat or moisture stress. 4-hr reentry. O
- Regalia (Group P5) at 1 to 4 quarts/A plus another fungicide on 7- to 10-day intervals. Does not benefit from the addition of an adjuvant. Preharvest interval is 0 days. 4-hr reentry. O
- Strobilurin fungicides (Group 11) are labeled for use. 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.
- Headline at 9 to 12 fl oz/A prior to disease development on 7- to 14-day intervals. Preharvest interval is 14 days. 12-hr reentry.
- Quadris Flowable at 6 to 15.5 fl oz/A on 7- to 10-day intervals. May be applied up to 7 days before harvest for processed mint and on the day of harvest for fresh mint. 4-hr reentry.
- Sulfur products. On Scotch spearmint, use wettable or flowable sulfur when plants are 5 to 6 inches tall or when the fungus first appears. Repeat application twice more at 3-week intervals. After harvest, repeat when mildew first appears or at four- to five-leaf stage; repeat at 30-day intervals as necessary to cover new growth or until growth stops in fall. Applying elemental sulfur increases sulfide concentration in mint oil; the length of time between sulfur application and harvest is more important than sulfur rates. Growers should have minimal problems with mint sulfide if they follow the rates and preharvest application interval as follows.
- Microthiol Disperss (80% sulfur) at 4 to 6 lb/A. Do not apply within 30 days of harvest. Do not apply if temperatures will exceed 90°F within 3 days after application. Do not use within 2 weeks of an oil treatment. 24-hr reentry. O
- Premixes of Group 3 + 11 fungicides are available for use.
- Quilt at 14 fl oz/A or Quilt Xcel at 10.5 to 14 fl oz/A. Apply when plants are 2 to 4 inches high or when conditions become favorable for disease development. Make a second application 14 days later. Do not apply within 7 days for harvest. 12-hr reentry.
Biological Control Efficacy unknown in Oregon.
- Ecoswing at 1.5 to 2 pints/A. Preharvest interval is 0 days. 4-hr reentry. O