By O. Neher and C. M. Ocamb
Cause The fungus, Erysiphe betae (syn = E. polygoni, Microsphaera betae), affects plants in the Polygonaceae. The disease has been widespread in several Western States since 1974. Fungal spores may blow in from overwintering sources. Overwintered infected seed beets may also be sources of infections. The fungus tolerates a wide range of environmental conditions.
Symptoms Small, discrete, white patches on both leaf surfaces. Patches coalesce until white mycelium and spores cover the entire leaf. Leaves later become yellow, then purplish brown, and finally necrotic.
Chemical control Begin applying when mildew is first observed. If necessary, repeat application to maintain 10% or less leaf area diseased throughout the growing season. Sulfur dust provides superior control to liquid applications and is the preferred treatment. Research in south-central Idaho indicates it is economically feasible to apply sulfur to fields developing initial powdery mildew infections up to 5 weeks before harvest (or about September 1 in that area).
- Copper formulations (Group M1) are registered for use but show poor efficacy and should not be used as stand-alone products.
- Badge SC at 1 to 4 pints/A on 10- to 14-day intervals. Do not tank mix with glyphosate or apply glyphosate within 7 to 10 days of Badge SC application. Preharvest interval is 0 days. 48-hr reentry.
- 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 at 7- to 10-day intervals. Might supplement a normal program when powdery mildew is first observed. 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.
- Demethylation-inhibiting (DMI) fungicides (Group 3) are labeled for use. Do not make more than one (1) application before alternating to a labeled fungicide with a different mode of action.
- Eminent VP at 8 to 13 fl oz/A. Do not apply within 14 days of harvest. 12-hr reentry.
- Inspire XT at 7 fl oz/A on 10- to 21-day intervals. Preharvest interval is 21 days. 12-hr reentry.
- Proline 480 SC at 5 to 5.7 fl oz/A on 14- to 21-day intervals. Preharvest interval is 7 days. 12-hr reentry.
- Tilt at 4 fl oz/A on 10- to 14-day intervals. Preharvest interval is 21 days. 12-hr reentry.
- Topaz (Group 3) at 4 fl oz/A on 10- to 14-day intervals. Preharvest interval is 21 days. 12-hr reentry.
- Topguard at 10 to 14 fl oz/A on 14-day intervals. Preharvest interval is 21 days. 12-hr reentry.
- Strobilurin fungicides (Group 11) are labeled for use but Erysiphe betae has developed resistance to Group 11 fungicides. 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. Preharvest interval is 7 days. 12-hr reentry.
- Quadris Flowable at 9 to 15.5 fl oz/A on 7- to 14-day intervals. May be applied the day of harvest. 4-hr reentry.
- Sulfur formulations (Group M2) Sulfur is manufactured under many trade names, and a few are listed here.
- Dusting Sulfur (98% sulfur) at 35 to 50 lb/A each application. For Oregon only.
- Micronized sulfur (92% sulfur) at 3 to 40 lb/A on 3- to 4-week intervals. For good coverage add a nonionic surfactant at about 0.125 to 0.25 % (volume by volume). It is recommended to use 5 lbs/A in at least 20 gallons/A as a tank-mix partner to help with the prevention of fungicide resistance. 24-hr reentry. O
- Sulfur 6L (52% sulfur) at 0.66 gal/A at 18-day intervals. 24-hr reentry.
- Thiolux (80% sulfur) at 10 to 20 lb/A on 10- to 30-day intervals. 24-hr reentry.
- Switch 62.5 WG (Group 9 + 12) at 11 to 14 oz/A on 7- to 10-day intervals. No more than two (2) sequential applications alternating with another fungicide with a different mode of action for two (2) applications. Do not allow cattle or other livestock to feed upon leaves of vegetables. Do not apply within 7 days of harvest. 12-hr reentry
- Topsin M WSB (Group 1) at 0.5 to 1 lb/A. Do not use alone; use another non-benzimidazole fungicide with Topsin M WSB for disease resistance management. Resistant strains can persist for many years. It is preferable to not apply more than once during the season. Make alternating applications with a non-benzimidazole fungicide. Do not apply within 21 days of harvest. 24-hr reentry.
- Trilogy at 0.5% to 1%. Not labeled for use in Oregon. Do not use above 90°F or when plants are under heat or moisture stress. Do not use when foliage is wet as good coverage is essential. Poor control as a stand-alone product. 4-hr reentry. O
- Vertisan (Group 7) at 10 to 30 fl oz/A on 7- to 14-day intervals. Do not make more than two (2) sequential applications before alternating to a labeled fungicide with a different mode of action (non-Group 7). Preharvest interval is 7 days; 0 days for forage and hay. 12-hr reentry.
- Premixes of Group 7 + 11 fungicides are available for use. Do not make more than three (3) applications per season.
- Priaxor Xemium Brand at 4 to 8 fl oz/A. Preharvest interval is 7 days. 12-hr reentry.
Biological control
- Ecoswing at 1.5 to 2 pints/A. Preharvest interval is 0 days. 4-hr reentry. O
References Bolton, M.D., and Neher, O.T. 2014. First report of QoI-insensitive powdery mildew (Erysiphe polygoni) on sugarbeet in the United States. Plant Disease 98(7):1004.
Braun, U. 1995. The Powdery Mildews (Erysiphales) of Europe. Gustav Fischer, New York, 377 pp.
Harveson, R.M., Hanson, L.E., and Hein, G.L. 2009. Compendium of Beet Diseases and Insects. 2nd Edition. St. Paul, MN: APS Press.