Cause The fungus-like microorganism, Peronospora farinosa f. sp. betae, causes disease on beet as well. The microorganism survives in infected crop residues and in wild or volunteer Beta spp. It also can survive in seed to a limited extent. Optimal temperatures for infection are 45°F to 59°F; little infection occurs above 68°F.
Symptoms All aboveground plant parts may be affected. Large, light green leaf spots develop on the upper leaf surfaces. Under moist conditions, a white to gray mold appears on the lower or upper leaf surfaces. Affected leaves may wilt, then die. If environmental conditions become unfavorable for downy mildew, plants may show recovery by developing healthy leaves, or other pathogens may be encouraged. During fall wet periods, most young petioles or leaves are covered with mycelial growth that can grow into the crown. Crown infections may permit other root-decay organisms to invade the plant.
- Increase plant spacing to provide better conditions for drying of foliage after rains
- Avoid excess nitrogen as leaves will be more easily invaded.
Chemical control Although fungicide applications don't protect perfectly, they are useful when disease conditions are severe.
- Copper formulations are not recommended as stand-alone materials.
- Cueva at 0.5 to 2 gal/100 gal water on 7- to 10-day intervals. May be applied on the day of harvest. 4-hr reentry. O
- Curzate 60 DF at 3.2 oz/A is available for use on Swiss chard seed crops in Oregon (SLN OR-150004) and Washington only (SLN WA-200001). Use only in combination with another registered protectant fungicide. 12-hr reentry.
- LifeGard WG (Group P6) at 1 to 4.5 oz/A on 7- to 14-day intervals for activating plant resistance. Refer to label for appropriate rate per application volume. Preharvest interval is 0 days. 4-hr reentry. O
- Mancozeb formulations are labeled for other diseases on chard and will aid with controlling downy mildew.
- Dithane DF Rainshield at 2 lb/A on 7- to 10-day intervals can be used on seed crops only. Washington and Oregon only (SLN WA-020028, SLN OR-020030). 24-hr reentry.
- Roper DF Rainshield at 2 lb/A on 7- to 10-day intervals can be used on seed crops only. Oregon only (SLN OR-130003). 24-hr reentry.
- Phosphonates (Group P7) should be quite effective.
- Fosphite at 1 to 3 quarts/A on 14- to 21-day intervals. See label for interval time requirement with copper-based products. May be applied up to the day of harvest. 4-hr reentry.
- Fungi-phite at 1 to 3 quarts/A on 7- to 14-day intervals. See label for interval time requirement with copper-based products. 4-hr reentry.
- Rampart at 1 to 4 quarts/A on 14- to 21-day intervals. See label for interval time requirement with copper-based products. May be applied up to the day of harvest. 4-hr reentry.
- Regalia (Group P5) at 0.5 to 4 quarts/A plus another fungicide on 5- to 10-day intervals. Does not benefit from the addition of an adjuvant. Preharvest interval is 0 days. 4-hr reentry. O
- 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. 4-hr reentry. O
- Actinovate AG at 3 to 12 oz/A as a foliar spray on 7- to 14-day intervals. 1-hr reentry. O
- Actinovate Lawn & Garden at 0.5 to 1 teaspoon/gal water. H O
- Romeo at 0.45 to 0.68 lb/A on 7- to 10-day intervals starting prior to infection. Preharvest interval is 0 days. 4-hr reentry. O
- Sonata at 2 to 4 quarts/A on 7- to 14-day intervals. Efficacy is unknown in the Pacific Northwest. Can be applied up to and on the day of harvest. 4-hr reentry. O
- Stargus at 6 to 8 fl oz per 1,000 ft row as an in-furrow treatment, 3 to 4 quarts/A as a soil drench (drip or chemigation) on 10- to 21-day intervals, or 2 to 4 quarts/A as a foliar spray on 7- to 10-day intervals. Preharvest interval is 0 days. 4-hr reentry. O
Reference Kioke, S.T., Gladders, P., and Paulus, A.O. 2007. Vegetable Diseases: A Color Handbook. Academic Press. 448 pp.