Cause A fungus, Cladosporium variable, is believed to be the primary pathogen. This fungus can grow under a wide range of temperatures, but 59°F to 68°F with relative humidity above 80% is most conducive to disease development. Spores can be moved by air, rain splash, or equipment.
Symptoms Early symptoms are numerous small, circular white to yellow spots, beginning on older leaves and progressing to younger ones. Spots often join together and then are irregularly shaped. When the fungus produces spores, spots change to an olive-black. Generally, older leaves die, but in severe infections all leaves may die and seed may be affected. Ripening seed on Swiss chard seed crops also may develop lesions and become shriveled.
Chemical control Fungicides used preventively help minimize the onset of infection and disease development.
- Bravo Weather Stik (Group M5) at 3 pints/A on 7- to 14-day intervals. For Swiss chard seed crops only in Oregon (SLN OR-200014) and Washington only (SLN WA-190001). 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 (Group M3) are labeled.
- 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.
- Dithane F45 Rainshield at 1.5 quarts/A on 7- to 10-day intervals can be used on seed crops only. Oregon only (SLN OR-090016). No portion of treated seed crop may be used or distributed for food or feed for 1 year (365 days) after the last application. 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.
- 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