Cause A fungus, Cladosporium variable. 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 in seed-crop crucifers. 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.
- 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