Answer: The leaf spots are caused by a fungus, probably Cladosporium variable. This fungus can grow under a wide range of temperatures, but 59 to 68oF with relative humidity above 80% is most conducive to disease development. Spores can be moved by air, rain splash, or equipment. 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. Fungicides used preventively help minimize the onset of infection and disease development but won't cure the already infected leaves. If you use a fungicide now, the new leaves should be disease free. Best wishes with your Swiss chard!
Q&A Library Searching Tips