A: The bright, orange spots you see on the oxalis are caused by a fungus, Puccinia sorghi, which is a type of rust. Generally, rust fungi must live on 2 or more plant hosts before they can complete their life cycle. They are quite complex even for such a simple organism. The rust found on the oxalis growing in your garden can be a real problem on corn, especially sweet corn. The rust fungus lives through the winter on wood sorrel (oxalis) and when weather conditions are perfect, the spores of the rust are blown onto the corn. The ideal conditions for rust development are long, cool springs with temperatures ranging from 60° to 74°F plus high relative humidity. Usually rust does not cause enough damage to the oxalis to warrant any type of chemical control. Although clipping the infected oxalis leaves would benefit any corn growers in the area and keep your oxalis looking neat and clean. An application of a fungicide on the oxalis when leaves begin to develop next spring might also help.

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