It’s the time of year when we see extensive spotting of yucca leaves. This fungal disease is caused by a species of Coniothyrium. It is not a major problem unless your yucca plant has been infected; then it’s a major problem. Spots appear as circular to elongated light brown areas with purple edges. The center of each spot is pale brown or tan and has many pinhead-sized black fruiting bodies (pycnidia) arranged in a circular pattern within the spots. These pycnidia contain the spores of the fungus.
Because spores are easily spread with water, avoid watering plants from above. When possible, water the soil around plants and avoid syringing foliage. Rain can be a very efficient vector of this fungus, and there is nothing we can do to stop that spread.
Disease management includes cutting off and removing infected leaves. Fungicides protect the healthy tissue but cannot help infected leaves. The copper fungicides are the only chemical options, and many of these are available for nonwoody ornamental crops. Go to your garden center and find a copper fungicide that is registered for use on yucca or lists a broader host range such as “ornamental plants.” Read labels carefully because many of these products warn of foliar discoloration or possible phytotoxic results on some varieties as a result of spraying. Always read labels carefully to check for crop clearance, disease clearance, rates, and timing, as well as warnings of injury. Spraying now probably won’t do much for the plant. If you have this problem every year, mark your calendar now to spray next spring as plants begin to grow.