We had several telephone calls at the Plant Clinic about yuccas looking sickly. Leaves are reportedly yellow to brown and dying over as much as three-quarters of the plant. In some cases, leaves have lesions, but that is not the current complaint.
Yucca filamentosa is the species usually grown in Illinois. According to Michael Dirr in Manual of Woody Landscape Plants, this species is very hardy and should survive Illinois winters. It is also forgiving of most soil types but does not tolerate wet areas. It appears that the injury we are seeing this year on yuccas is most likely winter injury, as discussed with trees and shrubs above. Yuccas in wet areas or areas of poor circulation seem to be affected the worst.
Cut out the brown or yellow leaves now to avoid problems with soft rot bacteria entering the dead tissue and moving to the stem. A light application of fertilizer can help the plant recover as well. Because this plant is so drought tolerant, supplemental watering is not usually necessary.
This is not an infectious disease situation. Still, yuccas are occasionally bothered by fungal leaf spots, including those caused by Cercospora and Coniothyrium, diseases that do not cause widespread death of leaves. If you are having problems with one of the leaf spot diseases, remove infected leaves. Avoid overhead irrigation, and consider using a fungicide to get the disease under control. Weekly applications of chlorothalonil (Daconil) or mancozeb helps stop disease spread. Be sure to check the label of the chosen product for clearance on yucca, rates, and timing before applying.