1) Coniothyrium or brown leaf spot (Coniothyrium concentricum)
Coniothyrium leaf spot first appears as tiny clear zones in older leaves of yucca. Lesions turn yellow and finally brown as they mature. Lesions are generally elliptical and scattered across the entire upper leaf surface. After about 4 months, black perithecia or pycnidia form in the sunken lesion centers and are easily seen with the naked eye. Sometimes a chlorotic halo and/or dark-purple or black margin forms around older lesions which rarely exceed 1/2 inch in diameter.
The most important aspects of control are removal of older infected leaves and elimination of overhead watering and exposure to rainfall. Since most yuccas are grown exposed to both overhead watering and rainfall, regular applications of fungicides are needed to prevent this disease.
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