Spanish bayonet (Yucca aloifolia) is a coastal plant, common in sand dunes in Gulf Coast states. This yucca derives its name from its sharp, pointed 2-foot-long leaves. It’s popularity is owed to the fact that it requires almost no maintenance once it's established. Spanish bayonet thrives in sandy soil and full sun and is hardy to U.S. Department of Agriculture hardiness zones 8 to 11. Although not susceptible to many diseases, improper watering or excessive periods of rain can cause fungal problems.
Brown Leaf Spot
Brown leaf spot is caused by a fungus (Coniothyrium concentricum). Symptoms include oval spots that are light brown at the edges and dark brown in the middle. The spots will turn dark brown and may join with others to form large spots. The disease first appears on the upper surface of the leaf. Control of brown leaf spot includes the removal of all infected leaves and applications of a fungicide, such as Daconil 2787 or Zineb. Avoid watering the foliage when irrigating the Spanish bayonet.
This disease, also called grey leaf spot, is another fungal disease caused by Cytosporina. Symptoms include grey spots and brown tips on the foliage. The spots can become quite large, up to 3 inches wide. Like brown leaf spot, tip necrosis affects older leaves, and treatment and management are the same. Remove infected leaves, avoid overhead watering and apply a fungicide, according to label directions.
Root rot, also called fusarium stem rot, is another fungal disease, caused by Fusarium spp and Nectria spp. There is no fungicide that is effective for this disease. When planting cuttings, use sterilized pots and soil and allow plenty of air flow around the Spanish bayonet. Fix any drainage problems that may be aiding the promotion of the disease by amending the soil with sand.