Slowly growing to about 10 feet tall, the Spanish bayonet (Yucca aloifolia) is a dryland tree-like shrub native to the southern United States, the West Indies and Mexico. The sharply-pointed spear-like leaves provide coarse texture in the garden, and in the summer the plant flowers with a spike of white blossoms. Grow this yucca in USDA winter hardiness zones 6 and warmer. The Spanish bayonet is susceptible to a number of diseases; to help treat and prevent these diseases, prune away infected foliage and avoid irrigating plants from overhead. If overhead watering can't be avoided, regular application of fungicide will help stop disease.
Brown Leaf Spot
The fungus Coniothyrium concentricum causes brown, oval, slightly depressed lesions to appear on the leaves of the Spanish bayonet. Newly forming spots are light brown but with a darker center while old spots are black and have concentric rings of light and dark. After about four months, the spots bear rounded black fruiting bodies.
Gray Leaf Spot
Also causing spots on the plant's foliage, gray leaf spots, caused by Cytosporina sp., are considerably larger than those of the brown leaf spot fungal disease. Besides spots up to 3 inches in diameter, this fungus more often causes the tips of leaves and leaf edges to turn gray with thin brown margins. Within both the spots and leaf tip and edges concentric circles of light and dark colors occur. After manifesting on the leaf tips, the disease spreads down the leaf blade, discoloring and eventually killing it.
Southern blight is caused by Sclerotium rolfsii. Often the white fan-like pattern of this organism is seen on the stem near the soil surface, but can be found on any plant part. Another indication of this disease is cottony, white masses the size of a mustard seed. Eventually they turn tan and finally dark brown and harden.