Yucca Fungus

Brown Leaf Spot

• Coniothyrium concentricum occurs frequently in certain areas of Colorado. This infection targets yucca and other cactus plants, manifesting first as splotchy yellow lesions that darken to brown. These lesions are often small, not exceeding a half-inch in diameter. Older yucca leaves generally become infected with this fungus more often, and controlling them involves the removal of old leaves. When this fungus appears on indoor plants, it is usually a result of overhead watering. Periods of heavy rainfall during which the plant does not dry out may also prompt this fungal infection.

Gray Leaf Spot

• The genus Cytosporina is responsible for a variety of fungus species resulting in the gray leaf spot found in the Spanish Bayonet Yucca. This fungus, far less common, manifests as necrosis, or dead and decaying cells, on the tips and edges of the leaves that become brown and gray. Cytosporina also produces circular lesions but, unlike Coniothyrium, these legions may be up to three inches in diameter and contain concentric light and dark rings. Like Coniothyrium, Cytosporina also infects older leaves more commonly.

Fusarium Stem Rot

• This type of systemic stem rot is common in yucca plants and is caused by the Fusarium genus of fungi. Both the leaves and stems become infected, quickly turning shades of brown and yellow as they decompose. Eventually the plant's stem weakens to the point where the plant falls over, possibly ripping it from the infected area. At this point the yucca plant dies. Though no fungicides exist that will treat this type of rot, its spread can be reduced by using sterilized potting soil and removing infected plants.

Southern Blight

• Sclerotium rolfsii, the fungus responsible for the southern blight infection, also attacks the plant's stem. It manifests as a series of delicate white veins that fan out from a central spore and may also grow along the soil. As Sclerotium matures, the white veins fade to brown then harden. The fungicide Terraclor 75 can treat it, but its administration is limited to once per year as it stunts the plant's growth. Southern blight can also be avoided by quarantining infected plants.

Cercospora Leaf Spot

• The rust-colored flecks of the Cercospora pathogen are often missed at first, as they initially are very difficult to see with the naked eye. As they mature, these spots expand to a quarter of an inch in diameter and are elliptical or oval-shaped with a tan center and a dark brown ring around the edges. Though no fungicides are identified as effective against this infection, the infected leaves can be removed. If caught soon enough, this will often save the plant. To minimize the possibility for this fungus to grow, avoid overhead watering.