The most common afflictions in yucca plants are caused by fungi and insects.
The usual suspects that cause discoloration on yucca leaves are Coniothyrium, Sclerotium, Cytospora and Cercospora. The spores of these fungi are spread from one leaf to another by water splatters, which is why overhead watering of yucca plants is not advisable.
The yucca plant
The yucca plant is part of the Liliaceae or Agavaceae family, and is native to the North American desert regions. It is a resilient plant that can withstand different soil conditions and even temperate climates that are far different from their desert home.
There are over 20 varieties of yucca and while they may look different from one another, they have similar characteristics that help you identify them as part of the yucca family.
Color
The leaves of the yucca plant are green, but they can range from pale blue-green to dark green. Some types of yucca can be variegated, meaning that they have yellow, white, or cream longitudinal stripes along their leaves. Most yuccas usually have the younger and more vibrant leaves at the top of the plant while the browning and dying leaves are closer to the base of the plant.
Shape
Yuccas are known for their signature leaves that look like swords with their pointed tips. The leaves can measure between one to three feet long. Most yucca plants have leaves that form a rosette starting from the base. The yucca plant’s trunk makes it easier to differentiate it from other plants that may be confused with it, such as agave plants.
Texture
The leaves of the yucca plant are thinner and more pliable than agave leaves. They are leathery and stiff and have serrated edges that stop at the tip of the leaf. The leaf spine is thinner than that of the agave. The tips of the yucca plant’s leaves can become frayed into filaments and hang from the edges of the leaves.
Yucca plant diseases or pests
Cercospora leaf spot
This condition is caused by the Cercospora fungus, which causes rust-colored spots on the yucca plant’s leaves. These small spots can grow to be a quarter of an inch wide and turn brown. You can keep the spores of the fungus from spreading to other leaves by watering the plant from the bottom, and avoid overhead watering. Make sure you remove all of the affected leaves and burn them. This will kill the spores of the fungus so that it does not spread to your other plants. You can also use a fungicide that contains thiophanate methyl.
Southern blight
The disease is caused by the Sclerotium rolfsii fungus, and while it attacks the entire plant, its effects are most noticeable on the stem. This fungus causes white threads, or mycelia, to grow on the plants and on the soil. These mycelia will form a fan shape. Eventually, as the disease progresses, circular lesions will form in the plant, called sclerotia. They will start out as white spots but will become brown as they mature towards being able to produce spores.
You can take care of this disease by cutting off all of the affected areas, bagging them, and then burning them. This keeps the spores from spreading to your other plants.
Gray leaf spot
This disease is usually observed on the Spanish bayonet yucca and is caused by the Cytospora fungus. In the early stages, the disease will manifest as graying and browning of the tips and edges of the leaves. Over time, the leaves will start to die and develop lesions three inches in diameter. As with other fungal diseases, make sure you do not water the yucca from overhead to avoid spreading the spores through water splatter. Remove any infected leaves and debris from the pot and burn them. You can also use fungicides to kill the fungi and its spores.
Brown spot
This yucca plant disease is caused by the Coniothyrium concentricum fungus and is characterized by brown spots on the plant’s older leaves. These spots usually have a purple border and the older ones even look black. Sometimes these lesions grow into each other and fuse, which ends up affecting entire sections of the leaf.
You can prevent brown spots by only watering the yucca plant from the bottom and never from the top. Make sure you remove all the infected leaves and burn them to kill the fungus and its spores. You can also use fungicides that contain both chlorothalonil or mancozeb.
Scale insects
One of the most common pests that can infest your yucca plant is the scale insect. These pests can also cause black spots to appear on your plant’s leaves. The black spots appear after the scale insects have fed continuously on the sap inside the plant’s leaves.
Scale insects can be eradicated by wiping the leaves down with a cotton ball soaked with rubbing alcohol. You can also use pesticides designed specifically for these types of insects. Because these insects go through a complicated life cycle, you have to reapply the alcohol several times to be sure that all of the insects have been killed and will not reinfest the yucca plant.
Systemic pesticides, which are those that make their way into the vascular system of the plant, are also very effective against scale insects: they will die after feeding on your plant.
You can make a homemade horticultural soap by mixing water, rubbing alcohol and dish soap into a spray bottle. Spray the solution on your yucca plant once a week for a month, and that should take care of these pests.
Conclusion
The yucca plant is most susceptible to fungal diseases. These are spread when infected leaves touch other leaves or through the spores being dispersed onto other plants when the infected plant is watered from overhead.
The most common fungi that infect yucca plants come from the families Cercospora, Sclerotium, Cytospora, and Coniothyrium. They can all usually be sorted out by using a fungicide. Make sure you remove all of the infected leaves and debris and burn them to kill all the spores.
Image: istockphoto.com / AdamRegan