Hi, I have had this yucca plant for many years, it blooms beautifully every year, but since last year it developed very ugly spots , especially the bottom spikes. It started cutting them off, but I am not sure if this is the best approach. Does it have a disease I should treat instead?
I appreciate your help with this!
Thank you for contacting the Toronto Master Gardeners.
From my research it appears that Yuccas are susceptible to both fungal and viral leaf spots. Cercospora, Cylindrosporium and Coniothyrium are the dominant suspects of yucca plant leaves. Coniothyrium leaf spot seems to be the most common of these. It is also known as brown leaf spot because the lesions turn brown. Often a yellow halo is present. Your first line of defense against this disease is sanitation. Remove leaves that are spotted or shriveled because the fungus continues to sporulate on those leaves. Pick off the infected leaves and dispose of these items in the city yard waste program, not in your own compost pile if you have one (unlikely to be hot enough to destroy fungus or bacteria). Clear the ground of plant debris around the infected plants to eliminate overwintering spores. This link shows various images of leaf spots.
On the other hand if you are seeing black/brown spots on the plant then it could be that you have an infestation of scale. Scale are sucking insects whose feeding causes damage to the leaves. Yucca plant bugs also feed by sucking the sap from the foliage. Their damage is yellowish-white, but the insects also deposit extract on the yucca foliage, leaving sticky black spots.
Scale, or armoured and soft scale bugs are tiny insects that attach themselves to the stems or leaves of a plant and remain in the same spot throughout their lives, feeding on the plant juices. Young scales or crawlers that have not attached themselves yet look like tiny yellow or orange spots. Armored scales do not secrete honeydew but soft scales do. They are commonly light tan but this varies depending on the species and life stage. Severe infestations can cause plant leaves to wilt, yellow and fall from the plant. If there is honeydew present it should be washed off.
Scale insects prefer shade so they are most likely to pop up on the part of your plant that is away from its light source. They multiply quickly. When removed, they leave a little mark where they have been feeding on your plant. Scale insects are difficult to control because the waxy or cottony covering serves as a protective barrier to traditional contact insecticides.
The following website provides quite a lot of good information on scale and its management.