Hi- Have has this plant for over 30 years- put it out on the patio in the summer and winter it inside. Great a lot this summer, has been in this container for 1.5 years. Lower leaves are yellowing- at closer inspection seems to have some kind of black fungus. Help! We call the tree Eyeore and he is like a member of the family. Have additional photo's but could only upload one
Thank you for your question to Toronto Master Gardeners.
Congratulations on keeping your Ponytail (Beaucarnea recurvata) alive for 30 years! You sound like an experienced gardener.
You guessed correctly that the black spots are a fungus commonly called Sooty mould. Fortunately, your plant will not die from this, especially if you take steps now to get rid of the problem. According to my favourite resource on indoor gardening, Houseplants by David Squire and Margaret Crowther, this mould “lives on honeydew excreted by aphids and other sap-sucking pests.”
First, remove the worst affected leaves and then use a damp cloth to wipe off as much of the mould as you can. Next, gently wrestle your Ponytail into the shower (or garage) and spray wash the tops and undersides of every leaf. Wipe the leaves dry and then spray with insecticidal soap. One session may not be sufficient to eradicate all the pests, so use a damp cloth to wipe away any new infestations and repeat the process in a couple of weeks. If you see the pests appearing, try to identify them and use appropriate safe controls. We have a great deal of information about this on our website. Just enter the key word into our search box and select from the list that comes up.
Recommended reading:
Houseplants, David Squire & Meredith Crowther. Chancellor Press, London, 1995.
Miracle-Gro Complete Guide to Houseplants. Meredith Books, Des Moines, Iowa, 2008