Two of our cactus plants have fallen sick this winter. One has become covered in light brown/beige spots. The other, a shorter round cactus has white foamy balls that have formed on it (pictures attached). They share a pot where they grow in cactus soil and are placed in front of a west-facing window. Other cacti and succulents in the pot (it is a potted cactus garden with ample space left around each cactus to let it expand) are doing fine. We have generally let the soil dry out, and then water thoroughly (every 3-4 weeks depending on the season). What could be wrong and is it possible to save them?
Thank you for contacting Toronto Master Gardeners.
Growing cacti gardens is certainly a rewarding hobby and requires less maintenance than other plants.
The usual sources of disease in cacti is over watering and introduction of a new infected plant. You do not mention if there have been any introductions to your cacti garden. If you do want to introduce new plants they should be quarantined separately for several weeks before introduction to the garden. I can not see the set up and drainage in your garden. Good drainage is important to avoid any standing water. Cacti need to fully dry out completely between watering. In the winter months they can go for much longer dry periods. All the sources suggest that under watering is better than over watering.
The Cacti with the brown on it is from the genus Cereus and could be several different things.
- Corking is a natural process of a maturing Cacti. It begins at the base of the plant and works its way up. It is a totally normal part of the life cycle. Do you recall if the brown began at the bottom? If it started more spotty higher up and spread down than it is more likely damage, fungus or spider mites.
- Brown on the plant can indicate physical damage from too much sun or cold. Was the plant exposed to the cold at some point or receive too much sun? If it is from damage the size and shape of the discolouration should not change.
- If the brown is caused by spider mites you should also see small threads like a spider would leave. They are difficult to treat and the plant should be separated from healthy plants. A local nursery should have something you can treat the pests with.
- There are fungal diseases that can cause the brown as well but are uncommon in your type of cacti. It would be accompanied with rot which you do not mention.
From the photo it is difficult to see the exact nature of the white foamy balls. chances are you have one of two infestations.
- Scale is a small insect that produces a protective casing around it as it latches on to the plant and eats. The main way to deal with scale is to physically remove them manually or with a jet of water. The plant needs to be monitored afterwards and an insecticide may need to be applied. Again your local nursery is the best place to find products.
- Mealy bugs are more common and a more likely possibility. Mealy bugs can attack all parts of the plant including the roots and is very difficult to treat. They have a higher resistance to pesticides so require stronger chemicals. These chemicals can damage the surface of the cacti leading to light damage later on so systemic treatments are recommended. They should also be removed with water and/or a toothbrush.
With some of your plants being healthy and some infected it may be prudent to remove and isolate the infected plants and clean and re-pot the healthy ones to avoid spreading of disease.
I am including several links below which explain in detail these issues and how to deal with them.