Saving a Large Potted Euphorbia from Black Rot

A 5-foot Euphorbia (often mistaken for a cactus) in a New York City apartment has developed black rot at the base of one trunk, extending up about a foot. The rot encircles the trunk, but tissue above appears healthy. The other two trunks in the same pot seem unaffected, but the shared soil and root system pose a risk.

The only reliable solution is to unpot the entire plant, despite the difficulty. Wrap the stems in rolled newspaper for handling, wear gloves (Euphorbia sap is irritating), and either cut the pot away or loosen the root ball with a knife. Remove all old soil, inspect roots thoroughly, and separate the infected trunk. Cut away any rotted roots on the remaining two trunks. Repot the healthy portions in fresh, well-draining mix (1:1:1 peat moss, perlite or sand, and sterilized soil) in a pot roughly one-third the plant's height. Do not water for a week.

The removed trunk can be salvaged by cutting at least 2 inches above visible rot, sterilizing the knife between cuts until only healthy tissue remains. Let the cutting callous for a week or more, then root in barely moist soil in a separate pot. Mist occasionally but do not water until new growth appears.

Discard all infected soil and plant material—do not compost. This aggressive approach offers the best chance to save the specimen.