Cactus Problems: Rot, Corking, Mealybugs, and Sunburn

Corking is not rot

Sooner or later a brown, dry, bark-like patch appears at the bottom of a maturing cactus and spreads slowly upward. Nearly everyone reads it as rot and either throws the plant away or starts cutting. It is corking, the normal woody tissue an older plant forms at the base to support its own weight, and it is a sign of a plant doing well.

The test is touch, not color. Press the area firmly with a fingertip. Corked tissue is hard, dry and slightly rough, like bark, and the edge between it and green tissue is sharp. Rot is soft, gives under the finger, is darker and often wet, and shades gradually into the healthy body.

Corking only ever moves upward from the base or outward from an old scar. A brown patch that appears part way up a clean stem and is soft to the touch is not corking, whatever it looks like.

Rot, and cutting a plant back to clean tissue

Rot follows water the plant could not use: a winter watering in a cold room, compost that stayed wet for a month, or a wound that got splashed. It usually starts underground at the root plate and reaches the surface only when a good deal of damage is already done, which is why a cactus can look fine one week and topple the next.

Once you see soft tissue there is no treatment, only surgery. The decision is how much to cut, and the answer is always more than looks necessary, because the fungal threads run ahead of the visible discoloration.

1. Unpot and look at the roots Wash the compost off. Healthy roots are firm and pale. Rotten ones are brown, hollow, and pull apart between the fingers. If the root plate is gone, the top may still be saveable as a cutting.
2. Cut across in slices from the top With a clean blade, slice down through the plant a little at a time from above the damage. Look at each cut face: any brown ring, streak or spot means keep going. Stop only when a whole face is uniformly clean.
3. Sterilize between cuts Wipe the blade with alcohol after every slice. Otherwise you carry the infection straight down into the clean tissue you have just exposed.
4. Bevel and dry the survivor Pare the rim of the cut face at an angle and stand the piece somewhere dry, warm and airy for one to four weeks depending on its width, until the face is hard right across.
5. Root it as a cutting Set the callused piece on dry gritty mix in a clean pot. Do not reuse the old compost or the old pot without scrubbing it. Water only once it has rooted.

Throw away the old compost rather than putting it on another plant, and scrub the pot in hot soapy water. Rot organisms survive perfectly well in dry compost until the next chance to get wet.

Mealybugs, above ground and below

Mealybugs are the pest you will actually meet. They look like small white cotton specks and they sit exactly where you cannot see them: in the woolly areoles at the base of the spine clusters, down between the ribs, and in the crown.

The worse version lives on the roots. Root mealybug shows as a fine white powdery deposit on the roots and inside the pot wall, and above ground it looks like nothing at all except a plant that stops growing and slowly loses color. This is why you check the roots at every repot even when nothing is obviously wrong.

1. Inspect under a bright light Once a month, look into the areoles and between the ribs with a torch, and check the underside of the pot rim. Catching a dozen insects is a five-minute job; catching a colony is not.
2. Spot treat with alcohol Touch each insect with a cotton bud dipped in isopropyl alcohol. It kills on contact and dissolves the waxy coat. Do this in the shade, never in direct sun.
3. Isolate the plant Move an infested plant away from the others immediately. Mealybugs walk between touching pots and spread across a windowsill within a few weeks.
4. For root mealybug, bare-root and wash Remove all compost, wash the roots under a running tap, cut away badly infested roots, dry the plant for several days and repot into fresh mix in a scrubbed pot.
5. Repeat on a fortnightly cycle Eggs hatch after the first treatment. Check and re-treat every two weeks for at least six weeks before you call it clear.

The rest of what turns up

Beyond mealybug and rot, the list is short. Most of it is manageable if you find it at the point where it is still confined to one plant or one patch.

Deciding when a plant is finished

There is a point where cutting further leaves nothing worth keeping, and holding on to a rotting plant on a shelf with others is how you lose several instead of one.

If the rot reaches the growing point, or if slicing down has taken you to within an inch or so of the base with brown still showing on the face, the plant itself is gone. Look for any clean offset around the sides, take it as a cutting, and discard the rest along with the compost.

Do not compost a rotted cactus with the pot standing among healthy plants while you decide. Bag it and take it out of the room the same day.

Records that pay off

Note the date you first saw any pest, on which plant, and what you did about it. Mealybug in particular comes back from eggs, and knowing that the last outbreak on a given plant was six weeks ago rather than six months ago decides whether you treat again or stop.

Also record the position of each plant when a problem appears. Rot on the same shelf two winters running is not bad luck; it is a cold, still, damp corner telling you to move the whole shelf.