Pests and rot on a Trichocereus
Rot is water, cold or both, and too much water is one of the most serious problems these plants have. Mealybugs, scale, spider mites, fungus gnats and snails each have a tell, and what turns up changes indoors, under glass and outdoors.
30-second version first, the whole guide below5 min read
The short version
- The pests are few and each one has a tell.
- A small lesion can be cut out and left to heal.
- Never stand a pot in a saucer of water.
- Let the mix dry between waterings, and use one that drains: over-watering on top of poor drainage is the recipe for losing a plant.
- Heavy clay holds water, and pumice mixed in, or a raised bed, helps it drain.
- Which insects arrive depends on where you are, and a local extension office knows the list for your area.
Your setup
Pick yours to see what changes in this guide.
Where it grows
In the ground
Heavy clay holds water, and pumice mixed in, or a raised bed, helps it drain.
In a pot
A pot can be lifted out of a wet spell, which a plant in the ground cannot.
Where it lives
Outdoors
Outdoors in a pot: snails and slugs are the ones that scar the growing tip.
Indoors
Indoors and under glass: mealybugs, scale, spider mites and fungus gnats are the ones that turn up on cacti kept as house plants.
The season
Winter
Rot follows cold on top of wet.
Not winter
Nothing in this guide changes outside winter.
Lines are from this guide, or from the cultivar library where one says so. For where they come from, see the end of the guide.
Introduction
Rot is water, cold or both. Too much water is one of the most serious problems these plants have, one extension service says. The pests are few and each one has a tell. A plant that drains well, gets enough light and is not stressed stands up to pests better than a stressed one, the same extension service and a botanical garden say.
Rot first
Rot is soft. A dark patch that is hard under a fingernail is a scar, growers say, and stays where it is. Dark flesh that gives way under a finger and leaves a dent is rot, growers say. A plant resting dry through winter goes a little soft all over, and that is not rot.
Hard, dark, healed
does not move
Soft, sunken, rotting
gives under a finger
- It follows soil that stays wet too long, whether from poor drainage or from watering too often.
- follows cold on top of wet.
- It follows green stem buried below the soil line, which, one extension service says, leads to poor establishment and death.
Growers say to cut it out with a sharp knife, wiping the blade with alcohol between cuts, and to take a margin of clean flesh with it. A small lesion can be cut out and left to heal. An older, weeping lesion at the base usually cannot be saved, one extension service says. A cutting from the clean part above it, growers note, is how the plant goes on.
The pests, by how they look
Growers and a botanical garden describe them this way:
- : white, cottony clusters on the column, around the and between the ribs, or on the roots. Wipe them off with a cotton swab dipped in alcohol, or wash them off with a strong stream of water, with the soil covered first.
- Scale: small, round white or tan spots like scabs, some with a darker center. They can be scraped or pried off.
- Spider mites: very fine webbing and small brown dots, worst on young growth. A strong stream of water washes them off; cover the soil first.
- Fungus gnats: small black flies. The larvae live in the soil and eat matter and roots. Yellow sticky traps deal with the adults.
- : small, oblong insects, with flying ones about. Yellow sticky traps help show what you have.
- Snails and slugs: scarring, or small chunks missing, at the growing tip.
Fungus gnats do their damage in a tray. A grown cactus is reportedly not bothered by them.
The adult
about 3 mm · like a tiny mosquito
The larva
about 6 mm · clear body · dark head
Not every mark is a pest
A raised, firm blister is the plant taking on water faster than it can use it, growers say, and nothing is eating it. It usually stays firm under a finger, which is how growers tell it from rot.
Figure
Four things a column tells you
The same plant, four ways it goes wrong. Each one is drawn from the sign a grower actually reads it by — which side, which part of the rib, and whether the skin is discolored or lifted.
Sunburn
too much light, too fast
Yellow on the lit side only, worst on the rib crests. The two darker patches have scarred.
Etiolation
too little light
New growth thinner than the old, pale, fewer ribs, stunted spines, reaching sideways.
Rot at the base
too wet, too cold, too deep
Soft and dark, starting below the soil. The pale margin means it is still moving.
Edema
water in faster than it breathes out
Raised blisters on skin that is otherwise perfectly green. Wetness and still air, not an infection.
What changes with the setup
Growers and extension services say it changes like this:
- Indoors and under glass: mealybugs, , spider mites and fungus gnats are the ones that turn up on cacti kept as house plants. Keep the air moving and the humidity low.
- Outdoors in a pot: snails and slugs are the ones that scar the growing tip. A pot can be lifted out of a wet spell, which a plant in the ground cannot.
- In the ground: rot is the larger risk. Heavy clay holds water, and mixed in, or a raised bed, helps it drain. Which insects arrive depends on where you are, and a local extension office knows the list for your area.
Keeping it from starting
- Look a new plant over carefully before it goes near the others, because pests come in on new plants.
- Never stand a pot in a saucer of water.
- Let the mix dry between waterings, and use one that drains: over-watering on top of poor drainage is the recipe for losing a plant.
- Keep the air moving.
Common mistakes, try it, quick check and sources
Quick check
Question 1: A dark patch at the base gives way under a finger and leaves a dent. What is it?
Question 2: You find white cottony clusters between the ribs. What do you do first?
Question 3: Which setup most needs drainage worked out before planting?
