(Photograph = Insect egg case, John Tann)
Introduction
In the wild and on windowsills, cacti project a kind of armor—spines, thick skin, and a knack for waiting out lean times. From a distance, they read as indestructible. Up close, however, they are living tissues with the same vulnerabilities other plants face. In nature, most plants do not look sick, yet a quick walk through any desert wash shows scars, galls, and bite marks; cacti are no exception.
The Details
In cultivation, the most common problem starts below the rim of the pot. In heavy mixes or in containers without a free-draining hole, roots sit wet, and fungi move in. Once roots rot, the stem starves: ribs soften, color dulls, and the plant collapses from the base upward. As a rule, water deeply, then let the soil dry; resist the urge to “help” a sulking cactus with frequent sips. In bright, dry air, roots rebuild; in cool, damp corners, they fail.
On the surface, pests cue you with small, telling signs. Cochineal scale lays down cottony patches on Opuntia pads; root mealybugs hide out of sight, but give themselves away with white fluff around drainage holes. Soft scales and mealybugs sap energy and invite sooty mold. Cactus bugs and other piercers leave pale stipples and ragged areoles. When you see honeydew, ants, or sooty smears, slow down and look closely into the axils and along the ribs.
Through wounds and natural openings, fungi and bacteria take their shot. Sunburn, hail, or a clumsy knock can open the door to black, wet lesions that spread under the skin. In heat waves, thin-skinned species blister; in cold snaps, chilled tissue turns gray, then black. In grazed country, wildlife and livestock break spines and bruise crowns; in towns, stray weed-whacker cuts set the stage for rot.
Prevention does the heavy lifting. In a pot, use a gritty, mineral-rich mix, and keep air moving around the plant. Between tasks, wipe blades with alcohol; do not share a dirty knife between a healthy cactus and a soft, oozing one. Quarantine new arrivals for a few weeks, and check them dry and wet—some pests only show when the plant is stressed. Outdoors, site plants on slight mounds or rocky rises so water sheds quickly, and give fast growers enough room that they do not shade one another into thin, tender skin.
In short, cacti are tough, not bulletproof. With drainage, restraint, and clean habits, they shrug off most trouble. Without those basics, even the best-armed barrel goes down the same way every houseplant does—from the roots up.
Additional Reading: Infections of Cacti
