If cacti and succulents are grown in appropriate conditions and properly cared for, they should remain healthy and vigorous. It is always a good idea to inspect cacti now and then, perhaps by watering them, so you can spot signs of trouble early on. Any symptoms or pests can then be treated before they become established and seriously affect the plant.
This section will be discussing information about many common pests, diseases, or cultural damage that may occur during the cultivation of cacti and succulents and ways to stop these pests.
Pest Problems
Pests can be controlled by chemical insecticides or, in greenhouses, often by natural predators. Systematic insecticides are absorbed into the plant’s sap, poisoning any insects that are feeding on the plant, and are more efficient than a contact application. Use a systematic insecticide 2-3 times during the growing season as a preventative measure even if your cactus is not infested.
You can identify if your cactus is infested by reading the descriptions below.
Once you have identified the pest if the directions tell you to use an insecticide, follow these guidelines for applying the insecticide: apply insecticide once a week for 3-4 weeks by drenching the cactus with insecticide with a solution that will allow the insecticide to reach the roots.
In addition to the pests, roots are also eaten by mealybugs, which leave powdery white deposits along the roots. To save the infested plant, stand its root ball in a bath of soapy insecticide, then allow it to drain.
Spider Mites:
Brown dots are a sign of spider mite attack on young growth. The tiny, yellow, red, or brown mites resemble ground pepper, thrive in dry greenhouses, and spine fine webs. Increase humidity or use the preditor Phytoseiulus persimilis in temperatures over 70 F (21 C).
Scale Insects:
Control these small, brown, moundlike insects with systematic insecticide (apply insecticide once a week for 3-4 weeks by drenching cactus with insecticide with a solution that will allow the insecticide to reach the roots). They usually appear on the stems and lower leaves of an indoor or greenhouse plant, and migrate up the stem. Plant growth is then checked and the sticky honeydew can encourage sooty mold.
Mealybugs:
An infested plant is covered in tiny, wooly white nests and sticky honeydew. The insects look like pale, waxy snowbugs. Treat with either systematic or soapy contact insecticide (apply insecticide once a week for 3-4 weeks by drenching cactus with insecticide with a solution that will allow the insecticide to reach the roots) or, in temperatures over 70 F (21 C), introduce the predator ladybug, Cryptolaemus montrouzeri.
Whiteflies:
These white, mothlike insects excrete honeydew on which sooty mold thrives, especially on soft-leaved succulents such as Euphorbia. Use an insecticidal spray (apply insecticide once a week for 3-4 weeks by drenching cactus with insecticide with a solution that will allow the insecticide to reach the roots) or soap; or in the greenhouse, introduce the wasp Encarsia formosa to the greenhouse. Wasps prey on young nymphs, which blacken when they die.
Vine Weevils:
Resistance to most chemicals, these increasingly troublesome brown beetles nibble leaves, and their fat, white larvae eat roots and burrow into stems, causing the plant to collapse and die. When repotting, look out for tunnels in the potting mix. If any exist, replace the mix.
Diseases
Cacti and succulents suffer mainly from fungal diseases. Fungal spores abound in the atmosphere and attack plants in damp or wet conditions, especially in a shady areas with similar daytime and nighttime temperatures. Some fungi actually grow through the plant skin. Most problems occur in late spring or autumn when condensation from cold nights fails to evaporate from the plants if the next day is overcast. In addition, overwatering or damp conditions can cause root/stem rot and the damping off in seedlings which makes them wilt and die. Damping off in seedlings can be avoided by lightly spraying the potting mix with a copper sulfate solution.
Fungal Leaf Spot:
Usually brown or gray, the spots may merge and cause leaf drop. Minute fungal soreheads develop where water droplets lie on the leaf, leaving sunken, brown scars. Discard all infected leaves; treat the plant with copper sulfate and, under cover, improve ventilation.
Corky Scab:
Brown, barb-like spots that usually develop near the base of a stem is a result of past disease damage or poor cultivation. Do not confuse these spots with natural corking on very old plants. If growing conditions are improved, the spots should not reappear.
Sooty Mold:
Caused by a soilborne fungus, this sooty black mold attacks plants, especially seedlings that are damaged, weakened by very wet conditions, or covered in honeydew. Discard infected seedlings and soil, treat plants with a systematic fungicide, and control any pests present.
Basal Stem Rot:
Plants that are cold, damp, or have poor roots are prone to rot. Rot fungus penetrates rapidly from the skin into the tissue, turning them soft and black. Cut off sound pot plant stems for cuttings and discard the rest. Apply copper sulfate to permanent plantings.
Cultural Damage
Although pests and diseases do affect cacti and succulents, the plants are much more likely to suffer physical damage caused by incorrect feeding and watering or adverse growing conditions, for example, inappropriate light, temperature, soil, or humidity. A plant that is grown in an unsuitable environment may produce unattractive distorted leaves or stems and healthy growth can lead to permanent physical damage or even death of the plant. Usually, if a plant fails to flower once it has reached the size at which flowering begins, it is not being grown in an appropriate environment, and all of its basic requirements have not been met.
Scorch Damage:
Hot, stuffy greenhouses, strong winds, or sudden bright sunshine on dewy plants can cause sunken brown patches where the tissue has collapsed. When moving plants to less sheltered positions, harden them off in shad or protect them with fabric or a flexible mesh.
Cold Damage:
Plants subject for a long period to temperatures that are too low for them can suffer stem tip damage, scarring, or even stem collapse. Rot often attacks the affected area. Increase the temperature so that dead tissue can dry out. Alternatively, cut out all the damaged tissue.
Etiolation:
Poor light, especially in warmth, as well as incorrect feeding, causes plants to become pale-skinned with elongated stems, or to have stunted leaves on spindly shoots. Healthy growth resumes once the plant is given sufficient light. Prune affected stems back to good tissue.
Drought:
If starved of water, leafy succulents shed their leaves, then start to die back to the stem tips or shed their branches, and eventually die. Watering usually reverses the process of decline. Cacti shrink into dormancy in a drought but rejuvenate rapidly once watered.
