What Could Be Causing Brown Spots on Indoor Cactus Plants?

Pests

• Red spider mites are smaller than 0.2 mm long and hard to see on a cactus or any plant. The plant may have webs, which is one indication of spider mites. As the spider mites feed on the cactus sap, the area first turns a whitish or yellowish color and later turns a brownish color. Wiping the plant down with a damp rag removes the mites and stops the infestation. It needs wiping other day until all the mites are gone. Overhead watering also cures and prevents spider mites. Scale doesn't cause brown spots, but the scales themselves resemble brown spots. The brown spot is a covering that protects the eggs and insect underneath as it sucks the juices from the cactus. After the scale feeds on the plant, the plant turns yellowish in color. Mild dish soap and a rag removes the brown spots when the infestation is mild. For heavy infestations, use a product especially formulated for getting rid of scale on cactus plants.

Environmental Problems

• Corky scab is an environmental or physiological problem. Cactus plants with corky scab have spots with a brown, corky appearance. The spots don't normally penetrate the cactus any further than the epidermis. Sometimes, the epidermis cracks open where there are brown, corky spots. When this happens, the epidermis splits open and eventually curls. Adjusting the humidity and light levels the cactus' needs solves the problem.

Disease

• Soft, mush brown spots are signs of rot. The rot is the result of bacteria or fungus around the plant's crown and travels upwards. If the plant has brown spots starting at the base, overwatering is the cause. If you catch the rot right away, cut off the top part of the cactus that is healthy and root it to make a new plant. Cactus soil should dry out between each watering to avoid this disease. During the winter, reduce watering and soil must be checked for dryness before watering.

Cactus Care

• Keep the cactus healthy by providing the proper care to avoid brown spots. Cactus plants need water when the soil is dry 1/2 inch down from the surface during spring and summer. During the winter, plants need a light watering about every two weeks. Each cactus variety has different humidity needs, but most prefer medium to high humidity. Use a fertilizer, such as 7-40-6 or 5-10-10, every three months during spring, summer and fall to promote growth. Cactus plants need bright sunlight, but some cacti do very well with indirect sunlight. The amount of light the plant needs will depend on the variety. Winter temperatures can be as low as 45 to 55 degrees Fahrenheit, but when the cactus is actively growing, it will need temperatures between 65 and 90 F. to grow.