Ask Mr. Smarty Plants

Most often when a white fungus-like substance appears on prickly pear it is an insect infestation not a fungus disease. It is easy to confirm this by seeing if you can scrape off the white fluff and if the white substance produces a bright red juice from the insect when it is squeezed between your fingers. If this is the case it is in fact the cochineal insect that is on your prickly pear. The Cactus Doctor, www.thecactusdoctor.com has a discussion about this insect on this website.  He writes, "When you see that your prickly pear and cholla cactus have white sticky mounds covering them, it means you have a cochineal (mealybug) problem. This is not a disease; it is actually an insect infestation. The white sticky mounds are the housing for cochineal bugs (also known as mealybugs). When these white sticky spots first start to appear it is best to spray them off the cactus pads with a power nozzle attached at the end of your hose. If the infestation begins to get out of control, I suggest treating the areas by scrubbing them with insecticidal soap or unscented dish soap. In small areas you can scrub with a toothbrush but for larger areas it is best to use a long-handled brush."

Since you didn’t find any sign of the cochineal insect, it’s time to investigate other possible problems with your Opuntia (prickly pear) cactus. In a previous Mr. Smarty Plants question, Barbara Medford discussed other problems of Opuntia.

From www.CactiGuide.com there is an article on Pests and Diseases of Opuntia, including Opuntia ellisiana (Spineless prickly pear). The Plant Pathology Lab of Texas A&M has also released a list of Opuntia diseases. Here are some extracts from that article:

Cactus Anthracnose (fungus - Colletotrichum (Gleosporium) spp.): This disease affects several kinds of cacti, Cereus, Echinocactus, Mammillaria, and particularly Opuntia (prickly pear). Infection results in a rather moist light brown rot which shows many light pink pustules on the surface. Spots are small at first, later enlarge and become covered by the small spore-producing pustules. Large areas may be affected, sometimes destroying entire plants. No satisfactory control is available, other than removing and destroying diseased cladodes as soon as noticed.

Scorch or Sunscald (fungus - Hendersonia opuntiae): This disease is common and serious on prickly pear cactus (Opuntia). Spots at first are distinctly zoned, later enlarging until entire cladodes turn a reddish-brown and finally die. The center of the disease area is grayish-brown and cracked. Other fungi may also be present in the diseased area. No practical control has been developed.

Scab (physiological): Particularly common on prickly pear cactus. Rusty colored, corky areas appear on the stems. Scab is thought to be a form of edema, resulting from overwatering and poor ventilation. Increase light and decrease humidity for control.

Stem Rot of Cacti (fungus - Drechslera cactivorum): Basal or top rot of seedling cacti that turns cactus into a shrunken mummy covered with brown spores. First symptoms are yellow spots. It can completely rot a plant in four days.

Summary: See each disease for suggestions for treatment, usually, cutting out the diseased portion and disposing of it in a way to prevent the disease being passed to others. Scab may be caused by watering the cactus, having it in too much shade and/or poor circulation. So, you could move it to a better location -- now there's a challenge!

There’s also a good website online with information about Growing Cold Climate Cacti outdoors that you might be interested in reading. It describes the many types of native cacti that can live in deep snow sites where temperatures drop well below zero degrees F.  There are tips on how to use rocks to best protect Opuntia and other cacti in garden situations.  The article also discusses damaging heat burns and the possibilities for fungi to be a secondary infection.

They write, "Rock gardens are typically located in sunny fast drying locations, which is ideal for most cactus plants including cold climate varieties. However, cold hardy cacti can get heat burns under certain conditions which can cause narcotic tissue. The development of necrotic tissue is dangerous, because fungi can establish themselves as a deadly plant infection originating in dead plant tissue. This kind of condition where dead plant tissue can cause the rest of the plant to die will also happen if cacti are exposed to cold winter winds, so both cold and heat burns need to be avoided."