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The Mexican Nopal, or in English "prickly pear cactus" or "Barbary Fig cactus" is a member of the group Opuntia Spp. within the Cactaceae family. Mold growth appears to be more common on cactii such as the Nopal when growing in higher and less arid regions such as the village of la Yerbabuena, near the foot of the volcano above Colima, Mexico.
This article describes mold growth on the Mexican Nopal cactus plants in the wild and on those grown as a food crop. With a fungal growth sample collected from Nopal growing in Yerbabuena, Colima Mexico. in 2011 we began seeking an accurate identification of the fungus, and an exploration of its properties both on the plant and as a possible seasonal contributor to the aerobiological milieu.
Our project will also investigate and document the causes and effects of mold infection of Mexican pricklypear cactus plants, and the identification of other mold genera/species commonly found growing on and affecting those important plants.
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The Mexican Nopal, or in English "prickly pear cactus" or "Barbary Fig cactus" is a species or group of species of Opuntia Spp. within the Cactaceae family. While some sources  claim over 200 species in that group, the USDA lists 59 species and 75 accepted taxa within the Genus Opuntia Mill.
I am particularly interested in fungi found on species of Nopal found at altitude in more wet or humid highlands of Mexico.
These include fruit-bearing pricklypear species (Opuntia ficus indica) that are widely used as a food (both the cactus fruit or Tuna and the younger cactus pads or nopalito) and drink product (an intense purple juice in water) in Mexico as well as an export product in the form of Nopal fruits, Nopal juice, and in power and cosmetic forms. 
Some might think that because cactus plants generally grow in dry locations that they never suffer from fungal attack, but that's certainly not the case. The page top photograph of a mold-infected Nopal (pricklypear) cactus plant was taken in Yerbabuena, Colima Mexico. Yerbabuena is a tiny village located at a comparatively high altitude and close enough to Mexico's Pacific coast to receive more rainfall than some other areas of the country. I have observed both superficial fungal growth on the intact skin of cactus plants and plants injured or destroyed by fungal attack. Experts report fungal invasion of cactus plants by other vectors such as through wounds, cuts, and direct penetration of the cactus.
Also see MOLD GROWTH on SURFACES, PHOTOS - What Does Mold Look Like on Various Materials & Surfaces? An extensive photographic guide to mold as it is found growing on various building materials & surfaces.
Appearance of Mold on Cactus: on the plant & under the microscope
Black & Other Dark Colored Molds on Cactus Plants
Mold growth on cactus is more common in areas where cacti such as the Mexican Nopal (below left) grows in higher and less arid regions such as la Yerbabuena, near the foot of the volcano above Colima, Mexico. (Photographs by DF, la Yerbabuena, Colima, Mexico, November 2011) Pricklypear cactus (Opuntia Mill. are also found in the U.S. in Florida and Hawaii as Opuntia cochenillifera.
Our Mexican Nopal cactus mold photo (at left, 1200x) seeks expert help with confirming its identity, and is discussed below.
Below (right) and tentatively identified as a Lasiodiplodia theobromae -like fungus are microscopic images (approximately 600x) of the mold we found growing on the Mexican Nopal cactus.
The conidia (spores) are obovate to pyriform, with a thick cell wall, dark brown, smooth, with a single transverse septum near the base. They appear to grow in opposed pairs on either side of the hypha. The upper larger segment of the spore is generally darker than its base.
We also found, no surprise, species of Cladosporium sp. on this cactus surface. Some experts report that superficial molds such as powdery mildew may appear on some cactus houseplants. Mildew on cactus will appear white or gray-white and is principally a cosmetic issue. 
Lab microphotographs and work to identify the black cactus mold shown above are in process - Ed.
List of fungi associated with cactus plants
At left our photo shows a typical colony formation on the surface of a cactus plant, viewed by stereo microscope. Here are some of the many fungal species associated with cactus plants and/or the soils around them. Keep in mind that many fungi may be helpful to certain cactus species both in soils and at cactus roots and at other plant locations or in controlling cactus pests.
But some fungal genera/species are indeed reported to invade or attack and damage or even kill cactus plants including:
White & Light Colored Mildew & Other Molds on Cactus Plants
If the cactus is being kept in a too-wet or too-humid environment. Mildew infection of a cactus is more likely for plants grown out of their native (dry) environment, and when the cacti are kept close to other mildew-infected plants.
Because the skin on most succulents is so thick, mildew may do less damage to a cactus than to other plants.
However other white and light colored molds found in buildings and on some plants can be harmful to people and animals as well, such as some species of Aspergillus sp. and Penicillium sp. See WHITE MOLD PHOTOS for photographs of white mold growth in buildings.
Other "white stuff" we see on cactus plants may be a left-over deposit from having sprayed or washed the cactus plant with vinegar or other solutions.
Watch out: some "white stuff" on cacti and certainly on other plants may be mealybugs not a fungal infection, but deserving action.
Our photo (above left) shows what mildew spores look like under the microscope.
Treatments for Cactus Mold Growth
Some, perhaps most molds molds observed on cactus plants may not actually harm the plants but may remain a cosmetic issue for hobbyists. Popular cactus mold cures include:
Other Pricklypear Cactus Nopal or Tuna Photographs
Nopalitos are sold in local markets as well as larger supermarkets and are exported as well. The young nopalito pads are harvested and cleaned of thorns for sale. (Left and below left).
Nopal is sliced into strips or diced, then cooked alone (boiled or grilled) or with a mixture of onions and other herbs, and consumed as a vegetable.
The Nopal fruit or Tunas (see photo near page top and below right) are harvested using a long pole on the end of which may be a forked nail-pair used to hook the fruits. Tunas may be peeled and eaten as a fruit but quite often are immersed in water, on occasion with added sugar, to make a fruit beverage.
Directories of 6 atlases or indices of building mold
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