Fungal / Mold Growth on Cactus
Opuntia Mill. fungal infections, white fuzz, mealy bugs

Mexican Nopal, or in English "prickly pear cactus"

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 [22] claim over 200 species in that group, the USDA lists 59 species and 75 accepted taxa within the Genus Opuntia Mill.[20]

I am particularly interested in fungi found on species of Nopal found at altitude in more wet or humid highlands of Mexico.

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 also investigates and documents the causes and effects of mold infection of Mexican prickly pear cactus plants as well as other types of cactus, and the identification of other mold genera/species commonly found growing on and affecting those important plants.

These include fruit-bearing prickly pear 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. [22]

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 (prickly pear) 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.

Watch out: while this article discusses cactus diseases that appear to be fungal or mold growths, some insect or parasitic cactus diseases maybe mistaken for mold.

At CACTUS COCHINEAL INFESTATION you will see examples of white fluffy or barnacle-like diseases on cactus that may not be funguses at all.

Above: small round black fungal colonies on a Nopal in Yerba Buena in Colima, Mexico.

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.

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Appearance of Black & Other Dark Colored Molds on Cactus: on the plant & under the microscope

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) Prickly pear 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. [1]

Lab microphotographs and work to identify the black cactus mold shown above are in process - Ed.

Question: cause & treatment for black mold on cactus

(Feb 5, 2015) Darlene said:
I have a Suharo that looks healthy but it has several hard black circles close to the bottom of the cactus. Is the mold? and what should I do to stop it?
Thanks

Reply:

Darlene

Over-watering or too much shade are factors that can increase the risk of mold growth on a cactus but of course by e-text I can't see what's going on with your plant.

If you like use the email found at our CONTACTS link (page bottom) to send me some photos of your Saguaro (Carnegiea gigantea) cactus common in the Sonoran desert and in the Organ Pipe Cactus National Monument in the U.S. A 5-armed saguaro is likely to be as much as 200 years old.

Below is a photograph of round black fungal attack on a different cactus, the species discussed on this page, the Nopal.

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List of fungal diseases of cactus plants

Just above 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.[16]

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[13][19].

But some fungal genera/species are indeed reported to invade or attack and damage or even kill cactus plants including:

• Fusarium sp. [2]
• Aspergillus fumigatis [through plant soil][12]
• Aspergillus niger [through plant soil][12]
• Aspergillus parasiticus [possibly]
• Colletotrichum sp, (anamorph: C. gloeosporioides) (cactus stem rot)[17]
• Fomes robustus (heart rot fungus) [18]

• Glomerella cingulata (cactus stem rot) [17]
• Lasiodiplodia theobromae - a member of the Botryosphaeriaceae/ B. rhodina family, wide ranging plant pathogen, may cause fungal (mycotal) keratitis and stem-end rot, a citrus plant disease, possible cause of nail and skin lesions on humans [5][6]
• Phialocephala virens
• Poria sp. [18]
• Sporothrix schenckii [12] [human pathogen , Rose-picker's disease]
• [Possibly] Yeasts [10][11] brought by insects
• 22 fungal species associated with cactus plants were identified by an assay of 900 endophyge isolates[14]

Comment: Apressoria or hyphopodia on cactus

(Sept 8, 2015) Ludovic said:
The mold you show here is none of the one suggested. The peg-like projection from the darkly colored hyphae are called 'appressoria' or 'hyphopodia', used to anchor on their substrate and also suck up nutrient from it.

The ones you show are characteristics of epiphyllous fungi in the Orders Meliolales (or black mildew) or Asterinales, depending the ascoma (=fruiting body, spore containing structure) associated with them. Did you find big spore with 3 to 5 cells (Meliolales) or small spore with only 2 cells (Asterinales)?

I love fungi but I have no idea how to treat plant that carry them. It seems to me that these fungi proliferate in particularly wet places (tropical?). I saw many of them on almost all sorts of plants in some very wet part of Costa Rica. I don't know if that will help.

Reply:

Thank you Ludovic, I've continued to collect fungal growth from succulents, particular cactus plants in central Mexico.

From the in-situ look these do not seem to be a mildew - the surface is very scaly and hard.

When I'm back I'll take a look through the microscope to see what spores we can isolate - if any. Possibly during wet weather they'll make a cameo appearance. But generally these are appearing at various altitudes up to 7000 ft but in a usually-dry climate.

I'd be grateful for any citations / references you can add for cactus-infecting cacti.

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White & Light Colored Mildew & Other Molds on Cactus Plants

White stuff found on both indoor and outdoor plants (photo at left) including cactus plants may be mildew,

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.

Our photo above shows what mildew spores look like under the microscope.

Photographs of mildew on plants are at MILDEW in buildings ? and advice on curing & preventing mildew on plants is at MILDEW REMOVAL & PREVENTION

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.

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White "Mold" on Cactus may be an insect scale: Cochineal, Not Mold

Shown here: Using a deer's tail as a brush, the worker is collecting Cochineal. The painting is by José Antonio de Alzate y Ramírez (1777), and the caption reads: Fig. 1, Indio que recoge la Cochinilla con una colita de venado, Fig 2. Dicha [colita de venado], Fig 3. Xicalpeftie en que aparan la cochinilla.

- source: Wikipedia, retrieved 2021/06/09, original source: https://en.wikipedia.org/wiki/Cochineal

[Click to enlarge any image]

Cochineal (Dactylopius coccus) is a white cactus scale insect that is both a cactus plague and a fascinating organism with a long history in Mexico.

Remarkably, if you smash a bit of this white scale on a cactus you may find an intense carmine red liquid (alizaron crimson) under your thumb.

While Cochineal (Dactylopius coccus) is a cactus parasite, it was also very valuable as a source of red dye that was so valuable as an export product that until the close of the Mexican War of Independence, Spain kept this source of red dye a state secret and held a monopoly of its production in Mexico. The Wikipedia article we cite above notes that

Pigments produced from the cochineal insect gave the Catholic cardinals their vibrant robes and the English "Redcoats" their distinctive uniforms. - Wikipedia Op. Cit.

Continuing today in the Mexican state of Oxaca, the intense red Cochineal dye is still used to give a stunning carmine color to hand-woven textiles.

In Mexico we find this parasite and dye source particularly on the prickly pear or nopal plant, though I have found it on organos and other cactus plants too, though on those plants it usually appears at a much lower volume and area.

Reader Question: tons of white mold on cactus trees

2016/08/22 Jane C. Ti said:

Dear Cactus Specialist, Our Cactus trees which are about more than 12 ft. tall, & they did have pink fruits, guess = Prickly Pears; are all sick with tons of white mold, please see the attached picture.

Would you please advise me, regarding: how to rescue these poor sick ones.

Thanks in advance! Sincerely looking forward to hearing from you at your convenience! Desperately, Jane

Shown here, la Cochinilla (Cochineal) on a Nopal cactus, illustrated in "el Nopal", Artes de Mexico, No. 59.

Reply:

Jane:

With regret, I'm a cactus-interested investigator but not a cactus expert by any means.

Your description sounds like one of the species of Nopal or prickly-pear cactus (Opuntia is the "prickly pear cactus" genus in the cactus family, Cactaceae). There are about 90 species of Opuntia.

You can use our page bottom CONTACT link to send us photos for posting or comment.

From what I've read as an amateur, fungal diseases on cactus plants are difficult to treat, in my opinion largely because the fungus is environmentally caused - all molds are everywhere all the time: it's the local conditions that make a mold flourish and become a problem.

In the left of this photo you see a light gray-white fungus on the nopal and a common parasite climbing on the organos cactus in the right of the same image.

These cacti were photographed in Pozos, in Guanajuato, Mexico.

at least one cactus "expert" (cactiguide.com) gives the depressing advice that you give up, take a clean healthy cutting and start a new plant.

In my view that's not gong to cut it if problem source in the local environment - too much water, shade, or something else isn't also found and corrected.

I'm doubtful that a fungicidal spray will be effective for dark scaly mold damage: the cacti I've examined with such black or dark molds were almost impossible to clean of mold growth.

But here's the good news. Well good of a sort.

Often a white "mold" or "fungus" people think they see on cactus is not a fungal disease but rather an insect. In fact often a white fuzzy "deposit" seen on Opuntia is cochineal (Dactylopius coccus) a scale insect in the suborder Sternorrhyncha, from which the natural dye carmine is derived.

Cochineal produces an intense red dye whose cultivation in pre-hispanic Mexico and whose later production was kept secret by the Spanish in Mexico.

Without over-pricking yourself, scrape off a bit of the white stuff and rub it between thumb and forefinger.

If your fingers become stained with an intense red dye, that's Dactylopius coccus (some texts refer to it as a "mealybug" ).

If that's what's on your cactus you might be able to clean it off with a soft long-bristle brush and dish soap or an insecticidal soap.

I'd avoid very strong power washing as it's likely to damage the cactus itself. Locally here in San Miguel de Allende our jardinero expert uses neem oil on several plant species to resist insect attack.

How to remove Cochineal from your cactus

We have successfully cleaned off small infections by the Cochineal scale insect using a soft brush and an insecticidal soap; those products are discussed in this article. You don't have to use a deer's tail.

Frankly I don't like to disturb the scale unless it's become a serious infection on a valuable plant.

White Growth on Melon Cactus in Iran

Reader Question: white growth on Melocactus diamantinus and other cactus

2018/10/07

im from iran
I have a few cactus and i have a problem (disease)

My Melocactus diamantinus and a few other cactus have a disease but I can't find a treatment.

I need to help find diseases and treatment ? I send few picture this email .
Thanks for help. - Anonymous by private email

Reply: diagnosis & treatment of white fluffy or barnacle-like infestation on cactus: mealybug / cochineal insects

As we illustrate starting at CACTUS COCHINEAL INFESTATION you will see examples of white fluffy or barnacle-like diseases on cactus that are insect-based rather than fungal.

Try scraping off some of the larger white growths on your cactus and mashing those between a thumb and forefinger. If you see a red dye in the result your cactus are infested with mealy bugs or cochineal insects.

For other readers, English names for Melocactus diamantinus include Turk’s Cap, Melon Cactus

Treatments for Dactylopius sp. - cochineal mealybug infestations on cactus

• For limited cochineal / mealybug infestations
  
  and those detected early you can often remove the insects using a garden hose with a spray nozzle, taking care to aim the spray so as to send the mealy bugs and their white fuzz into a harmless area of the ground, not down into the soil around your cactus plant.
• For more firmly-attached barnacle-like white cochineal growths on cactus
  
  you may need to use a stiff bristle brush or on smaller cactus plants, a toothbrush, to scrub off the growth. Scrub using an insecticidal soap or neem oil mixture.

What are Dactylopius sp. - mealy bugs, or cochineal bugs ?

[Mealy bugs] are small scale insects that feed on the cactus.

They produce fluffy white wax that hides their bodies as they feed on the cactus and protects them from the elements (drying out, in particular) and from predation.

The fluffy wax also serves as a sail or balloon to float on the wind and take the bugs to a new patch of cactus.

The bugs produce carminic acid that also helps protect them from predation, especially from ants.

This carminic acid in the bugs has been used by indigenous peoples of southwestern North America, Central America and sub-tropical South America to make a brilliant red dye for centuries, perhaps millennia, to produce beautifully colored textiles.

Originally, the cochineal bugs were limited to the New World. When the European explorers visited and saw the beautiful red cloth of the natives, they took the cochineal bugs back with them and now they occur all over the world. - source: Ladybird Johnson Wildflower Center, 4801 La Crosse Ave., Austin, TX 78739 USA, Tel: 01 512.232.0100, https://www.wildflower.org/expert/show.php?id=9592 retrieved 2018/10/07

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How to Remove a Mealybug Infestation on Cactus

Watch out: some "white stuff" on cacti and certainly on other plants may be mealy bugs not a fungal infection, but deserving action. Our mealybug photo below was provided by Iranian reader P.C. 2018/10/12.

• The insecticides discussed earlier on this page
  
  are helpful at dealing with mealybug infestation. Alternatively, cleaning the infected area with a soft brush and a spray of 70% isopropanol (alcohol) can be effective at removing mealy bugs from cactus and other succulents.
• Using a soap solution and brush
  
  can also rid mealybug infections.
• Neem oil,
  
  as we discuss in this article, can be sprayed as an effective mealybug control.
• "Commercially available beneficial insects,
  
  such as ladybugs, lacewing and the Mealybug Destroyer (Cryptolaemus montrouzieri), are important natural predators of this pest.", according to planetnatural.com.
• Pesticides such as Bonide Chemical 952 Number-1 2-Percent Systemic Granules
  
  can be applied to the soil to (ultimately) provide systemic protection against recurrent mealybug infestations.
• Natural predator insects
  
  that attack mealy bugs and soft scale include Cryptolaemus montrouzieri, aka dark brown ladybugs, small predatory beetles sold by garden suppliers. These insects, in both larval and in their adult forms, feed on aphids, mealy bugs, and scale.
  

Treatments for Cactus Mold Growth - may also be helpful with mealybug infestation

Some, perhaps most molds, observed on cactus plants may not actually harm the plants but may remain a cosmetic issue for hobbyists. Popular cactus mold cures include:

• Removal and disposal
  
  of the mold-infected cactus part if feasible - suitable for cactus house plants;
• Using a cotton swab moistened with vinegar
  
  or with 70% alcohol to clean small infected areas of cacti used as house plants.
  
  Watch out: cactus horticulturists point out that vinegar can be purchased in varying strengths for horticultural and other uses (thus its effectiveness as a fungicide may vary), and it is also reported that using vinegar on cactus may both help and harm the plant, depending on the strength of the vinegar solution, its application, and the plant species.
  
  Also, there may be a connection between subsequent cactus damage and acetic acid bacteria. [8] Stan Starbuck reports that while vinegar has been used successfully to treat certain fungal infections on soil surfaces

  Some plants such as Aloe, Haworthia and Euphorbia suffered small amounts of cosmetic skin damage. Other plants sprayed in the same manner such as Hurenia, Echinocereus, Mammillaria and Rebutia suffered fatal results.[7]

  Watch out: furthermore, often "white stuff" that appears to be "growing" on the surface of soil around plants, particularly houseplants, may not be a fungus at all, but rather a white mineral deposit left on the soil surface from watering activities.
  
  The "vinegar cure" for this condition is not killing off a mold infection. Instead it is dissolving the mineral salts back into the soil, just as we use vinegar to de-scale a coffee maker.

• For cacti grown as crops,
  
  different approaches are needed including attention to mold sources (piles of organic debris nearby) and perhaps dusting the crop with a Bordeaux mixture.
• Avoid over-watering cactus
  
  at any time but particularly if it is suffering from mildew or mold growth.

Question: treatments for white fungus on cactus

(May 20, 2014) mark said:
White mold or fungus treatment on Nepal cactus

(Oct 11, 2015) steven said:
have a 12 foot cactus and i see white mold on it. How do I save it or is it doomed?

Reply:

Mark, I guess you meant to type Nopal. Indeed there can be more than one genera/species of fungi on a plant, or the same fungus may appear differently at different growth stages.

I'm doubtful that a spray fungicide is the best approach for the plant - at least not before diagnosing the cause of the fungus growth. For example an over-wet plant may continue to have trouble.

Australia's Garden Web (www.au.gardenweb.com) has listed these fungicides used successfully on cactus plants

• Benlate
  
• Fongarid
  
• Hortico Rose dust fungicide and insecticide
  
• Sulphur powder
  
• Yates Garden Spray insecticide fungicide

Depending on where you live, different products may be available.

Watch out: if you are harvesting nopal for food use be sure to read the MSDS and toxicity data for the fungicide that you are considering.

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More Prickly Pear Cactus / Nopal & Paddle / 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.

Below, two more common afflictions of nopal cacti in

San Miguel de Allende, Guanajuato, Mexico.

Below: parasitic attack on Organos cactus

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Cactus Rot Prevention & Cure

Watch out: the number one killer of cactus plants kept by many enthusiasts is over-watering. Cactus rot kills of the roots of the plant first so by the time you see rot there may be more damage than is obvious.

But it may be possible to save your rot-damaged cactus by judicial cutting, drying, sun exposure, and avoiding over-watering.

Below: Organos cactus that are telling you they've had enough water. Stop watering these plants.

In the case of Organos that appear in several photos on this page including just above, the cactus will show you that it has had plenty of water by its swollen appearance.

On 2021-06-03 by mak.church (mod) - black rot on cactus tip

@Heidi Sebastian,

You're on the right page and I would direct you to the section: on this page:

Treatments for Cactus Mold Growth or mealybug infection. Or rot.

Your spray might be helpful but as we both will read in cactus mold treatment advice from experts, one needs to match the spray or other treatment to the specific pest: molds, insects or,

if the problem is over-watering that causes root rot. But rot can appear at the cactus at any location, near its base, along its stem, or as in your case, at its tip.

I think your cactus has stem rot showing up at the tip, better called "cactus tip rot".

Cactus pros say you have to act quickly to remove the rotting tip of the cactus.

Use a sharp and clean knife or other cutting tool (disinfect it with soap and water or in 95% alcohol before using it) to cut off the damaged tip.

When you cut off that tip, look at the cross-section of the cactus top that remains: if there is any rot in its center, you need to cut a bit lower down on the plant until you see only healthy tissue.

Now keep the cactus out of the rain and give it sunlight until the exposed cut area has dried and become callused over - it will turn a thick gray colour.

Don't over-water your cactus.

On some of our organos in pots in Mexico we have had to stop watering them entirely as even when rains are intermittent more watering was killing the cacti.

On 2021-06-02 by Heidi Sebastian

My cactus has recently got black spots and more so at the tip. Can you please advice how to treat? I’ve cut some of it out and sprayed with an organic garden spray.

[Photo above]

On 2020-06-08 - by (mod) - saving a cactus paddle or pad by re-planting

Emili

That looks to me less like mold than like cactus rot. Over-watering or too much rain can indeed cause rot. The cactus roots will be totally gone.

If I were trying to save that pad I'd pull the over-wet, partly-rotted end out of the planting soil and instead flip it over, put the stem side down (assuming its dry), and keep watering to a minimum for a time;

the rot-damaged area won't get pretty but you may get some new growth off of the sides of the paddle.

On 2020-06-08 by Emili

What should I do with this Santa Rita pad?

It got rained on one day and I believe this caused it to mold

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Other Black Damage on Cactus

Anon from Feb 2: re: black cactus fungus - Sun Scorch

That looks to me like Henersonia opuntiae a fungal infection aka "sun scorch" (which isn't really due to sun).

You should remove those infected Nopal cactus or "paddle cactus" pads by cutting or twisting them off (we use cooking tongs to hold the paddle).

Then dispose of the paddles as bagged garbage or burn them so as to avoid spreading this fungus to the rest of your cactus plants.

On 2020-02-16 by Anonymous

I have white substance on my cactus and when I scrape it off it looks like a cross between soft rice and very small styrofoam. How do I get rid of it?

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Cactus Scale Damage & Scale Removal & Treatment

These Organos cacti, gift of my sister-in-law Lili, live in pots, get pretty-good sun, and were sometimes over-watered. ( I didn't do it!)

Definition: Cactus scale is a small light colored or white flattened "armored" (hard-surfaced) scale that is circular and has a dark center spot. It's found growing only on cactus, though of course there are other armored scales found on other plants.

Above: you can see that this cactus has got a lot of water; its pleats are quite expanded. It's also got a lot of scale that developed over several years. At first we just watched it and hoped (in futility) that the scale would magically go away.

Left alone, the scale simply has a fantastic time growing all over the cactus. More sun helps fight it but that's not enough. Below: feeling aggressive and frantic we decided to use, household food-grade white vinegar, straight out of the bottle, as a cleaner. That's a bit strong.

I decided not to use the white brush shown in the photo above as its bristles seemed a bit too stiff and scratchy. Instead I used the softer natural bristle brush shown below.

I scrubbed, but not too hard. It's better to scrub gently, with more liquid cleaner, and take more time. You want to leave the cactus' skin un-harmed. The result is below.

The photos show a successful effort to remove cactus scale growth from these fellows. I used household food-grade vinegar, but will describe some other remedies below.

Treatments for Cactus Scale

A common treatment for cactus scale is gentle cleaning of the surface using either a commercial insecticide or treatment sold for cactus scale such as those listed here.

• Insecticidal soaps
  
  such as this product sold by Garden Safe [photo above] can be sprayed on all sorts of plants and plant parts. This product is said to control aphids, mealy bugs, mites, Leafhoppers, Psyllids, Scale insects, Thriips, Whiteflies and "other listed pests".
  
  Manufacturer's product description:
  
  Potassium Salts of Fatty Acids, the active ingredients in Garden Safe(R) Brand Insecticidal Soap Insect Killer, are made from plant-derived sources.
  
  The active ingredients damage the cell membranes of many soft-bodied insects, such as aphids, whiteflies and mites, killing them through contact.
• Neem Oil Spray -
  
  an "organic" insecticide, fungicide, and miticide (mite Killer) (shown below) - we use this also on our peach trees but not close to harvest. Be sure to use Neem oil as directed by the manufacturer. Shown here is Neem Oil spray from Bonide.
  
  Bonide, Ortho, and other manufacturers sell a wide range of fungicides and pesticides for plants. Other examples are copper fungicides, a mixture of Neem oil with fungicides and miticides, Be sure to read the manufacturer's label about proper selection of the treatment as well as how it should be applied.
  
  Bonide's description:
  
  An all purpose insecticide, miticide, fungicide for organic gardening. Derived from the Neem seed, Neem Oil is an excellent choice for use on virtually any plant, including roses, flowers, vegetables, herbs, spices, houseplants, trees, turf and shrubs. Kills all stages of insects - eggs, larvae and adults. Makes a great dormant spray.
  

A spray mixture of 50% rubbing alcohol (76 to 95 %) and 50% water and 4-5 drops of dishwashing detergent to make a total of about 16 oz of liquid (2 cups).

Put that into a clean spray bottle, spray the scale, and wipe it gently with a soft cloth or a sponge. On our tall Organos cactus which are the iron-men of San Miguel de Allende I used a natural-bristle scrubbing brush that worked quite well.

• Systemic insecticides for cactus scale:
  
  some cactus experts suggest that if your cactus have a lot of scale you might prefer a systemic insecticide such as Imidacloprid applied as a "soil drench" to the soil around the cactus (+ normal watering).
  
  
• Natural predator insects
  
  that attach some, but not all scales, may be effective on soft cactus scale: Cryptolaemus montrouzieri, dark brown ladybugs, sold by garden suppliers in both larval and in their adult forms.
  
  Really? Well not everywhere. These insects feed on aphids, mealybugs, and soft scale and are effective in greenhouses where humidity may be kept high (over 70%) and temperatures kept between 64 and 80 F; they will probably not survive very hot conditions such as the Arizona desert in July nor very dry conditions such as the high plains of Guanajuato.

If you clean off the scale before it coats most of the cactus you can probably save it.

I also avoid over-watering and too much shade.