General Info, Succulents -


Root rot is one of the main killers of succulents and cacti and is due to too much water.

Too much water can kill your plants by either creating conditions for ‘bad’ fungus to thrive or by creating oxygen-deficient (anaerobic) soil. If your succulent or cacti is planted into the correct soil type with adequate drainage the latter shouldn’t be an issue. But root rot fungi such as pythium and phytophthora can still be.

We can control our own watering regime to minimize this risk but we have no control over climatic conditions.

When prolonged rains come, as they sometimes do, what pro-active steps can be taken to protect your outdoor succulent or cacti garden?

The adage of prevention is better than cure could be applied here. And that is because if root rot gets a hold of a plant it may initially show no really visible symptoms and can be impossible to deal with once it has ‘stolen the march’ on you. Which it can do extremely rapidly.

(And I should add, root rot diseases in plants generally are more common than might be thought).

The prevention of root rot is in the form of a systemic fungicide.

Most of the commonly available systemic fungicides used to control root rot (Yates Anti Rot, Searles Root Rot etc..) have phosphorous acid as an active ingredient and are applied as a foliar application.

When foliar applied the phosphorous acid (in the form of mono & di-potassium phosphonate) will be taken in through the leaf and will then translocate (move) throughout the plant. It’s chemical makeup allows it to be very mobile within the plant, even translocating into new growth.

(However, if the plant is not actively growing then there will be limited translocation. So don’t apply if the plant is either dormant or stressed).

Typically, in relation to ornamentals, the directions on a product label for a phosphorous acid based fungicide will state to “re-apply every 4-6 weeks when conditions favour disease”.

So, for instance, you could apply a systemic fungicide such as Yates or Searles prior to a predicted/prolonged wet period on the basis that, barring a ‘Noah’s Ark’ event, the residual effect of the application will carry you through until the weather settles again. This could be such as in key periods during a wet La Nina summer.


Because for a start cacti, of course, don’t have any visible leaves so you are effectively spraying the stem.

I contacted a number of manufacturers to get their take on it.

One went away to confirm with a grower. Another emailed us back an answer. Another didn’t respond but when I dropped by their office it appeared they were having an issue as to whether cacti by definition were classed as ornamental. Ultimately I never got a response from this company, so perhaps they never came to agreement on this question?

In the case of both who did respond, they stated that you can use their product on succulents and cacti at the ornamental rate and as a foliar application.

But they emphasised following the instructions for use which includes (for ornamentals) “testing on a few plants of each species prior to main application” (or wording close to that dependent on manufacturer). In one response the company stated it was “best to perform a test patch firstly in an inconspicuous area (of a plant)”.


Many of us love our succulents and cacti just too much to ‘test’ something on even a part of one, let alone completely on one of them!

In reality though, they are just covering themselves, which is understandable.

Personally, we have used a soil drench systemic fungicide (which has two active ingredients, neither of which is phosphorous acid) for a long time now with excellent results.

So can’t we just soil drench a phosphorous acid product as well?

After all, at least one online gardening source states that it can be used both as a foliar application and soil drench.

And it is true some trial work back in the 1980’s with avacadoes in Australia did appear to show positive results to soil drenching (as well as direct injection into the tree trunk).

Interestingly, when you look at the instructions for use today from a supplier of a very highly concentrated phosphorous acid product intended for horticulture use, direct injection is given as a method to apply to avacadoes but soil drench is not.

So, did something change between then and now? I don’t know, but I encourage anybody who does to drop us a line and enlighten us.

All I can tell you is this:

Manufacturers emphasise following the instructions for use and not deviating from same and so do we.

Which means that you either foliar apply what is a pretty environmentally safe product in a phosphorous acid based systemic fungicide (after having tested it first on some plants or parts of plants. OR NOT!) or you use a systemic fungicide that is applied as a soil drench (these are specialist products from horticultural suppliers and aren’t cheap).

Whichever way you go, when it gets consistently wet, appropriate use of a systemic fungicide to control root rot can reduce the chances of losing a plant and avoids the inherent heartache of losing plants you not only love but which may have taken a long time to attain their size.



Fungicide resistance.

Australia has some very extensive rules for the use of fungicides in agriculture that relate specifically to rotating the fungicides used to help prevent fungi developing resistance as a result of overuse of any one particular chemical.

Although overuse of a fungicide by a home gardener and any subsequent resistance that might potentially build up does not have quite the same commercial ramifications as in broadacre cropping and the like, it should still be taken into account.

The systemic fungicide we use is classed as a Group 1 14 fungicide and there are clear guidelines for its use in this regard.

In respect to phosphorous acid as a fungicide, the international organisation FRAC (Fungicide Resistance Action Committee) class it as low risk for resistance build up.

Thats a big plus for these types of products.


nb. Phosphorous acid and Phosphoric Acid, although they are very close in both name and chemical formula, have vastly different properties with the latter classed as a hazardous and corrosive material.

Do not get the two confused.




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