When things go wrong

It’s not all plain sailing in the wonderful world of cacti and succulents. Plants die and they gets pests and diseases. Don’t panic! It happens to be best of us and it’s just one of the challenges of growing these plants in an artificial environment.

One of the most valuable things I learnt from joining the BCSS is that any plant may just curl up its toes and die at any time. You’ve probably done nothing wrong and there is no clear reason why it has given up the ghost so and there’s no point in feeling responsible.

Even the most experienced grower finds this happens. For them it may be much more tragic in being a plant they have been nurturing for 20 years or more, or something particularly rare and expensive. One day they find it has become a pot full of mush or a hard, shrivelled lump.

I get more cacti that die after losing their roots rather than from rot. In habitat during the dry season the roots die back and regenerate when the rains come. The same happens in cultivation but roots are contained in tightly packed compost and may not be able to regenerate. Cacti can look fine for several years without roots although they begin to shrivel and the plants gets to the point of no return.

Don’t despair – they are only plants! Even with a sensible approach to cultivation there is always the risk that a plant will either not thrive or it will die (they are usually the same thing). Pests and diseases are a completely different matter. You haven’t invited them in (although you may have been careless) but they have nevertheless invaded your growing space. But what to do about it?

THE SICK PLANT
If a plant is not thriving or not growing at all, take it out of the pot and have a good look at the roots. Assuming there are no unwelcome visitors, the root ball may be compacted or the roots may have begun to rot or have died altogether. Wash off all the compost with a jet of water and let it dry out for a few days.

If the roots are dead you could try cutting off the bottom of the plant until you reach fresh tissue. Let the cut surface dry out for a couple of weeks and replant it in gritty compost (or neat Seramis or cat litter) and place it in a shady place and water very gently. If the plant as reached the ‘point of no return’, which is more than likely if it’s been in this state for a long time, this will not work.

If a plant has rotted, cut off the good bit well above the rotten part, let it dry off for a week and treat it as a cutting. You can dust the cut end with sulfur but that’s not essential. Rooting compounds do not, apparently, help.

The golden rule is “Don’t give up on a plant until you have at least tried to rescue it.”

PESTS & DISEASES
There is a lot of information available but here’s my take on the subject. Unfortunately, with changes to the regulation of pesticides many effective products have become unavailable in the UK, particularly systemic insecticides. The neonicotinoids insecticides have mostly been withdrawn in the UK although there are still a few brands available. While they are undoubtedly harmful to the bee population, used in the enclosed space of a greenhouse on individual plants they are unlikely to have any impact.

There are still effective chemical pesticides available in the UK but no longer for domestic use (although you obtain and can use them if you take a very expensive pesticides course and get a licence). Some are still obtainable in mainland Europe but it is illegal in the UK to use such products without a licence.

There are plenty of contact ‘insecticides’ that don’t actually kill all forms of the pest but employ physical action to reduce their numbers.

Things with 6 or 8 legs
Mealybugs
By far the biggest pest of cacti and succulents, there are several (indistinguishable) species but they all are equally annoying. There is probably no collection without at least a small population and you have to keep an eye open for them. Setting aside the root mealybug for a moment, here are some comments about prophylaxis and treatment.

Mealybugs are plant sucking insects of the order Hemiptera (which also includes aphids) and their effect is to damage the plant through their feeding activities and from the honeydew they produce. If there is a severe infestation the damage can be permanent but catch them early and you should be able to avoid this.

It is worth noting that an infestation can be the sign of a sick plant as mealybugs are attracted to them and/or more easily take hold. This is different to the occasional mealy bug or egg case on a plant.

My first action is to douse them with 70% isopropyl alcohol (IPA) which is obtainable from eBay and less foul smelling than methylated spirit (which you can also use). I apply with a small spray (you can buy these from your local chemist) or you can buy 1 litre spray bottles of IPA. I keep a couple in my greenhouse fully loaded and ready for action.

If there are a lot I usually isolate the plant, use alcohol first, and after 10 minutes spray it with an insecticide that is active against mealybugs. You can find these pests throughout the year but there is a surge in late summer as they seek a place to over-winter. The small fluffy eggs nests are very distinctive and should be removed and blitzed with alcohol. You may find them on the underside of pots so inspect them carefully.

Insecticides with claimed activity against mealybugs include Bug Clear Ultra. This is what the RHS has to say on the matter.

Many growers have started using soap-based products that work on contact by interrupting the waxy surface of the insects, but this has no action on eggs. SB Plant Invigorator is currently very popular but it requires frequent application and, like all physical insecticides it can leave a residue on plants. It is, however, very safe. One approach suggested to me is to spray on two consecutive days in the hope that the effect on the mealybugs on the first day will be strengthened by a second application, but this is just a theory.

I once bought diatomaceous earth, extremely fine silica powder which can block the breathing tubes (spiracles) of insects and give them a seriously bad hair day. It is used as a dusting powder in the poultry industry to control mites. However, this also leaves a residue and does not kill eggs.

Root mealybugs
One thing to clear up here is that your standard above-ground mealybug can be found below the soil but they do that to over-winter. They only venture as far at the space between the compost and the pot, not down into the root ball.

True root mealybugs (Rhizoecus species specifically) are very distinctive and live in the heart of the root system. They look like orzo pasta, completely clean-shaven things, very different to aerial species.

You will only find them when you repot a plant, or investigate why a plant is not thriving or is dying. The best treatment is to wash the roots completely, spray them with an insecticide and/or dip them in IPA, and let the roots dry out before replanting in fresh compost. After a few months, tip the plant out of the pot and have a poke around to see if they are still there – they probably won’t be as they appear to be more susceptible to insecticides.

Red spider mite
Red spider mite can result in devastating damage to cacti in particular, marking them to such an extent that they are only fit for propagation. The mites (related to spiders) appear suddenly during the middle of the summer in hot, dry weather, but it’s difficult to prevent them as they are present in the garden naturally.

The other problem is that the first sign of an infestation may already be too late. The epidermis of the plant becomes brown and you might think it’s sunburn but if you have a powerful magnifying glass take a look. With bad you may see a fine mesh of cobwebs (but many spiders produce similar ones). On the positive side, they tend to be isolated into one area, perhaps even just one plant. They die off in the winter and do not seem to remain in the greenhouse, unlike mealybugs.

Treatment involves immediately isolating the infected plants and those around them. Insecticides have no effect but you can try spraying them with IPA and/or SB Plant Invigorator. In fact, in 2020 I started using this weekly as prophylaxis during hot, dry weather. The mites don’t like damp conditions so even lightly misting your with water in the evening may help. Check out the RHS website for more advice.

Some cactus genera seem to be particularly susceptible to red spider mite such as Rebutia species and Lophophora. My R. steinmannii got a bad case a couple of years ago (see photograph) which was a bit upsetting as I grew it from seed in the 1970s. I’m letting it grow out and will take cuttings.

Other pests
Scale insects used to get a mention in the 1970s but I have never seen them on my plants. I have not yet come across Western Flower Thrips but know that some growers have. I have had larvae of vine weevils in the soil of some outdoor succulents, including a Sedum which they chewed off at the roots. They are particularly common in the south of England. Snails sometimes crawl through the window of the greenhouse and leave their tell-tale slime. They have been known to graze the tops off spineless cacti but they seem more interested in the paper labels on my seed pots. Occasionally, moths lay their eggs and their caterpillars graze on seedlings.

Biological control of insects and mites
This is of no benefit in a collection of cacti and succulents because there needs to be an active background population of the pest species for the control species to be able to flourish. So unless you are willing to tolerate mealybugs all year round this won’t work. For spider mites it’s definitely not an option because you need to isolate the plant immediately.

Biological control is more suited to crops grown under glass, and although many botanical gardens use these control agents in their collections I suspect they are either more for show or they are more willing to accept a pest population than the average grower.

Fungal diseases
Black mould is a potential problem, particular in the cold, damp winters in the UK. I have had the odd case of mildew and black mould and have found that Bayer’s Fungus Fighter is useful. Many older agents such as copper-based fungicides, as now banned in the UK.

Some cactus species have extrafloral nectaries which exude sugars that attract black mould. I’ve not found a way of removing the black coloration or stopping it from happening, which is a shame as it spoils the look of a plant.

Some opuntias are particularly prone to what is said to be a fungal disease, particularly Tephrocactus bonnieae which I have never been able to grow as a clean plant. My two Quiabentia zehntneri specimens have both been stricken by a similar lurgy but continue to grow.

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When things go wrong