Sooty mould

One of the most frequent questions I get asked is about sooty mould.

I always try to illustrate my posts but, oddly, I can’t put my fingers on a photo of sooty mould. So I will ask you to use your imagination. You have a camellia, bay tree or, quite likely, another large-leaved evergreen in a pot, and the leaves are covered with a matte black coating. One particular question recently was classic – ‘What is the cause of the black soot like coating on the leaves of my camellia and what can I do to remove it?’

Well the black material is a fungus called sooty mould. If it is a bad infestation it can be so thick that it interferes with the light getting to the leaves and will weaken the plant. It remains all year and tends to be impervious to water. But it is possible to wash off the mould with a washing up sponge and weak washing-up liquid solution. The plants afffected tend to have thick, plastic-like leaves and will not be harmed by this process as long as you use a sponge and not a scourer. You should rinse off the leaves after removing the mould too.

But this is not the whole problem. The mould damages the plant only because it reduces light getting to the leaf and it does not actually invade the tissues and do direct harm, unlike black spot or mildew. The sooty mould is growing on, and is sustained by, sticky, sugary sap excreted by scale insect.

So if you see sooty mould you can be sure that scale insect is present. These limpet-like insects are largely stationary and live on the underside of the leaves, often along the main veins, and on stems, where they suck sap – or rather just have sap pushed into them by the plant, and excrete the excess as ‘honeydew’. Aphids are much the same and most people who have walked or parked under lime trees, infested with aphids, will know about the constant dripping of honeydew.

So although you can get rid of the sooty mould you also need to eliminate the scale insects. Like most pests, they seem to thrive best on plants under stress. So you tend not to get bad scale insect on a bay or camellia in the garden border but very commonly on plants in pots. When in a pot it is easy for the plant to be underwatered and underfed, especially if planted in multipurpose compost. These plants, often with yellow leaves through underfeeding or too much sun and insufficient water, are the most prone to scale insect attack. So what to do.

If you have time and patience you could pick or scrape off the scale but it is probably better to spray in this case. If you use an organic spray that suffocates rather than poisons the pest, you must cover the whole plant, especially the undersides of the leaves. If using a chemical then you may need to repeat spray several times. Systemic sprays, that are absorbed into the sap and kill even the individuals that you don’t hit with the spray, should be used with caution but this is a case where they are useful. The flowers are not especially attractive to bees and other pollinators and if grown in pots the chemical is not contaminating the rest of the garden.

But bad scale maybe a sign that the plant needs planting out, repotting or better general cultivation.

Bay trees are also very commonly affected by scale and sooty mould but be careful if using insecticides on these if you are using the leaves for cooking – though I wouldn’t use them if covered in scale or mould anyway.

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8 Comments on “Sooty mould”

  1. tonytomeo
    December 17, 2022 at 7:41 am #

    Bingo! It happens among citrus, and is particularly common among grapefruit, as if scale prefer grapefruit to the other citrus.

    • thebikinggardener
      December 17, 2022 at 9:05 am #

      Yes I forgot about citrus. I don’t know where scale comes from. I only have a lemon at the moment but it seems impossible to eradicate despite spraying now and then and scraping off obvious scale now and then – I suppose I should be more dedicated. I didn’t know that grapefruit was the tastiest for scale!

      • tonytomeo
        December 17, 2022 at 11:39 pm #

        Scale will infest any citrus, but grapefruit seems to be their favorite. My colleague’s ‘Eureka’ lemon is always infested with scale, partly because he can not keep them out. He can spray the trees with water, but that only mildly annoys the scale. The trees are very sheltered from wind, and mingle with adjacent vegetation. I know that I could get rid of the scale, but he would not like it. I just prune citrus trees in such a manner that they can not touch other vegetation or infrastructure, and put a barrier around the base of the trunk. ‘Tanglefoot’ is ideal, but I just use common axle grease at home. (Axle grease eventually weathers and deteriorates, so must eventually be applied again.) For severely infested specimens, I prune them aggressively to temporarily increase air circulation. As they recover, I do not repeat that part of the process. If the ants that cultivate aphid can not get into the infested trees, the scale are controlled by predatory wasps. Even if the scale survive, they are controlled to an acceptable degree.

  2. Jaye Marie and Anita Dawes
    December 17, 2022 at 10:42 am #

    This does not sound like what I see on my favourite camellia. It has black spots that don’t rub off. The only way to get rid of them is to remove the leaves. But the spots come back. Any ideas?

    • thebikinggardener
      December 17, 2022 at 11:27 am #

      There are a couple of fungi that affect camellias and cause brown leaf spots, that can expand to blotches. As these age you may find black ‘spores’ on the affected areas. They can cause die-back of twigs too. You may be able to control it by removing affected leaves or by spraying with an insecticide to prevent spread.

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