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.