Sooty mould - What is it, and how to control it organically - © Duncan Smith MSc

A lot of people wonder what the black stuff is that they sometimes find on the trunks, branches, leaves and fruit of some garden plants and trees and also in some our native forests. It’s called sooty mould, because of its colour, and caused by several different microfungi, including Capnodium, Alternaria and Cladosporium spp. They quickly colonize the sticky, sugary secretions found on some plants from the feeding activity of many different sap-sucking insect species on them. Also, some plants, including citrus trees, can secrete sugary solutions naturally. These secretions, called honeydew, are very sweet and an ideal medium on which these fungi can flourish.

Ants farming insects

A sign that you have an insect problem is that you may see ants running up and down your plants. If you turn the leaves over, or look carefully on the rest of the plant, you might find a range of sap-sucking insects hiding there. They might include aphids, whitefly, mealybugs, scales or even mites (mites are not insects but belong to the spider family). Ants have developed a symbiotic relationship with these other insects and are herding, or farming, them for the honeydew they produce. By stroking them with their antennae, ants can induce a honeydew droplet from the back end of these pests. It’s a highly nutritious food for them, and in return, they tenderly look after and protect their charges from predators.

They have been known to move these insects to areas on the plants where the best sap is found. If it rains, they may take them to more sheltered places, sometimes into their own nests. Some ant species clip the wings off aphids to prevent them from flying away, and some use chemicals found in their feet, to drug aphids and prevent their wings from developing altogether.

Sap-suckers: the cause of the problem

Close-up of sooty mould peeling away from a citrus leaf. © Duncan Smith

Sooty mould can be easily scraped off with your finger to reveal the green surface of the plant underneath. Sometimes your car can be spattered with drops of honeydew if you park under a tree infested with sap-sucking insects. This mould can be easily removed by gently washing it off plants or cars. However, washing it off could be time-consuming and you may also damage the plants' protective, waxy surfaces.

So, to get rid of sooty mould, you need to get rid of the cause of the problem – the insects – any new plant growth after that will be clean and healthy.

Aphid feeding with siphon-like mouthpart (stylet). © Duncan Smith.

These sap-suckers all possess one thing, a siphon-like needle mouthpart (stylet) that they plunge into the stems or leaves of plants to feed on the sap. They don’t have to suck, as the plant pushes sap into them. If you look carefully at a colony of aphids, you may actually see them doing a little dance up and down as the plant pulses sap into them, just like a small Mexican wave!

Sucking insects commonly transmit serious pathogenic viruses and bacteria directly into the plant tissue when they feed. This can cause various types of disease.

Aphid producing live young. © Duncan Smith

These insects are all seeking protein for the production of their eggs or live young. However, plant sap contains mainly sugars and very little protein, so they have to take in vast amounts of fluid, and excrete the sugars. These rain down onto the understory below, covering trunks, leaves and fruit with a thin layer of sweetness. Fungal spores of various moulds land on this, grow and flourish, and turn the plants black and sooty.

Sooty mould in native forests

Sooty mould growing on a New Zealand native tree

In New Zealand’s native forests, severe sooty mould infestations can be seen on the trunks, branches and leaves of beech, manuka and other native trees. In beech forests, if you take a close look at the blackened trunks you may see tiny hair-like structures protruding from them. These are the anal tubes of scale insects feeding on the sap. At the tip of these ‘hairs’, you may see a tiny, glistening, translucent pearl of honeydew. This is an attractive food source for nectar-feeding New Zealand’s native birds, such as kākā, bellbirds and tūī.

Sadly in New Zealand, honeydew is also appealing to German and Common wasps, which are very serious pests. In South Island beech forests it is estimated that these wasps consume about 90% of this honeydew, thus depleting this food source for our bird life. Wasps also feed on other insects and invertebrates too, which are essential food sources for our native fauna. They are even known to kill baby birds in their nests, for their protein, to raise their own young.

Controlling sooty mould in the garden

In your garden, while low levels of sooty moulds look unsightly, they are just cosmetic and aren’t harming the plants themselves. However, if the mould is very severe it can block out the sunlight, reducing photosynthesis and weakening the plant. This makes the plant more susceptible to diseases or pests.

Sooty mould is commonly seen on trees, but it can also be found on shrubs like azaleas; gardenias; camellias; crepe myrtles; laurels and fruit trees, especially citrus.

Sooty mould growing on greenhouse tomatoes

Occasionally, it can be seen on vegetable plants, particularly in greenhouses, where a severe build-up of sap-sucking insects like aphids and whitefly have been allowed to flourish.

There are products out there that are said to control these insects. In New Zealand, I sprayed a light mineral oil called Conqueror oil (BioGro certified from Yates) on my citrus trees with some success at reducing these pests. Other products, which I haven't tried, include Organic Super Spraying Oil (BioGro certified from Kiwicare) and Neem Oil (such as the BioGro certified neem oil from Green Trading). Insecticidal soaps, such as Nature’s Way Vegie Insect Spray (BioGro certified from Yates and based on natural vegetable oils) are all said to be effective.

A word of caution. With all these products do read the label and follow the instructions carefully. Test them first by spot-spraying a small area of your plants to make sure that they don’t damage them. If after a few days the plants look OK, then it will probably be safe to continue spraying the rest.

Mineral oils

Mineral oils are derived from highly refined petroleum products and are mixed with water before applying to plants to control insect and mite pests. The mode of action of mineral oils (and neem) is to block the little holes along the sides of the insects’ abdomens, called spiracles, through which they breathe. The insects will suffocate and die. Adults, nymphs and some eggs will be controlled. Scale insects stick on your plants like limpets, so you may think that the spray hasn’t worked, but it has. Their bodies will remain on the plant like dried husks.

Neem oil

Neem oil is derived from the seeds of a tree in the mahogany family that is native to the Indian sub-continent. It is sometimes called nimtree, neem or lilac tree (Azadirachta indica). Neem oil is an organic, broad-spectrum product. It can be used as a fungicide (controlling sooty mould, black spot, rust, mildew and scab); as an insecticide (controlling whitefly, aphid, scale, and mealy-bugs); and as a miticide (controlling spider mites). It is also a natural insect repellent for use on house plants, flowers, vegetables, trees, shrubs, fruit both indoors and out. It is biodegradable with no researched evidence of toxicity to mammals, birds, bees or earthworms.

In some literature, neem oil is said to be safe on beneficial insects. As a plant pathologist, I wouldn’t agree with this. Beneficial insects have the same physiological characteristics as the target insects and are likely to succumb to these sprays too. Some people reduce the concentration of neem oil from 2% to 1%. in their sprays. It reduced the risk of plant damage and has been found to be just as effective. However, this is something I haven’t tried for myself, so always ask for advice first and read the label carefully.

With both mineral oil and neem oil, you must cover all of the plant, especially the undersides of leaves as they are both contact sprays, i.e. they have to hit the insect target. Always spray in the early evening, when there is no direct sun and is cooler. Where pest infestations are severe, you may need a follow-up spray two weeks later.

Always read the label and, if you are not sure if your plant might be sensitive, spray a small test area of your plants first. If your plant is OK after two or three days then it is probably safe to spray the rest of your plant. In recent years newer oils have been developed which have a much lower risk of plant damage.

Insecticidal soaps

Insecticidal soaps are soft soaps that be used to control soft-bodied insects such as caterpillars, aphids, mealybugs, spider mites, thrips, scale insects and whitefly. Soft soap is usually a liquid form of soap and easily soluble, e.g. Lux flakes or dish-washing liquid. These are made by saponification with potassium hydroxide.

Hard soaps, or bar soaps, are made by saponification with caustic soda (sodium hydroxide) and common salt (sodium chloride). These should not be used as they will damage your plants.

The mode of action of insecticidal soaps is not exactly understood. Possible mechanisms are:

- They enter the insect via their mouthparts, disrupting and damaging their cell membranes, leading to the contents of cells leaking out;

- They may dissolve the waxy layer of the insects’ cuticles which leads to water loss by evaporation and ultimately dehydration;

- It could block the insect spiracles causing suffocation;

- It may interfere with insects’ growth hormones or affect their metabolism.

Some people spray insecticidal soaps to clean sooty mould off their plants. Commonly, they use one tablespoon of dish-washing liquid to 4 litres of water. After spraying they leave it on the plant to soak for a while, which loosens the mould. Then they either gently rub the mould off or gently hose it off. NEVER scrub the plant surfaces as they will be damaged. Some people do this a few times on stubborn moulds.

My concern would be the potential damage to the protective waxy leaf surface. This layer is the plants’ mechanism to prevent disease. Once it is stripped off or damaged, it makes it much easier for fungal and bacterial diseases to penetrate and take hold.

NOTE - A significant disadvantage with neem and mineral oil, and many insecticidal sprays, is that they are broad-spectrum. They can also kill the ‘good guys’ in your gardens, predating on or parasitizing the ‘bad guys’ on your plants. These include, of course, ladybirds and their nymphs; lacewing and hoverfly larvae; predatory mites; parasitic wasps and others.

Also, please, please, please make sure there are no bees about!

Successful recovery

Once these sap-sucking insects have been controlled, any new plant material should be fine, green and healthy. Any black leaves or stems remaining will eventually recover as the mould dries and flakes off. A gentle hosing, or rainfall, will help remove these flakes and speed up the recovery process. If branches are severely coated with mould it might be wise to gently remove them, if possible, so they don’t weaken the plant. Trim off any seriously damaged plant material.

Happy gardening!

Further reading - If you have a problem with PSYLLIDS in your tomatoes and/or potatoes, then you might like to read my article describing what the pest is and how to control it organically. https://duncangoeswalking.squarespace.com/blog-pages/psyllids-a-major-pest-of-tomatoes-and-potatoes-and-their-control