Honeydew is a sweet sticky liquid excreted by aphids and other pests as they process the large quantities of sap they suck from a plant. This nutrient-rich liquid forms an ideal medium for certain moulds.
There are typically two types of sooty mould growth. The first is found on leaves and survives for as long as the leaf is alive. The second lives longer and is found on the stem. It grows from the previous season’s mycelium, which is the vegetative part of a fungus. It comprises branching, threadlike hyphae that give the mould its shape.
As sooty mould spreads across the leaf, it reduces sunlight exposure and affects the leaf’s ability to photosynthesise. This can lead to stunted growth, premature aging of the leaves, leaf drop or even the death of the plant.
Numerous Ascomycetes fungi, including the genera Antennariella, Aureobasidium, Capnodium, Cladosporium, Fumago, Limacinula and Scorias, are responsible for the formation of sooty mould. Sooty mould is more common during warm months, and seems to thrive the hotter and drier it becomes. This is partly because, during periods of drought, the pest infestation of sap suckers tends to increase, leading to an increase of the mould.
Furthermore, as there is less rainfall, the honeydew is not washed off. Indeed, a plant under water stress is likely to suffer more from mould coverage than a healthy, well-watered one. Sooty mould has modified cell walls that help it to adhere to the plant/leaf surface. These also help to prevent the mould from drying out.
Prevention is better than cure
Several methods can be used to treat this problem, but it is more important to control the pests causing it. Many of these have been covered in previous editions of Farmer’s Weekly. Ask your extension officer for help in identifying these and advice regarding the control. A useful control method is to control ants in the vicinity. These guard the aphids and other pests from predators in exchange for their honeydew.
Ring the stem of the plant with petroleum jelly. If ants can be controlled in this way, sufficient numbers of predators will appear and feed on the pests that allow sooty mould to flourish. If predators do not keep the pests under control, use an application of insecticidal soap, neem oil or a horticultural oil. Depending on the infestation, it may require several applications.
Once you have dealt with the cause of the mould, turn your attention to the mould itself. With no supply of food, the mould will disappear of its own accord over time. Where a heavy coating is present, remove the mould by washing the affected part of the plant with a very weak soapy water solution. Leave the solution on for a few minutes, then rinse off with clean water. Fruit or vegetables covered in sooty mould are still edible, despite their appearance. Simply wash the produce in warm soapy water.