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A Close-Up on Sooty Mold


Have you ever noticed a plant, fruit or a sidewalk with a black, powder like substance on its surface? Chances are you were looking at Sooty Mold. As its name implies, this fungal disease has a soot-like appearance which one might mistake for an accumulation of dust or the like on a plants’ surfaces



The term “Sooty Mold” does not refer to a specific organism as is mostly the case, it is a collective term for a variety of fungi that colonize on a sugar-rich exudate known as honeydew which is produced by sapsucking insects on the infested host plant. Luckily, all is not lost, and this disease is treatable with the correct care.


The organisms which are responsible for Sooty Mold have been identified as a wide variety of fungi classified under the phylum Ascomycota. Fungi in this phylum all have common physical characteristics: They produce sac-like structures (called asci) which contain the spores needed for sexual reproduction. Not all species under this phylum are plant pathogenic, and many have been found to be beneficial such as Saccharomyces cerevisiae, which is the yeast we use for bread and beer fermentation.



Common genera which have been found to be included in sooty mold fungi are: Aethaloderma, Capnodium, Cladosporium, Eautennaria, Scorias, Trichomerium, and Alternaria, but many others may also be included. These fungi feed on the mineral dense honeydew, produced by insects, to survive and multiply.


The characteristic black look of sooty mold is due to dark-coloured pigments in the mycelium of the fungus that is present. The fungi will continue to grow and spread via its spores that are blown with the wind or splashed with water to other parts of the plant covered with honeydew where it will colonize further. Warm temperatures and dry conditions will hasten the spread of sooty mold spores as aphids generally produce more honeydew when plants are undergoing moisture stress.


The specific species of sooty mold-causing fungi is determined by combination of environmental factors, host plant and insect species present on the host. Some sooty mold fungi can colonize on any surface and on honeydew from any insect, while others are highly host specific and only feed on honeydew from specific insects. As an example, some sooty molds will only grow on the honeydew produced by soft scale insects feeding on a citrus tree, when the honeydew is produced by aphids, or any other insect the sooty mold fungi will not grow.


Honeydew is a sweet and sticky substance produced by sap-sucking insects, classified as such for their sucking mouthparts with which they pierce the surface of plant structures to feed on the nutrient rich phloem (plant sap). When these insects feed, they ingest a large amount of sap in order to absorb the amounts of proteins and nutrients they require. The excess sugar water that they cannot consume, they will then excrete in the form of honeydew. Insects such as aphids (Aphidoidea), psyllids (Psyllidae), whiteflies (Aleyrodidae), mealybugs (Pseudococcidae), leafhoppers (Cicadellidae) and soft scale (Coccidae) can all produce honeydew in both their immature and adult life stages.


Sooty mold, or the fungi that causes it, does not infect the host plant or surrounding plants. Some of these fungi are plant-parasitic in nature, but in the context of sooty mold growing on the honeydew produced by the above insects, generally they do not infect the plant on which the honeydew is found. However, secondary effects of sooty mold are the decline of the photosynthetic ability of leaves covered with sooty mold. Light cannot penetrate the fungi, thus causing a lack in photosynthetic ability. This has been known to cause leaf yellowing, premature leaf drop and stunted growth in severe cases.



A very Interesting mutualistic symbiosis exists between ants (Formicidae) and aphids (Aphidoidae). It has been found that ants feed on the honeydew produced by aphids, and in turn the aphids receive protection from the ants against their natural predators as well as improving the hygiene of the aphid population.


Furthermore, a study on the symbiotic relationship between ants and aphids revealed that there exists a mutual selfish manipulation between the two insects. The study found that the honeydew produced by aphids contained higher concentrations of dopamine, causing ants to behave more aggressively resulting in better protection against natural predators. In another study, it was shown that attending ants selfishly manipulate the reproduction rate of green morph to obtain a higher quality honeydew.


The control of sooty mold is quite simple. The primary problem is obviously the insect population creating the honeydew, which is then colonized by the sooty mold fungi, therefore if the insect population is reduced, so too will the instance of sooty mold reduce. This can be achieved using chemical interventions with pesticides, and it is recommended to always use the least toxic approach possible and allow natural predators to maintain the population.


If interventions were successful, the honeydew production will slow down and stop, resulting in the decline of sooty mold and ultimately the disappearance thereof. In the event of a smaller degree of infection, affected shoots and leaves can be removed via careful pruning. Affected leaves can also be wiped with a soapy solution to remove the honeydew and sooty mold, but the main cause should still be addressed.


Vegetables or fruit covered with sooty mold are still entirely edible and can be washed to remove the honeydew and sooty mold. If ants are present, it is important to discourage them from feeding on the honeydew either by placing a sticky barrier around the trunk or through chemical means as these ants will impede any biological control from natural predators.


Affected leaves that dropped will be replaced naturally once the tree is not under physiological stress Anymore. An important fact to consider is that stressed plants tend to be more susceptible to insect infestation. Therefore, take care to fertilize and water plants correctly and tend to issues as soon as possible.

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