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| Initial signs of Sooty Mold growth on mango tree appear as blotches of white and black on leaves, eventually the tree will become covered with fungi that have a soot-like appearance. |
Central Florida has been warm and dry this fall and we've been several years without any freezing weather. So one day you notice your mango trees look like mine (below).
My reaction was "WTF?!"
This horrible-looking black stuff is a rather common condition of mango trees in the subtropics called "Sooty Mold." The mold isn't the problem. The mold is a symptom of pests (insects) that are hard to see on large trees. The insects are creating a sweet, sticky substrate on which the fungi grow. In the case of my tree the insects are scales and mealy bugs. In other's backyards you might find whiteflies and aphids causing the same condition; Mango Black Sooty Mold.
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How could something that sounds so inviting be so disgusting? Honeydew is essentially insect poop excreted after the bugs suck sap from sweet trees like the mango.
Honeydew is a sweet, sticky liquid that plant-sucking insects excrete as they ingest large quantities of sap from a plant. Because the insect can’t completely utilize all the nutrients in this large volume of fluid, it assimilates what it needs and excretes the rest as “honeydew.” Wherever honeydew lands—e.g., leaves, twigs, fruit, yard furniture, concrete, sidewalks, or statuary—sooty molds can become established.
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| A healthy (mostly insect free) Florida mango tree will bloom and fruit from January into spring. |
Its not all that easy, however.
Ants, like the large carpenter ant above, feed on the honeydew and when ants are observed, one can assume that they are killing off beneficial insects in order to protect the supply of honeydew coming from the not-so-desirable insects. Those insects like ladybird beetles (ladybugs) are no match for these mighty ants. So the mealybugs and scales flourish.
Add to that over 175 species of armored scales occur in Florida. They are distinct from other types of scales because they secrete a waxy covering over their bodies that is not attached to the body. The scale lives and feeds under this covering.
Sooty mold is the common name applied to several species of fungi that grow on honeydew secretions on plant parts and other surfaces. The fungi’s dark, threadlike growth (mycelium) gives plants or other substrates the appearance of being covered with a layer of soot.
Sooty molds don’t infect plants but grow on surfaces where honeydew deposits accumulate.
Although sooty molds don’t infect plants, they can indirectly damage the plant by coating the leaves to the point that it reduces or inhibits sunlight penetration. Without adequate sunlight, the plant’s ability to carry on photosynthesis is reduced, which can stunt plant growth. Coated leaves also might prematurely age (senesce) and die, causing premature leaf drop.
Fruits or vegetables covered with sooty molds are edible. Simply remove the mold with a solution of mild soap and warm water.
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| Mango leaves infested with Mealybugs and Scales high in the canopy of the tree. |
Fungi that most commonly cause sooty molds in garden and landscape situations are in the genera Capnodium, Fumago, and Scorias. Less common genera include Antennariella, Aureobasidium, and Limacinula.
The species of sooty molds present are determined by a combination of the environment, host, and insect species present. Some sooty mold species are specific to particular plants or insects, while others might colonize many types of surfaces and use honeydew produced by several kinds of insects.
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| Mealybug nymph (Pseudococccus viburni) To the naked eye these insects appear as scales on the underside of plant leaves. |
A number of insects can produce the honeydew sooty molds need for growth. Their common characteristic is that they all suck sap from plants. The insects include aphids, leafhoppers, mealybugs, psyllids (including eucalyptus lerp psyllid), soft scales, and whiteflies. Both the immature and adult stages of these insects feed by sucking sap from plants, producing honeydew.
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| Green Peach aphids (Myzus persicae) and their cast skins. |
How go Kill the Insects and Remove the Sooty Mold
In my garden the insects have proven so difficult to control that I destroyed some trees while trying to treat others. Avocado were so infested and the bugs so persistent that I destroyed the trees. Mango trees I've treated with a solution of dish detergent and vegetable oil and with insecticidal soaps. However, because the trees are 30+ feet tall it is difficult to treat the upper canopy of the trees.
Some of the insects have dropped off but many remain. There is the problem of the endless summer in Florida, too. More insects are emerging from the soil underneath the trees every day. In some orchards black plastic is put under trees to bake the insects out of the ground.
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| A photo of my then-healthy mango tree in January 2018, as it began blooming. Since then mealybugs and scales have infested the tree. |
I haven't resorted to anything so drastic, yet.
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| Ants tending a colony of brown soft scale (Coccus hesperidum) |
I would not encourage anyone to kill ants as they perform an essential function in the environment, but ant management is a serious consideration when it comes to Florida mango trees. Without ants protecting sucking insects the insects would be controlled by natural predators.
Ants are attracted to and use honeydew as a source of food. Because of this, they will protect honeydew-producing insects from predators and parasites in order to harvest the honeydew. In many cases, predators and parasites are sufficiently abundant and quickly begin feeding on and reducing populations of scale insects, aphids, psyllids, whiteflies, or mealybugs once ants have been eliminated. If populations fail to decline, apply horticultural oils, neem oil, or insecticidal soap to suppress the problem insects without killing everything.
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| A hard freeze like the one pictured above in January 2010, would remove all the foliage from the mango tree without killing the tree. Removing the foliage would be a most beneficial way to control the problem insects. However, we haven't had a hard freeze in years. |
Sometimes vigorous pruning can be helpful in removing most of the infested plant parts. Also, keep ants out of trees and away from honeydew-producing insects by applying a sticky compound around the trunk and trimming limbs touching buildings or other access points. Baits, such as ant stakes placed under trees and shrubs, may help reduce ant foraging in some cases.
Once honeydew-producing insects are suppressed, sooty molds will gradually weather away. In some instances, if necessary, sooty molds can be washed off with a strong stream of water or soap and water. However, it can be difficult to remove sooty mold even with soap and water.
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| Brown Soft scale (Coccus hesperidum) and nymphs. |
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| Adult Rose leafhopper (Edwardsiana rosae). |
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| Whiteflies (Trialeurodes vaporariorum). Two adults with nymphs. |
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| The Hispaniolan solenodon (Solenodon paradoxus) is a 76-million-year-old venomous, nocturnal, burrowing, insectivorous mammal. Photo: Lucy Emery |
How a Curious Mammal Evolved Its Venom
The Hispaniolan solenodon is a wondrously strange creature.
About the size of a guinea pig, it has a long, hairless snout, sharp little teeth and, to top it all off, venom-laced saliva. Highly endangered, it lives quietly in the forests of the Dominican Republic and Haiti, and scientists have been hard pressed to understand much about its habits and evolution. Recently published, "Solenodon genome reveals convergent evolution of venom in eulipotyphlan mammals," by Casewell et. al. (a diverse group of researchers writing in the Nov. 2019 issue of PNAS) outline intriguing conclusions they reached about how the solenodon got its dangerous spit, after they sequenced its genome and analyzed its venom.
What happens when hermit crabs confuse plastic trash for shells? An ‘avalanche’ of death.
A study that called attention to a remote cluster of islands off Australia’s coast was met with international concern when it published in May. In a harrowing account of their trip to the Cocos (Keeling) Islands two years prior, researchers recalled seeing beaches that were “literally drowning in plastic.”
But Jennifer Lavers and her research team now say they made another startling observation while digging through copious amounts of litter on that 2017 trip: Many of the bottles, cans and containers were not empty. Scores of hermit crabs, mostly dead, were trapped inside.
In the first study of its kind, an IMAS-led research team estimates that around 570 000 hermit crabs have been killed on the Cocos (Keeling) Islands in the Indian Ocean and Henderson Island in the Pacific after being trapped in plastic debris. Published in the Journal of Hazardous Materials, the study was carried out by researchers who previously revealed that Cocos and Henderson islands are littered with millions of pieces of plastic. (Image, right, Dr Lavers with Cocos Island hermit crabs in discarded plastic buckets. Credit: Silke Stuckenbrock)
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