Sometimes late in the summer, leaves of several species of plants turn black. On closer inspection, their appears to be a covering of coal soot all over the surface of the leaf which can be wiped off with a damp cloth. Perhaps the leaf is also sticky to the touch. What is the problem?


The growth is actually a form of a fungal (mold) growth called sooty mold. However, that is not the real problem. Sooty mold is actually a sign of a serious infestation of some type of sucking insect such as aphids, scales, mealybugs, whiteflies or occasionally, spider mites.

These critters are sucking the juices out of leaves above and then their droppings fall onto the leaves below. This substance is commonly called "honeydew" and it is full of sugars and is very sticky. Once it accumulates, the fungal spores of sooty mold begin to grow and turn the leaf or needle black.

Although the mold is generally on the same plant that is infested by the insects, it may also appear on plants growing beneath. Often, you might see hosta plants that have sooty mold on their leaves that is caused by an infested tree above.

In severe cases, the mold may build to a thickness that blocks the light from entering the leaf. Then, it will cause the leaf to lose its chlorophyll, turn yellow and lose energy. This may also weaken the plant further.


Periodically observe your susceptible plants in mid summer and, if necessary, control the insects that suck the juices from these plants.


Once you see sooty mold on leaves later in the summer, inspect the bottom of other leaves to see if sucking insects are present. If so, you may choose to treat the plant for these pests. This will prevent further sooty mold and there are no fungicide treatments needed for the mold itself.

Note: We have provided some general information and observations on this topic aimed at the home gardener. Before you take any serious action in your landscape, check with your state's land grant university's Cooperative Extension Service for the most current, appropriate, localized recommendations.


Certain species of plants are more susceptible to infestations of sucking insects that will result in sooty mold later in the summer. Among these would be tuiliptree (Liriodendron), magnolia, and Scotch pine (Pinus sylvestris).

 

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