Got Mold? Here’s What It’s About

Leaves, twigs and fruits of many deciduous and evergreen plants can become covered with a charcoal black coating that is caused by a group of fungi known as sooty molds.  These fungi are not plant pathogens and do not directly harm the plants they are growing on.  They live and grow on “honeydew,â€� a sweet substance excreted by insects including aphids, scales, mealybugs and whiteflies.  As these insects feed by sucking plant juices, they excrete honeydew which drops onto the plant surfaces below.  Often shrubs such as azalea, rhododendron, holly, and others growing under trees are covered in honeydew and sooty mold because of insects feeding in the trees above them.  Although sooty mold fungi do not cause direct damage to the plants they grow on, a heavy coating can reduce the plants ability to photosynthesize by blocking sunlight.  Combined with damage from the insect pests, this can interfere with growth and health of the plant.

   In Connecticut, sooty molds are also commonly observed on many plants including tuliptree, magnolia, maple, oak, beech, pine, spruce and yew.  On tuliptree, two honeydew-producing insects, the tuliptree aphid and the tuliptree scale, can reach high population levels by late summer.  Large amounts of honeydew will coat the upper sides of leaves, the trunk and surfaces below the tree and sooty mold will grow on it.  The Fletcher scale infests yew and high populations lead to stunting of growth, production of honeydew and sooty mold growth.  The populations of this scale are usually kept in check by beneficial parasites.  Control measures for the black vine weevil, another sometimes serious pest of yew, can eliminate these parasites.  

   A fascinating sooty mold can be found on American beech in Connecticut.  Instead of a thin black coating or crust, this sooty mold develops into large, spongy masses on the twigs.  Initially, the spongy mass will be yellowish in color and will become black as it matures in the fall.  September is a good time to look for these on the lower branches of beech trees.  You may first find the aphid associated with it, known as the beech blight or beech woolly aphid.  Branches will appear as if they were covered in cotton because of a white waxy substance on the aphids. This aphid lives in colonies, concentrated in high numbers in a small area, and the high amount of honeydew produced within that area is what allows this sooty mold to form such a large mass of growth.  The spongy masses can sometimes be as large as footballs.  This sooty mold, Scorias spongiosa, is unique in that it only grows on honeydew from the beech blight aphids, and hence is only found on beech.  Most sooty mold fungi will grow on the honeydew of several different insects.

Next week, ways to control sooty mold. 

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