Sooty Mold is a black  slimy powder that occurs on the upper surfaces of Linden and Magnolia leaves .It can show up on other trees like Lilac and Honeylocust but these are the Linden and Magnolia arethe main trees in Ontario.  The Mold present is a result of a  mold feeding and growing on sticky leaf sap that is on the surface.  The Mold is a member of the Ascomycetes in case you are interested.

​​

 The Mold actually is not the problem, rather it is a sign of a underlying larger issue of plant sucking insects such as aphids and scales. In other words if you have sooty mold - you definitely have a aphid or scale problem.  Aphid and scales are insects that have sucking mouthparts not unlike mosquitoes.  Tree sap doesn't have a lot of food for the insects, so for aphids and scales to get enough nutrition, they pump out a lot of tree sap in the form of poop (for lack of a better word) to get what they need.  This sticky waste covers the leaves and the mold then starts to grow on it and the black look is the end result.

​​

Aphids and Scale are a result of  one of two things.  A) a periodical flare up of a particular aphid or scale population from time to time or B)  a tree under stress that does not produce enough secondary metabolites to fend off the bugs becoming established. Overcrowding and lack of sunlight are very common reasons why linden and especially magnolia gets aphids and/or Scale insects.  Severe infestation or repeated scale/aphid infestation can lead to tree decline and mortality.

​

 Safer's Soap for aphids and Dormant Oil for scale are two ways that a landowner can control these insects if they become established or you can retain an Arborist that can apply a systemic pesticide.  More importantly, it is most important to keep the tree healthy by giving full sun as both linden and magnolia do not do well in shade. 

​​

​​

​