Question
Jim,

Our extension agent has identified the black
deposits on my deck as lycenium or mycelium scale.

I have tried several products including trisodium
phosphate and bleach, oxyclean, deck brightener,
and a commercial mold and fungus remover. EAch one
lightens the black deposits a little with each
application.

Do you have any suggestions?
Also is there deck finish that will repel the deposits.

The leaves on the maple close to the deck are coated with
black and will soon fall delivering their black soot again.

Thanks,

Carol

Answer
The Maple tree is the source of the sooty mold. The tree sounds like it has an infestation of an insect called aphids or possibly scales. These insects suck the plant juices from the plant and secrete a substance called honeydew which is high in sugar content and a sooty mold will grow on the honeydew. This is the black you are seeing. You need to control the aphids or the black will return after you clean the deck. I would treat the tree with an insecticide called Bayer Advanced Tree and Shrub Insect control. This is applied around to the soil around the tree and the roots will transport the insecticide to the twigs and leaves. Being it is late in the year you will get some  action from the insecticide to the twigs but not much to the leaves. I would go ahead and apply it now and then again in the early spring. That way you should eliminate the insect problem and stop the production of the honeydew and the sooty mold.

I would use a power washer on the deck to remove the black mold. A pressure washer is generally used for cleaning larger decks. These power machines do the job efficiently and within a short time. Pressure washers give you the option of using cleaning solutions. However, this is not required as their washing action is capable enough to get rid of the grit and grime. However, these machines generate extreme pressures that may remove the wood finish. This will decrease the durability of the wood, leaving it with a rough surface. Experts recommend a low power pressure washers (1500 psi or less) that have a wide fan tip. They do nice a job of cleaning decks without harming the wood finish.

Then use a solution of bleach and water (The recommended ratio is 6 oz of bleach to every gallon of water) and spray the power washed deck to kill any left over fungi. After it dries I would check with your local paint dealer and ask for a sealer for the wooden deck. This will help protect the wood from future deposits.

Here is a web link to the Bayer product. Check with your local nursery for this product.

http://www.bayeradvanced.com/product/Tree-Shrub-Insect-Control/concentrate.html