The only fungal disease known, that affects JM's (and many other trees & woody plants), is Verticillium Wilt. It develops on the roots and blocks the uptake of moisture & nutrients to the foliage. Once it becomes well established, the tree will die. There is no known treatment or cure.

Almost any of the JM's you purchase, is already infected with the disease, since propagators don't usually test rootstock before grafting. The one's in Oregon are particularly known for using dirty rootstock.

That being said, you can usually prevent the disease from multiplying and seriously affecting the health of your tree.

Select a location that is shielded from the sun during the hottest part of the day.
Keep the soil moist, not damp or soggy, in the root zone. Do not allow it to dry out.
Do not overfertilize the tree. Apply a light application of balanced fertilizer (12-12-12 is fine) in early spring, when new growth begins. Use compost in late summer, for additional nutrition.
Apply mulch over the roots, to conserve moisture. Leave an air gap of 3-4 inches around the trunk, to prevent insects and diseases from entering the tree trunk.
Do any pruning in early winter, as the sap rises early in Maples.

Check any tree before purchase, for signs of disease, such as dead branches, curling or yellow (chlorotic) leaves.

The Coral Bark JM ('Sango Kaku') is a good choice. Very colorful in the winter, with it's red bark. Mine is about 20 years old and it receives a "haircut" almost ever winter, to keep it in shape and height limited to around 8ft. It's at the drip edge of a White Oak and a Southern Magnolia.

In addition to the tips Ladywindsurfer gave, don't replace the soil. Amended soil is almost never beneficial and frequently detrimental to woody plants. The link below will give additional info.

Here is a link that might be useful: Planting a Tree or Shrub

"Almost any of the JM's you purchase, is already infected with the disease, since propagators don't usually test rootstock before grafting. The one's in Oregon are particularly known for using dirty rootstock."

Ladywindsurfer,
That comment is interesting. Do you happen to have more information/documentation about that?

Hey -- thanks!

I think I will give this a try!

An additional thought. Verticillium wilt which is naturally present in soil enters a tree through cut/damaged roots. When planting JM's (and dogwoods) ignore the usual advice about teasing/spreading the roots. Set the soil ball into the planting hole and gently water around it to loosen the feeder roots without breaking them.

Once planted never dig or edge under a JM. Never do anything which may cut a root. Mulch it and leave alone. If a ground cover is desired plant that at the same time the tree is planted. Anthracnose can also be a problem. The fairly new organic Actinovate fungicide will control it. Follow package instructions.

I don't think maples can get blackspot. It is for the rose family, which can include apple and pear trees. The only thing that has negatively affected my maples is that hard frost in Easter of 2007. Darn near killed them.

Are you sure your knockouts had BS? I've heard of them getting powdery mildew bad, but not black spot.

Hey Buford..... It was some sort of funky crud! Leaves began to yellow and get black spots -- it spread throughout the plant. Then the stem ends got black also. I treated it with Bayer All in One (which I love, and which has always kept my roses really healthy.) But I think I was too late for that. It is dead as dead can be!

Maybe it wasn't black spot, but I think it was some sort of plant disease. Icky.

:)

I have two 'Sangu Kako' Coral Bark Maples and they are easy and very trouble free! Dig a big hole at least 2-4 times bigger than the root ball, so the soil is loose and will allow the roots to grow. Plant the root ball no lower than the soil level and maybe an inch higher for better drainage. Use only the native soil. Once planted, apply a nice hardwood mulch 2-3 inches deep, but keep it from contacting the trunk of the tree. Water long and deep at least once a week until established. Knock Out Roses are usually very trouble and disease free unless they are in soil that stays too wet. I have many and the only one that is not happy is on the bottom of a slope where it stays too wet. You are right to try to find a plant that will be happy in the site you have rather than keep some unsuitable plant alive with chemical intervention, and poisoning your garden.

recently over the past few weeks i have noted a black mildewy discoloration at the base of my super nice sangu kako coral bark maple slowly creeping upwards. The leaves seem fine so far, but I am worried this will progress. I have sprayed an antifugal but does not seem to help. What do you think this might be? And more importantly how should i proceed? Thanks !