Judging by the description and time of year I'm guessing Powdery Mildew. I'm going to give you a link to a .pdf file that lists common dogwood diseases in Alabama and remedies. In this case the link will suggest a fungicide to protect an already flowering tree like yours.
(if you cant get the link to work let me know)

Here is a link that might be useful: Dogwood Diseases of Alabama

I thought powdery mildew at first too, but 6" square and sap? Sounds more like the trunk. I guess it could be borer damage, but white growth doesn't fit. I'd love to see a picture of it.

Another spot has appeared today about 2 inch square, bumpy, like a growth on the trunk it is really white. It is thick. I'll try to get a photo.

Mollie, it certainly isn't powdery mildew, which affects the foliage of plants, especially the younger foliage. Would it be possible for you to post an image of this ailment?

If no picture can be forthcoming, you'll need to come up with some more details. Does this 'lumpy white growth' scrape off readily? Does it appear to be part of the woody trunk or simply resting on top of the surface? Etc.

Okay, yesterday I cut and dug out the three spots. It was the size of a baseball, it was thick and like cutting through a rubber ball or the firmest thickest mushroom ever. It was very white on top and the under side when I dug it out was black/grey with black/grey hairy pieces. It finally came off the tree with a lot of work, it was on there very firm. Today I sprayed it (soaked) with a fungicide since it looked mushroomy. I am thinking tomorrow I need to seal up the areas with some tree tar sealant. What do you think? The leaves look fine and the tree is old but looks healthy.

How old is this old tree? In the world of trees, Cornus florida has a relatively short lifespan.

I am very curious what this problem is, hope someone familiar with it offers more info.

Wound dressings (pruning paints, tree tar) are not recommended. From aggie-horticulture:

Wound Dressing

In recent years, much has been written about the advantages and disadvantages of using a wound dressing on large cuts. Traditionally, wound dressing or pruning paint is used only on cuts larger than an inch in diameter. However, scientists have found that wound dressings are strictly cosmetic and have little to do with preventing insect or disease damage to the wound area. Pruning paint may, in fact, slow down the healing process. In general, wound dressings are not recommended or necessary,

There are lots of publications out there from horticulturists recommending against it.

Mollie, mollie.

I so wish that you could have posted some images. I'd never have suggested that you perform major surgery on your tree, under any circumstances.

It sounds like the dogwood might be 'infested' with a fungus, but whether or not it's a disease causing agent or a decomposing organism taking advantage of normal dead wood is unknown. What's certain is that trying to remove a large portion of the tree, like a surgeon removing a cancerous tumor, is never (ever) the way to recovery.

Add to that the facts that fungicides will (cannot) not help, and any kind of sealant is not recommended. Tree paints, tars, and other goop have been on the 'BAD FOR TREES' for many years. They impede the plants innate ability to heal from within, and do nothing to prevent the further spread of disease.

Healthy trees are composed largely of dead wood. As a matter of fact, what we commonly think of as 'wood' is mostly non-living cells and tissues. It is quite common and natural for ubiquitous saprophytic fungi to find a way inside the tree body through bad pruning cuts, bark injuries, etc., where they are happy to begin the decomposition process....only on the dead wood. Trees can live for many years (generations) with their entire insides gone! It's the outside layers of a woody plant that are 'living'.

However, there are other fungal organisms that require living cells in order to exist. These eat away at the shell of living tissue surrounding the tree column.

Both of these fungi groups will typically present, at various times of the year, an outward sign of their activity. Mushrooms, conks, and the like will suddenly show up on the trunk and branches of our trees. Those growths are simply a flag. The fungus is deep inside the tree body, hard at work.

The saprophytic fungi are nothing to worry about and the parasitic fungi cannot be 'cured'. We try to prevent an infection of the latter with proper pruning, and by doing all we can to avoid damage to the skin (bark) of our trees. But as trees age, they become more susceptible to damage....just like animals.

I'm terribly sorry that you've cut into the tree and I hope that the damage isn't considerable. I strongly urge you to allow the plant to handle this on its own terms, something that woody plants are more than capable of doing (physically and chemically), if given the opportunity.

I'd maintain a healthy 3 to 4 inch layer of mulch around this tree, at least to the drip line, if not more. Never pile mulch right up against the trunk however. Mulch is probably the single most beneficial thing you can provide for your tree.

The house was built in the 1930's but have no way to know how old the tree is as it was here when we bought the house. The tree looks old but healthy. I don't see any dead parts on the tree but I definitely won't put the tree sealant on it. It is a large dogwood tree and I am hoping it isn't totally infected but it sounds like there may be nothing I can do.

Try not to change anything (too much) in the tree's immediate environment. This means above AND below ground. No trenching, building of flower beds around it, specialized fertilization, etc. Mulch will greatly enhance the root/soil system, giving the tree some added empowerment to live long and prosper, even though it may be infected with an internal fungus.

I assure you, that your tree has lots and lots of 'dead' wood. All trees do. These non-living portions are the heartwood, and much of the interior, non-functioning portions of the wood. It's only the outer shell (a few inches on a large tree) that still consists of living cells. This is simply how trees and other woody plants evolve, and it is perfectly normal and healthy.

However, since those non-functioning cells are prime targets for both saprophytic and parasitic fungi, we simply need to do what we can to prevent superficial injury, stress, etc.

I have a 75 year old Wild Dogwood in my yard. It is 30 feet tall and the canopy is 30 or 40 feet round. That might help you figure out how old your tree is.. We have had a lot of rain lately. I am not sure where you live or if you have had a lot of rain. BUT, a lot of rain can cause lots of little things to pop up.

Did you cut the bark off the tree when you removed the mystery growths??? If you cut the bark off you do need to repair the tree. Now I do not know if my following method will be considered kosher by the Horticulture experts. BUT, it worked for me. I used exterior grade wood putty to repair my dogwood, that was 7 years ago. The tree is fine. I even painted the wood putty to make it look like bark. The repair is still there.

I do give my Beautiful OLD Dogwood Tree Miracle Grow Tree spikes. 3 once a year. and I put time released, season long, insect control around it. Every summer, after the tree blooms I prune out any dead branches.

Your trees resistance to fungus, etc. depends on if it is a WILD Dogwood or a Nursery grown Dogwood. At least that is my understanding. A few years ago my neighbors Nursery Dogwoods started dying..Then another neighbor. It was some kind of Dogwood illness I can't remember the name..I thought for sure I would lose my Dogwoods but, I lost ONE, the only Nursery Dogwood that I had. The Wild ones had resistance to it.

mollie, please do not apply anything to 'repair' the tree. Cosmetic applications do not repair, fix, cure, or mend a tree injury and MAY prevent the natural healing that trees do all by themselves.

And I must respectfully suggest that tree fertilizer spikes can do damage to the sensitive roots that happen to be near the spikes. Those spikes are made up of fast release fertilizers that are capable of burning nearby roots.

I second rhizo's suggestions, do as little as possible at this point and allow your tree the opportunity to heal itself. Anything further will more likely hasten it's demise than to help it to heal. Best of luck with it!!!!

"Add to that the facts that fungicides will (cannot) not help, and any kind of sealant is not recommended."

Sorry for straying from the topic; but why wouldn't fungicides work?

That's a bit of a complex answer, but I'll try.

Whether this is a disease causing fungus (parasitic) or a saprophytic fungus feeding on the interior dead wood, there is no way to get a chemical to the source. Those white growths are merely a symptom that this fungus is present and active within the tree, they aren't the active fungus.

So, no spraying...no injecting of chemicals can get rid of these ubiquitous organisms.

It would be very unusual for an older tree NOT to be colonized by fungi of some sort. And those trees can live for many more years, as long as the fungus is saprophytic. Remember, most of the interior of the tree body consists of dead wood. Trees can live just fine having a hollow center. It's when the fungal organism is parasitic that there's a problem.

These kinds of fungi (the parasitic ones) can kill a tree, sometimes in a very short time. They typically work within the vascular system of a plant, causing die-back, wilt, and other noticeable symptoms. Mollie has stated that this tree appears to be healthy, and I'm going to take that as a good sign.

What we should all concentrate on is prevention. Avoid damage to the bark (by careless mowing or weedingwhacking). Learn how to make pruning cuts properly. Don't over fertilize (or fertilize at all) your trees. But we all need to understand that these fungal spores (the good ones and the bad ones) are everywhere, all the time, in great quantity. There's simply nothing we can do about it, other than trying to keep our plants healthy.

Thanks for the reply rhizo and don't worry I understood it just fine. Realizing the white fungus was a 'symptom' made it click.

Hello,
I also have a white'fungus on my dogwood tree. The leaves are dead only at the tips??

Mollie: If you are really curious about what the growth is and still have the part you cut off, you might want to take it to your extension center. Someone there may be able to ID it for you and tell you what to expect. If you have already disposed of it, showing them a picture might suffice.