Since aphids are very easily controlled with a sharp water spray I would think that an oil spray would be overkill. Maybe even a band of sticky material early in the season, before the aphids climb up the tree, could help. Anytime you need to control an insect pest start with the least toxic method and if that does not solve the problem then move, slowly, up the chain.

I have used Bonide (All Seasons Horticultural Oil). What I like about it is that it can be used during the growing season. Although, as kimmsr said, I prefer to try all alternative methods first.

How toxic is All Seasons Horticultural Oil? I'm just following the recommendations of my local Co-op.

Maybe you can help me to figure out if aphids are definitely my trees problem. Here's a nice collage of the tree. What do you think caused the sooty mold?

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I think you might have a scale problem. Scale insects can also produce honeydew. If it is scale the treatment would be the same. Dormant oil spray applied in winter.

Great photos by the way.

Dormant oils are considered 'soft pesticides' because they have little toxicity. One product I've heard about is PureSpray* Green which is organic and apparently very good, but I'm not sure if it is available in garden centers, or where exactly to get it. Here is a link that describes your Linden tree problem.

Here is a link that might be useful: Linden leaf mold

What I see is scale, not aphids, and for scale oils are an appropriate solution. Which to use depends on when the application takes place and while plants are dormant, dormant oil sprays are what you want. When plants are growing and you need to apply oil sprays to control scale at that time superior oil sprays are what to use.
Also look into the soil and the nutrient balances since most often plants that are under stress are attractive to insect pests, plants that are not as healthy as they should be.

Ok. All Season is literally an All Season oil and it is made with superior paraffin based oil. So I'll use that.

I had a soil test done for turf purposes and I am following the suggestions in order to establish the right NPK and PH levels, but is what my trees need different from what my grass needs? If so, it would be pretty difficult and costly to take a soil sample at every tree and try to do a different soil treatment for the areas within the drip line of every tree. I'm hoping what I'm doing for the grass will help the trees as well....

Horticultural oil applications are an excellent and important way to interrupt the life cycles of scale and aphids and should be applied now. Oil sprays are a much used step in a good IPM program. You'll be smothering eggs in all of the nooks and crannies. Even spider mites eggs can be killed.

I also see evidence of Japanese Beetles, but I'm sure you know that! Boo! Other than that, I think your trees look very healthy, other than the stress that black sooty mold has caused.

A heavy layer of that stuff inhibits the photosynthesis processes in the leaves, resulting in a less than vigorous appearance. Keeping up with the aphids and/or scale will also mean that the sooty mold will go away, allowing the leaves to be a nice, healthy green.

I'd like to see images of stems and new growth this spring, so a proper ID can be made should this (these) pest (pests) come back. But again, I strongly urge you to spray your trees with the horticultural oil product of your choice. Your trees will greatly appreciate it, I promise you! Prevention: it's a good thing.

I think you are on the right track looking for a honeydew producer as the cause of the sooty mold. I see one scale in one of the pictures, but that doesn't mean that it is the source of the honeydew. I think the oil is a good product for eliminating many sucking-insect eggs and should help address the problem whatever it is. The cluster of eggs in the one photo is a hemipteran, possibly a plant pest, but not a source of honeydew. It would be good to do a closer inspection to see what is on your tree that could be producing honeydew. Your local extension office can help.

The fertilizer your trees need is different from what your lawn needs. In particular, your lawn needs a lot of nitrogen and your trees do not. If you are fertilizing the lawn around the tree, I would not put more on the tree. It may need more of other nutrients which are very low in lawn fertilizers. A separate soil test and fertilizer program for each tree is completely unnecessary. If your trees are growing poorly or display poor color, then a soil test may be helpful. I would look at the soil test for the lawn first and see if there is anything lacking there. If your trees appear nutritionally healthy, the expense of a soil test may not be worth it.

I do not agree that all insects are attracted to stressed trees. Sucking insects in particular are using the sap in the phlom and leaf vascular tissue as a food source. If the tree is under stress, the food source for these insects is poorer and they will move on to other plants. Aphids will produce more winged adults when fed on a stressed plant. Overfertilizing with nitrogen will encourage these pests.

As a general rule people that are healthy and have a good immune system do not get sick, and do not attract insects, so why would not the same thing happen to healthy plants. Every garden writer I have read in the last 20 or so years has told us that if your plants are healthy, growing in good healthy soil, they will be less attractive to insect pests and diseases. Even Rebecca Coles, Paul James, and a whole host of people on the Home And Garden Channel programs have said that.

How about if we emphasize the phrase, " as a rule of thumb" and apply it to this? Because we all know that people who are healthy and have a good immune system DO get sick....ALL of the time! But, as a rule of thumb, we all know that it is wise to be as healthy as we can and to build up and protect our immune systems. Some diseases can be avoided that way, but by no means can all be.

This also applies to our plants. If we do all that we can do to provide our plants with a good above and below ground environment so that physical stress is not a daily routine, they are much more able to tolerate the usual pressure of this pathogen and that insect pest.

It is also one hundred percent true that SOME pests are designed to take advantage of plants under stress. One example would be phytophthora, a ubiquitous root rot disease that probably won't become a problem unless a SUSCEPTIBLE plant is growing in poorly drained soils. Pine borers also come to mind, which will move into acres of stressed pine trees a year or two after a hurricane or flood.

However, there are simply some plant pests out there that can and will infest perfectly healthy plants. Those are the plants that will be best able to recover, by the way. There are also plants with known genetic predispositions to certain disease and insect pests. We can't do anything about that, except avoid their use in sensitive locations.

So, our oath should be to 1) do all we can to keep our plants healthy (and this largely means take care of the SOIL), 2) select plants that are suitable to the specific site 3) avoid plants with known susceptibilities and weaknesses. Look for resistant cultivars, when possible.

The solution to keeping the pesky critters under control is multi-faceted and multi-dimensional. Sort of like a 3-D puzzle. It's terrific when we can make it work, and believe it or not, it's based on some very simple concepts, all of which have been put in place eons ago by Mother Nature! We measly humans just can't seem to put the pieces all together sometimes, lol!

OMG! How could I have missed this, lol! You're missing one of the single most benefical things you can do for your trees. MULCH!

No, viche....trees and turf do not have the same needs, but when those big lugs are planted right in the middle of a lawn, there's not much we can do about it other than try to avoid some of the very worst of practices. A healthy layer of mulch an mitigate some of the trials and tribulations that trees suffer when surrounded by turfgrass.

Is your email address valid? I'll be happy to send you an article about trees and turf, if you are interested. It's short!

Mulch...really? that's funny because I've read that mulch can be the cause of some problems because it acts as a home or medium of passage for pests. What benefits does it offer other than moisture retention?

Mulches can be a home to pests but it is also home to many beneficial insects that eat the insect pests. Making an environment that attracts a diverse population of insects assures you that you will have both, and since there are around 8,000,000 insect species in the world and only 8,000 insect pests, the good guys (and gals) outnumber the pests greatly and if we really allowed nature to work the benficials would clean up the pests for us.

Yes, the 'pest' factor with mulch is very overblown, especially considering all of the essential benefits. Mulch buffers soil temperature (hot and cold, providing a much better environment for those sensitive (and shallow) roots as well as about a BILLION jillion important microorganisms that our plants rely on for health and vigor. Mulch can feed those critters, too.

Mulch will keep the mowers and weedeaters away from the trunks of trees, where continuing damage can result in serious problems from even mature trees.

Also, grasses compete with tree roots for space, water, and nutrients. Yes, the shallow fibrous root system of trees are only about 6 to 10 deep. Not only that, but many species of turf grass exhibit allelopathic properties that trees can be sensitive to. Giving your valuable trees as much grass-free root space as possible is an enormous benefit.

I won't even mention weed and feed fertilizers. ;-)