how to keep0 the aralia plant from growing tall?+ ++
Cut back the entire Japanese aralia shrub in late winter or early spring, just before it begins to sprout with new growth. Snip the branches 2 to 4 feet back, just above the leaf nodes, shaping the bush as desired. This type of pruning helps maintain a shrub that is dense, upright and full of blooms.
https://www.gardeningknowhow.com/houseplants/aralia-plants/aralia-plant-information.htm
All of the old foliage had dropped; any help would be appreciated.
Here are the most common reasons plants drop their leaves. It's usually over or underwatering, temperature change, humidity, or fertility. I don't think your plant spray affected it, but aphids are sucking insects, they don't consume the plant, so make sure what you used was appropriate for sucking insects.
https://www.gardeningknowhow.com/houseplants/hpgen/houseplant-dropping-leaves.htm
https://www.gardeningknowhow.com/houseplants/aralia-plants/aralia-plant-information.htm
I stop it from spreading? The leaves on that particular branch fell off, but the other parts of the plant seem healthy. The “rot” is now on the main trunk. Should I quarantine this plant?
If you have it in a pot, it certainly would be a good idea to move it away from others till it is healthy again. I also would remove the branch that you first noticed the bark issue. If the "rot" on the main trunk isn't too far down, you can also remove up to and beyond the "rot" to healthy stem. Also, in case of root or stem rot, repotting may save it.
There are more than 70 varieties of aralia, but I'm guessing you're referring to one of the large tropical ones. The aralia needs protection from wind, a full sun location and regular watering, but it won't do well in soggy soil.
Here's an article that may help:
https://www.gardeningknowhow.com/houseplants/aralia-plants/aralia-plant-information.htm
WE purchased two japanese aralia plants,and after being in the ground for two days, one of the plant leaves are turning dark gray and dropping.
Unfortunately, your photo did not come through. This can happen with the soil remains too saturated, but most commonly it happens when the plant receives too much light. This shrub is best suited for mostly shade to deep shade.
Here is an article that will help:
https://www.gardeningknowhow.com/houseplants/aralia-plants/japanese-aralia-care.htm
Shall I cut the plants back, or wait to see if they resurrect themselves? Thank you.
I would not cut them, just yet. Wait until you can see whether some of them perk back up. In a week, whatever does not shape back up can be cut back.
Until then, here is an article that will offer some tips for this plant:
https://www.gardeningknowhow.com/houseplants/aralia-plants/japanese-aralia-care.htm
The aralia plants were planted this past spring.
Typically, these are pruned completely for restorative purposes. It would benefit them to have this done right before they start actively growing for the year. It is getting close to that time, so doing so now would benefit your plants.
Here is an article that will help you with the, general, care of your plants:
https://www.gardeningknowhow.com/houseplants/aralia-plants/japanese-aralia-care.htm