Is there a way to remove fungus without dismantling the lens?

UV light will kill the fungus, so all you need to do is let the lens be exposed to sunlight for a few days. Be sure to remove any UV filter you might have attached to the front of the lens.

Unfortunately, killing the fungus is not removing it. If it does not affect image quality, then that might not be a short-term concern. But you'll likely need to continue to expose the lens to UV light regularly. Since fungus is a spore, it can lie dormant for very long periods of time, then flourish again when the conditions are favorable. Those conditions are heat, moisture, and protection from UV light. Just another reason to remove any UV filter you might have on the lens for no real logical reason.

Beyond that, some lenses make it fairly easy to remove the front element from the front. Depending on how your lens is constructed, you might be able to do that, clean the back of the front element, along with the front of the next element, and put it all back together relatively painlessly. I've done it with a Tamron SP 17-50mm f/2.8 Di II. Just be sure to mark the position of each piece relative to the piece it is attached to before you loosen anything, particularly anything that looks like an optical adjustment.

For some hints on how lenses are put together and how to do simple cleaning of the easy-to-get-to internal parts of some of them, take a look at this blog entry from Roger Cicala, the founder and chief lens guru at lensrentals.com.

UV lamps are available in the market. It would be better to put the lenses in an ordinary stainless steel container and put the UV on for few hours; don't look at it while it's on. I think it's better than exposed open to sun as some dirt and moisture from the environment will make the lenses dirty. What I did purchased one cylindrical drum (use for keeping grains in the kitchen) and put a half kg of silica gel in a small container in the bottom, and covered with another steel lid that has holes so that the silica would be able to absorb moisture from lenses are sitting above the silica container.

As the silica gel absorbs moisture, it gradually changes colour from blue to white. When the silica is white, take out the silica gel and reheat it so that the absorbed moisture will be evaporated. When the silica dries out, its colour will become the original blue.

By doing this, my dozens of lenses of telescope and binoculars are ok, and have been for the last 30 years. But once a year silica gels needs to be loaded.

My supplier in India uses sunlight only. He is using a cannon camera since 30 years and puts it in sunlight every two months. He is very happy and also I .