Question: I have black spots all over my vinyl siding—thousands of them, about the size of a ballpoint pen tip. They are hard, and when you pick them off, they leave a small brown stain. They are even as high up as my gutters. It seems they are only on the siding above my mulch beds. I have heard that the mulch can spit its coloring out and also that it could be mold from the mulch. I have tried house and siding cleaner and a pressure washer, to no avail. I also tried Krud Kutter and a Scotch-Brite sponge, which works, but it would take five years to clean the siding that way. What are these spots and what can I use to remove them?
Answer: The black tar-like spots that are all over your vinyl siding are more than likely a fungus called Sphaerobolus stellatus, better known as artillery, or shotgun fungus. Although it's most often found on the east coast, it can grow anywhere and especially thrives in areas that have cool, damp Springs, or rainy Fall weather conditions.
As for where it grows or how it found your siding, artillery fungus is a notorious wood-dwelling fungus that's usually found growing in wood-based mulches, especially those that are a combination of bark and hardwood. This may be where it's stemming from in your yard as you mention that the fungus seems to be limited to growing on the siding closest to your mulch beds. This fungus can grow on multiple surfaces including wood siding, fences, decks, and cars.
Artillery fungus is difficult to get rid of because of its sticky properties. That's why, if it's at all possible, it's best to wash the fungus away when the spores have freshly landed, which is normally when the weather is cool and damp and the temperatures are between 50 and 68 degrees Fahrenheit.
How to Remove Artillery Fungus
There are a lot of "magical' remedies you will discover online, including specialty soaps and anti-fungal treatments, that claim they'll get rid of this type of fungus just by being sprayed onto the siding. According to the leading horticulturists that study artillery fungus every day, these kinds of statements are not true. What it really comes down to elbow grease. Anything safe for your siding can be used to help loosen it up a bit, but it truly comes down to scrubbing by hand to get it all off.
That being said, what we can do other than tell you to keep doing what you're doing, is to offer you some suggestions that others say have worked for them in the past.
Most Common Recommendations for Removal
One thing almost every suggestion included was to keep the surface wet while you cleaned it. Use a scraper or putty knife to remove the fungus before trying to scrub it, and then once you've picked a cleaner, allow it soak into the surface from a few minutes, up to a few hours before you begin trying to scrub it away. No matter which you try, should you try any, just be sure that you test it first by trying it in a place on the siding that won't matter should it be discolored, as it likely will with most of these recommendations.
Bleach - The most common trial and error method in removing artillery fungus is bleach, however, it doesn't always work and seems to be a temporary solution. Also, it still requires a lot of scrubbing. Keep in mind that it might also bleach the paint on the siding. Do a test spot somewhere inconspicuous.
Peroxide - Hydrogen peroxide has also been said to work, especially at higher strengths. Apply it, allow it to bubble, and then brush it off with a wet scrub brush or sponge.
Mouthwash - Mouthwash is something many swear by for cleaning artillery fungus off of vinyl siding, especially mint-laced varieties. As with bleach, saturate the area with the mouthwash and allow it to penetrate before scrubbing it off. Others have tried a combination of bleach and mouthwash; just be careful mixing any chemicals together.
Melamine Foam - Better known as the Mr. Clean Magic Eraser, melamine foam pads work best after the area is sprayed with one of the cleaners above and allowed to soak into the siding for a few hours. Then, scrub with the pad to erase the remaining spores. This method may be expensive depending on the size of the space you're working on, since Magic Erasers tend to fall apart rather quickly. Note that the bleach seems to get rid of the actual fungus, while the Magic Eraser removes the stain left behind by it.
How to Prevent Artillery Fungus
To prevent the regrowth of fungi on your siding once you remove what's already there, you will need to use mulches that are non-wood based. If the mulch is being used purely for landscaping and visual appeal, some alternatives include stone products like pea gravel, lava rock, or crushed stone.
Using compost with no wood in it is also a good alternative. Studies have shown that the Artillery fungus spoors did not grow when only compost was used.
If you wish to keep the wood-based mulch, you can try to keep the fungus at bay by adding a layer of fungus-resistant mulch such as cypress or cedar nuggets to keep the fungus from spreading. Raking the mulch often during the moist months to expose the fungi to the dry air can keep the spores from spreading.
Keep in mind that fungus-resistant mulch is a bit of a misnomer because, although they're labeled as such, these are not completely anti-fungal and will not prevent fungal growth entirely. Using wood mulch will always carry a high risk of having artillery fungus.
Getting rid of the fungus is possible, but it's not going to be easy. Find a good cleaner that works for you and begin scrubbing. Or, better yet, hire someone to do the scrubbing for you.