Artillery Fungus on Siding: What It Is, How to Remove It, and How to Keep It Away

By Aaron Koszyk, Owner, Double A Power Washing · 8 min read

Walk along the north side of a house in Mishawaka or South Bend in late spring and you will often find them: hundreds of tiny black dots, each about the size of a pinhead, stuck to the vinyl siding in a band from the ground up to about 20 feet. Homeowners scrub them, pressure wash them, and call us convinced it is mold or a paint defect. It is neither. It is artillery fungus, and it comes out of the mulch bed under that wall.

We see it on dozens of homes every year across Michiana, and it is one of the few exterior stains where the honest answer is that prevention beats removal. This guide covers what artillery fungus actually is, whether it can hurt you, how to remove it with the least damage to your siding, why power washing makes it worse, and how to keep it from coming back next spring.

What Artillery Fungus Actually Is

Artillery fungus is the common name for Sphaerobolus stellatus, a small wood decay fungus that grows in damp, shaded organic material. In a residential yard that almost always means shredded hardwood mulch, especially the dark dyed mulch that holds moisture. You will also hear it called shotgun fungus, artillery mold or cannonball fungus. They are all the same organism.

The fungus itself is tiny and cream colored, about the size of a pinhead, and you will never notice it in the mulch. What you notice is how it reproduces. Each fruiting body builds up pressure and fires a single black spore mass called a gleba toward the brightest nearby surface. That is why the dots cluster on light colored siding, white garage doors, car paint and window glass, and why they are always thickest on the side of the house facing the mulch bed.

In northern Indiana the fungus is most active in cool, wet weather, so the spores show up in April through June and again in September and October. A bed that is heavily shaded and never dries out can fire year round.

Is Artillery Fungus Harmful to People, Pets or the House?

No. Artillery fungus is not toxic, it does not cause allergies the way indoor molds can, and it does not eat siding, wood trim or paint. It is a nuisance, not a health hazard. You can touch the spots, kids and dogs can play in the mulch, and there is no reason to treat it like a mold remediation job.

The real cost is cosmetic and financial. The spore masses are coated in a natural adhesive, and once they dry in the sun they harden like a speck of tar. Scrub too hard and you take the finish off the siding with them. Some homeowners have voided vinyl siding warranties with aggressive cleaning, and a heavily spotted house can be a real problem at resale. A few of our customers first found the problem when a home inspector flagged it.

One more thing worth knowing: the spores can stick to cars parked near a mulch bed and are just as hard to get off automotive paint. If your driveway runs beside a mulched bed, park on the other side while the mulch is fresh and wet.

How to Remove Artillery Fungus From Siding

The right method depends on how long the spores have been there. Spores that landed in the last few weeks are still slightly soft and come off with far less effort than spores that have baked through a summer. Whatever the age, the sequence is the same: soften, agitate gently, rinse with low pressure, then repeat rather than pushing harder.

1. Test a hidden spot first. Pick a few dots behind a downspout or below a deck and try each step there before working across the wall.
2. Soften the spores. Wet the area with warm water and a mild detergent, or a purpose made exterior siding cleaner, and let it sit for ten to fifteen minutes. Do not let it dry on the siding.
3. Agitate by hand. A soft nylon brush or a melamine sponge, used with light pressure, will pop most fresh spores loose. Work in small sections. A plastic putty knife held nearly flat can lift older spots, but never use metal.
4. Rinse at low pressure. A garden hose is enough. If you own a pressure washer, use the widest tip, keep it at least three feet from the wall and point it downward so water does not drive behind the siding.
5. Repeat, do not escalate. Two or three gentle passes beat one hard one. Stubborn spots will often leave a faint tan shadow. That shadow usually fades over a season and is far better than a scuffed panel.

On painted surfaces, stucco, brick and stone the spores bond even harder, and scrubbing can lift paint or scar the surface. Those are the situations where it pays to have a professional look before anyone touches it.

See a Mishawaka Job From the Air

Drone footage from our work at Commercial Cleaning Graffiti Removal City of Mishawaka

Why Power Washing Does Not Remove Artillery Fungus

Every spring we get calls from homeowners who rented a pressure washer, blasted the dots at 3,000 PSI, and now have a wall with the same black spots plus streaks and a few loose panels. High pressure water hits the spore mass and drives it deeper into the texture of the vinyl instead of lifting it. It also forces water up behind lap siding, which is how you end up with wet sheathing and mold on the inside of the wall.

Be careful with any company that promises to power wash artillery fungus off without scrubbing. That is either high pressure, which damages siding and voids most vinyl warranties, or harsh chemistry that fades the color. Our approach is a low pressure soft wash house washing: the whole wall is treated with a siding safe detergent that loosens the adhesive, the spotted band is agitated by hand with soft brushes, and everything is rinsed at garden hose pressure. It is slower than a normal wash and we price it that way, but it is the only method that removes the spots without leaving worse marks behind.

Even done correctly, the honest expectation is that fresh spores come off almost completely and older spores come off mostly, sometimes with a light shadow. Anyone promising a perfect result on a two year old infestation is not being straight with you.

DIY vs. Hiring a Pro

How to Keep Artillery Fungus From Coming Back

Cleaning the siding does nothing about the source. If the mulch bed stays, the fungus fires again next spring. Prevention is about the bed itself.

• Replace shredded hardwood mulch near the house. Stone, river rock, rubber mulch or large pine bark nuggets do not support the fungus. Even switching the three feet closest to the foundation makes a real difference.
• If you keep wood mulch, keep it fresh and dry. The fungus lives in decaying wood, so old mulch is the problem. Remove the old layer rather than topping it, replace it every year, and rake it every few weeks so it dries out.
• Avoid dark dyed mulch on the sunny side, and any mulch on the shaded side. Dyed mulch holds moisture, and shaded beds never dry, which is exactly what the fungus wants.
• Use a barrier. Some homeowners have good results with a strip of stone between the mulch and the wall, or a low fabric fence during spring, since the spores only travel toward light.
• Inspect twice a year. Walk the house in May and again in October. A dozen fresh spots wiped off with a wet cloth is a five minute job. Five hundred baked on spots is a service call.

If you are already planning landscaping work, this is a good reason to do the beds nearest the house first. It is the one change that ends the problem for good.

Looking for more than house washing? We also offer house washing, concrete cleaning, roof cleaning, and gutter cleaning throughout the Mishawaka area.

Why Choose Double A Power Washing?

Double A Power Washing is a local, family owned exterior cleaning company serving Mishawaka, South Bend, Granger, Elkhart and the surrounding Michiana communities. We have removed artillery fungus from hundreds of homes, and we will tell you up front what will and will not come off before we start. Every wash is low pressure, siding safe and fully insured, and we are 5.0 rated on Google.

What Our Customers Say

Frequently Asked Questions

Artillery fungus is one of the most misunderstood exterior stains in Michiana. It is harmless, it is stubborn, and the wrong cleaning method costs more than the spots ever did. If you have a wall full of black dots in Mishawaka, South Bend, Granger or Elkhart, send us a photo and we will tell you honestly what a soft wash can and cannot do for it, and what to change in the mulch bed so it does not happen again.

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