Mold vs Mildew vs Algae on Rochester Siding: Why the Organism Matters for the Treatment

The black-green streaks spreading down the north face of your vinyl siding are not all the same thing. Rochester homeowners see three distinct organisms on their home exteriors — mold, mildew, and algae — and they look similar enough from the driveway that most people (and some pressure washing contractors) treat them identically. They shouldn't.

The organism determines the correct treatment concentration, the dwell time, and what "success" looks like. Get it wrong and you either waste cleaning chemistry on something that needs a different approach, or you strip visible growth while leaving behind spores and root structures that restart the colony in six months. This matters in the Rochester market specifically because our lake-effect humidity — Lake Ontario sitting thirty miles north and feeding moisture-laden air into Monroe County through nine months of the year — creates conditions where exterior biological growth returns fast if you only clean the surface.

Gloeocapsa magma: the black streaks on your roof (and sometimes upper siding)

The most dramatic exterior staining in Rochester is the black vertical streak pattern on asphalt-shingle roofs that migrates down the fascia onto upper siding. This is Gloeocapsa magma, and it's technically a cyanobacteria — not an alga and not a mold. Cyanobacteria are photosynthetic organisms that produce a dark pigment (the melanin-like sheath that gives G. magma its characteristic black color) as UV protection. That pigment is the stain you see, and it doesn't pressure-wash away.

G. magma feeds on the limestone filler in asphalt shingles. On siding, it typically arrives via spore drift from the roof and colonizes any surface with sufficient moisture and shade. It thrives in Rochester's climate because it tolerates wide temperature swings — it's active from about 45°F up — and the lake-effect cloud cover that gives Monroe County its grey spring and fall skies reduces UV irradiance enough that G. magma's UV-protective pigment is an advantage, not an overinvestment.

The treatment: sodium hypochlorite. ARMA TR-7-Bulletin — the Asphalt Roofing Manufacturers Association's guidance document on roof cleaning — specifically recommends a 50:50 water-to-bleach mix (approximately 6.25% SH applied concentration when starting from 12.5% SH) for heavy G. magma colonization on shingles, with a 15 to 20 minute dwell time and a low-pressure rinse. On siding, a lower concentration (1.5 to 2% SH) is standard because vinyl and painted surfaces don't need the roof-shingle kill concentration. The chemistry is the same; the ratio differs.

The key is dwell time. G. magma is not on the surface — it's anchored to the substrate by holdfast structures analogous to plant roots. Short-dwell cleaning strips the top layer of pigment and leaves the holdfast structure intact. This is why a surface that looks clean after a fast pressure wash restreaks within six to twelve months; the organism survived below the visible surface and re-grew. Correct dwell time — 15 to 20 minutes before rinsing — kills the structure, not just the pigment.

Cladosporium: the olive-green fuzzy growth on shaded siding

Cladosporium is a true mold — a fungus in the Cladosporiaceae family. Under a microscope it looks like small clusters of dark olive-green to black spores; on siding it appears as a fuzzy or powdery growth, often concentrated in the lower third of a wall section where shade, splash-back from soil, and minimal airflow create ideal conditions.

Cladosporium is ubiquitous — it's the most common airborne mold genus in the Northern Hemisphere — and in Rochester it's active from early spring through late fall. It colonizes vinyl siding, painted wood, window caulk, and any organic substrate that holds moisture. North-facing walls below the roofline overhang, where rainwater splashes and drips and shaded air barely dries, are the primary habitat.

Unlike G. magma, Cladosporium is truly allergenic. People with mold sensitivity can have respiratory responses to heavy exterior Cladosporium growth adjacent to HVAC intakes or open windows. This is one practical reason exterior mold on a Rochester home is not just a cosmetic issue.

Treatment: sodium hypochlorite at 1 to 2% SH applied concentration, similar to house-wash standard. Cladosporium's cell wall is susceptible to oxidizing bleach chemistry — the hypochlorite ion disrupts the fungal cell membrane and kills the organism. Dwell time of 10 to 15 minutes on saturated surfaces. One application handles light to moderate colonization; heavy growth (visible furring, growth into caulk gaps) may need a second application.

The important difference from G. magma: Cladosporium does not have a melanin sheath, so the staining is a physical presence (the organism itself) rather than a pigment deposited in the substrate. When you kill Cladosporium, the visual evidence is gone once rinsed. This means the before-and-after visual improvement is often more dramatic than G. magma treatment on siding.

Aureobasidium pullulans: the black-pink organism on wood and painted surfaces

Aureobasidium pullulans is the organism you're least likely to have heard named, and the one most commonly encountered on wood decks, wood fences, and the painted trim on older Rochester homes. It shifts color as it matures: yeast-like and nearly transparent in early colonization, turning pink-to-salmon in adolescence, then a deep dark brown or black in mature growth. The pink stage is the tell — if you see rosy or salmon-colored patches on unpainted wood, that's Aureobasidium at the intermediate growth phase.

Aureobasidium is a black yeast that thrives at moderate moisture and temperatures — the exact profile of a Rochester late summer. It's particularly aggressive on outdoor wood because it can metabolize the tannins and cellulose in wood fiber as a carbon source. This is the organism that drives genuine wood degradation on unprotected decks: it doesn't just sit on the surface, it consumes the wood over time.

Treatment for Aureobasidium on wood requires the full soft-wash protocol: SH solution (0.5 to 1% on wood, lower than for vinyl because wood is more sensitive), full dwell, soft rinse, and — critically — the oxalic acid brightener step afterward. The brightener neutralizes the high-pH bleach environment that, if left uncorrected, will gray the wood and interfere with stain adhesion. Aureobasidium on wood is one of the primary reasons the brightener step is not optional: you've killed a wood-consuming organism that left its mark on the grain, and the brightener is what brings the wood back to a viable staining substrate.

The Lake Ontario factor: why Rochester's biological load is heavier than most markets

Humid continental climates (Rochester's Köppen classification: Dfb) have more exterior biological growth than drier markets at comparable latitudes. Lake Ontario amplifies this by:

1. Extending the moisture season. Lake-effect precipitation — rain and snow — runs from October through April in Monroe County, but the lake's thermal mass also generates elevated humidity through the shoulder months. The exterior of a Rochester home is wet or near-wet for substantially more days per year than an inland market at the same latitude.

2. Moderating winter temperatures on the south shore. The lake pulls Monroe County's average January temperature up compared to points 50 miles inland, which means organisms like Cladosporium and Aureobasidium survive winters that would kill them at colder inland locations. Rochester's pest and mold season is longer than it would be without the lake.

3. Reducing UV irradiance. Monroe County averages around 156 sunny days per year — one of the cloudiest metros in the contiguous US. Reduced UV exposure means UV-sensitive organisms (including mold spores on the surface of siding) survive longer between rains than they would in a sunnier climate. It also slows the natural UV degradation of spores deposited between cleaning cycles.

The practical implication: an 18-month cleaning cycle that's appropriate for a home in inland central New York may be a 12-month cycle for an Irondequoit or Webster lakefront home. The service areas guide notes that lakefront Irondequoit properties see 12 to 15 month mildew return cycles on north-facing siding compared to 24-plus months for inland south-facing walls in the same suburb.

Why it matters that your contractor can tell the difference

Most Rochester homeowners can't distinguish Cladosporium from Aureobasidium from G. magma at the driveway. A competent pressure washing contractor should be able to, because the diagnostics drive the quote. Surface mold (Cladosporium, Aureobasidium) on well-maintained vinyl siding responds to a single standard house-wash treatment. G. magma that has established on siding for multiple seasons, or Aureobasidium that has penetrated into painted wood, may need higher concentrations, longer dwell times, or a second application.

The tell is when a contractor quotes a flat rate based on square footage without asking about the type of growth, the age of the staining, the substrate, or the history of previous cleaning. Surface area matters for pricing — more square feet means more time and chemistry — but the organism type affects whether a standard application will produce the outcome the customer expects.

BF Home Services carries both PWRA and UAMCC credentials and explicitly soft-wash-first protocols, which matters when distinguishing biological treatment from mechanical dirt removal. Upstate Roof Cleaning is the specialist for G. magma on roofs specifically — RCIA-certified technicians who understand the cyanobacteria biology, not just the bleach application.

For the house soft wash service detail — what's included in a standard treatment, what the 18-month mildew-return guarantee covers, and what triggers a second-treatment recommendation — see the service page.

What "killed at the root" means in practice

You'll see "kills algae at the root" in pressure washing marketing. This is the correct framing when it applies, but it's worth unpacking what "root" means for organisms that don't technically have roots.

For G. magma: the holdfast structures anchoring the colony to the shingle or siding surface. These structures are the reason G. magma survives a surface rinse — they're anchored below the visible pigment layer. A kill-at-root treatment means the SH solution has penetrated the holdfast and oxidized it, not just decolorized the pigment layer above it.

For Cladosporium and Aureobasidium: the mycelium network — the root-analog of fungal organisms. On vinyl siding, this is superficial (fungi don't have cell-wall-penetrating enzymes for PVC). On wood and painted surfaces, mycelium can penetrate into the substrate matrix. A kill-at-root treatment on wood means the SH solution has dwelled long enough to reach and oxidize the mycelium at depth, not just the surface sporulation.

The distinction matters for warranty framing: an 18-month mildew-return guarantee on a house soft wash means the contractor believes the treatment was deep enough to kill the organism rather than surface-clean it. Contractors who offer 6-month or no-warranty cleaning are either running shorter dwell times, lower concentrations, or both.

Questions about what's growing on your specific exterior — or why a previous cleaning didn't hold? Reach us at connormeador@gmail.com.