Alternaria Mold: The Outdoor Spore That Peaks in Fall

When people talk about fall allergies, ragweed gets most of the attention, but alternaria mold is the other half of the late-season story. Alternaria is a genus of fungi found in soil, on plants, and in decaying vegetation almost everywhere, and its spores routinely rank among the most common outdoor mold spores recorded at counting stations across the United States. Its season runs later than most pollen seasons — building through summer, peaking in late summer and fall — which makes it a major reason autumn counts stay elevated even after the ragweed peak has passed.

Key takeaways

• The National Allergy Bureau (NAB) lists Alternaria among the most frequently counted outdoor mold spores at its U.S. stations.
• Alternaria grows on soil, plant surfaces, grains, and decaying vegetation, according to the AAAAI's mold resources.
• Spore release typically peaks in late summer and fall, on warm, dry, windy afternoons, per NAB reporting patterns.
• Pollen Atlas data shows mold readings in Gulf Coast cities staying elevated weeks after weed pollen fades in the fall.
• Researchers studying the November 2016 Melbourne thunderstorm asthma event examined ruptured pollen grains and fungal spores, including Alternaria, as part of the airborne load that day.

What alternaria mold is and where it grows

Alternaria is not a single species but a large genus of fungi, with Alternaria alternata among the most studied. The fungi colonize plant material — living leaves, ripening grain, grasses, and the dead vegetation that accumulates on the ground — and produce dark, club-shaped spores that are large by fungal standards and distinctive under a microscope, which is why counting stations can identify them reliably.

Soil, plants, and grain fields

Outdoors, Alternaria is essentially a plant-associated fungus. It appears on cereal grains as they ripen, on tomato and other garden plants, on grasses going to seed, and in the leaf litter under trees. Agricultural regions can generate very large spore loads at harvest time, when machinery disturbs colonies on grain and stubble. Suburban and urban areas are not exempt: compost, mulch, and untended leaf piles are all suitable habitat, a point the AAAAI makes in its general mold guidance at aaaai.org.

Why warm, humid regions see more of it

Alternaria needs moisture to grow but releases spores most freely in dry, windy conditions — a combination the Gulf Coast supplies for much of the year. Houston pollen and mold counts and New Orleans pollen and mold counts regularly show mold as a leading component of late-season readings, and Miami pollen and mold counts show mold persisting in months when northern stations report almost none. The pollen and mold heatmap shows this regional split clearly in October.

When alternaria mold spores peak

Alternaria follows a seasonal curve that lags the spring tree pollen wave and overlaps the fall ragweed season. Colonies build on growing plants through spring and summer, and spore production reaches its highest levels from mid-summer into fall, when the most plant material is mature or decaying.

Daily timing: the afternoon release

Unlike many pollens, which peak in the morning, Alternaria spore counts often peak in the afternoon. Spores detach as humidity falls and air warms and stirs, so dry, breezy afternoons — especially after a humid spell that fed colony growth — produce the highest readings. Harvest activity can create local spikes on top of this daily pattern.

Weather that raises and lowers counts

Rain has a two-stage effect described in NAB station reporting: counts drop while it is raining, then can surge in the following days as moisture feeds new growth and drying air releases spores. Thunderstorms add a separate mechanism studied in the literature: strong storm outflows can lift and concentrate airborne particles, and researchers examining the 2016 Melbourne event — the deadliest thunderstorm asthma episode on record, reviewed in the Asthma and Allergy Foundation of America's resources — documented how storm conditions can rupture pollen grains and redistribute fine airborne material, including fungal spores, across a city in under an hour.

How alternaria mold counts are measured and reported

Mold counts come from the same sampling network as pollen counts. National Allergy Bureau stations use volumetric samplers that draw in air and deposit particles on a coated surface; trained counters then identify spores under a microscope and report concentrations. The AAAAI's NAB pages at pollen.aaaai.org publish station readings and explain the categories used for mold levels.

Mold reporting differs from pollen reporting in one important way: spores from many fungal genera are counted, and Alternaria is reported as its own line item at many stations because of its consistent presence. Pollen Atlas folds mold into its daily index presentation alongside tree, grass, and weed pollen, so a high fall reading can be traced to its source rather than treated as a single unexplained number. For background on the season as a whole, see the guide to mold allergies in fall, and for a city where mold counting has a long public record, the companion piece on the Chicago mold count.

Readers looking for a provider can search the Find an Allergist directory by city and specialty; it is a directory reference, and the count data remains the daily tracking tool.

Station records also show why Alternaria rewards attention in its own right rather than as part of a generic mold total: its seasonal curve, rising through summer and peaking in fall, is distinct from the flatter profile of some other common spores, and its daily afternoon release pattern differs from the morning pattern of most pollens.

Frequently Asked Questions

What is alternaria mold?

Alternaria is a large genus of fungi that grows on soil, plants, grains, and decaying vegetation. Its spores are among the most commonly counted outdoor mold spores at National Allergy Bureau stations in the United States.

When do alternaria mold spores peak?

NAB reporting patterns show Alternaria spore levels typically highest from mid-summer through fall, with daily peaks often in the afternoon on warm, dry, windy days. Warm, humid regions such as the Gulf Coast see a longer elevated period.

How are alternaria mold counts reported?

Counting stations collect airborne particles with volumetric samplers, and trained analysts identify Alternaria's distinctive spores under a microscope. Results are published through the National Allergy Bureau and incorporated into daily index presentations such as Pollen Atlas's.

Is alternaria mold only an outdoor fungus?

Alternaria is primarily described as an outdoor, plant-associated fungus in AAAAI resources, though its spores are airborne and therefore present wherever outdoor air circulates. Its signature season and habitat are outdoors in soil and vegetation.

Conclusion

Alternaria mold is the quiet driver of late-season counts: a plant-associated fungus whose spores build all summer, peak on dry fall afternoons, and linger after frost has finished the ragweed. Knowing that alternaria mold follows its own calendar — later, longer, and more weather-sensitive than most pollen — makes fall readings far easier to interpret. Pollen checks you daily. Check it back — free counts and 5-day forecasts, worldwide: PollenAtlas.com

Sources

• American Academy of Allergy, Asthma & Immunology (AAAAI) / National Allergy Bureau, mold count reporting and Alternaria resources (2025).
• Asthma and Allergy Foundation of America (AAFA), mold allergy resources (2024–2025).
• Peer-reviewed reviews of the November 2016 Melbourne thunderstorm asthma event and airborne fungal spore research (2017–2023).