Sclerotiniaceae Molds: Their Effects on Plants, People, & Air Quality

A hand holding a strawberry with no mold and one infected with white mold. (Sclerotinia Sclerotiorum)

The Sclerotiniaceae family make terrible dinner guests. They happily help themselves to all the produce in your home or restaurant, leaving only sniffles and coughs as a parting present. Allow ActivePure to introduce you to this fungal family tree via two infamous branches: Sclerotinia sclerotiorum (white mold) and Botrytis cinerea (gray mold.)

Sclerotiniaceae and Plants

The Sclerotiniaceae family of mold takes its name from the tendency of some species to form “sclerotia.” These are clumps of fungal fibers (hyphae) that harden together to keep the fungus alive as its food source is depleted. Eventually the sclerotia “fruit” in the soil releases new spores that either find their host plant or are carried off via wind currents, which keeps the infection cycle in motion.

What is being infected? Many members of the family are necrotrophic plant parasites, meaning they infect living vegetable matter. Under the right conditions, spores which land on leaves, stems, fruit, or flowers begin secreting chemicals that allow them to evade that plant's immune system and digest its cells.

These molds cost the economy (specifically agriculture) a lot of money each year.

“Collective crop losses to S. sclerotiorum (white mold) exceed $200 million annually.”

Similarly, gray mold’s cost is so large that it is difficult to calculate; however, estimates suggest that the real cost could range from $10 billion to $100 billion worldwide. Gray and white molds can also thrive on produce after it has been picked, allowing the molds to financially impact restaurants and individuals.

Sclerotinia Sclerotiorum

White mold lives in the soil, sending out spores under cool and moist conditions. It is one of several related species referred to on plants as “white mold.” According to the American Phytopathological Society (APS,) “White mold is easily identified by the characteristic white cottony mycelium of the pathogen that grows on the surfaces of infected aerial tissues. The hyphae produce enzymes and oxalic acid, creating water-soaked lesions, frequently with a distinct margin.” In other words, when it flourishes on plants, white molds look like cotton over a dead, wet spot.

The APS continues, “At later stages of the disease, the cottony hyphae of the pathogen aggregate into (typically) pea-sized clumps of mycelium[...]These clumps eventually mature into hard black sclerotia.” Per the APS, white mold tends to infect “legumes, sunflowers, canola, most vegetables (e.g., potato, tomato, lettuce, and crucifers,) tobacco, many flowering bedding plants, and stone fruits.”

Botrytis Cinerea

Botrytis cinerea — known to mycologists as early as 1729 — is one of the most thoroughly studied molds. While gray molds are members of the Sclerotiniaceae family, they form their own genus. According to Iñaki García of CANNA Gardening:

“The species name Botrytis cinerea is derived from the Greek word ‘botrys,’ which means ‘cluster of grapes,’ and the Latin word ‘cinerea’ refers to the ashy color of the mold. Viewed under a microscope, using a little imagination, the structures carrying the fungal spores do indeed look like bunches of grapes.”

When it comes to crops, gray mold is the second most detrimental fungal pathogen. It is only outranked by Magnaporthe oryzae, which is responsible for rice blight. Gray mold tends to lie dormant in dead ground vegetation, and then reinfect live plants as the weather gets warmer. When allowed to flourish, these molds form a gray fuzz, which is where it got its nickname “gray mold.” While it infects over 200 species, gray mold is most famous for feasting on strawberries (even after they have been picked.)

One beneficial feature of gray mold is, when controlled it can make better-tasting wine. This “noble rot” intensifies the grapes’ sweetness, making them perfect for ice and dessert wines. According to the website Wine Folly, noble rot tends to occur on grapes close to a river, especially in colder weather.

Sclerotiniaceae and Air Quality

The effect of gray mold on humans has been heavily researched, but the effect of white mold on humans is almost unstudied. Under laboratory conditions, white mold has been shown to release toxins that damage human cells, but this is a risk to food quality, not indoor air. Nevertheless, the following information on gray molds will give you an inkling of what future studies may discover about other members of the Sclerotiniaceae family.

Infection: Sclerotiniaceae molds are not frequent colonizers of humans. Most human fungal infections originate with other genera, such as Aspergillus, Mucorales, or Candida. There is one recorded case of a related Botrytis species infecting a healthy adult, but we were unable to find any examples of human gray mold infection.

Hypersensitivity Pneumonitis: There are two recorded cases of hypersensitivity pneumonitis (HP) linked to gray molds, both occurring in vineyard workers around 1987.

Asthma: A 2022 paper found a “strong positive correlation” between gray mold and the body’s creation of an inflammatory protein associated with asthma (interleukin-17A.)

Allergies: Gray mold is an extremely well-documented allergen, both indoors and out. Indoors, gray mold allergens can be found in “groceries, flowers, dust, carpets, wooden material, and furniture.” While it only makes up 1.1% of all allergenic fungal bioaerosols outdoors, many people find they react to it. Because of this, a 2009 study concluded that “B. cinerea is as important as the more prevalent mold [sic] genera Cladosporium and Alternaria, and we suggest that it should be included in standard allergic tests panels.”

Reducing Sclerotiniaceae Molds Indoors

To reduce Sclerotiniaceae spores indoors, you’ll want to pay attention to the following factors:

1. Control Moisture: When it comes to mold and moisture, an ounce of prevention is worth a pound of cure. For instance, watch for hot or cold spots created by the HVAC system, which could generate condensation, including pipes and ducts. Reducing moisture is a quick and easy way to curb spore production.

2. Monitor Plants: Monitor indoor plants and flower bouquets for signs of mold infection. While individual mold spores aren’t visible, pruning or naturally treating mold colonies can reduce the number of new spores entering your air. You can also help prevent mold growth in the first place by not overwatering your plants.

3. Contact ActivePure: Devices with ActivePure Technology have been tested against both the airborne ascospores of white mold and the airborne conidia of gray mold. Over a two-hour period, an ActivePure device inactivated 92.5% more B. cinerea conidia than the control chamber. In another test, an ActivePure device inactivated 98.1% of S. sclerotiorum spores (versus the control) in 4 hours.

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