Gray mold (Botrytis) on dormant flytraps and pitcher plants

Dead leaves, dormancy and the mold that follows

Winter is when carnivorous plants carry the most dead tissue. Outdoor Venus flytraps die back above ground and rest in their rhizomes ^[4], and Sarracenia pitchers turn brown as the plants go dormant in fall ^[5]. That dead material is exactly what gray mold needs.

Gray mold is caused by the fungus Botrytis cinerea. It survives on dead plant tissue, produces masses of dusty gray spores that travel on air currents and splashing water, and readily attacks soft tissue. To get into tougher, healthy leaves, its spores need a food source such as a wound or dead and dying tissue ^[1]. UC IPM describes the same sequence: Botrytis does not invade healthy green leaves or stems unless an injured or dead area is present, or it can grow in from a food base such as a fallen leaf, which it colonizes first ^[2].

That gives a practical line between normal and trouble. Gray fuzz confined to leaves that were already dead is a cleanup job. Fuzz spreading from dead leaves into green tissue, the leaf bases or the growing point is a problem that needs action now. If the base of the plant is soft, read rhizome and root rot as well.

Why winter storage invites gray mold

Cool and damp suits it. Botrytis can grow from about −2 to 32 °C (28 to 90 °F), slowly at the extremes, with an optimum around 21 to 25 °C (70 to 77 °F). It is more active below that range than above it, and it is most troublesome at moderate temperatures with high humidity. Free water is needed for spores to germinate, and moisture is often a bigger limit on the fungus than temperature ^[2]. Wisconsin Extension also notes that gray mold is more common in cool, damp conditions ^[6].

Still air and old leaves add to it. The ICPS lists cold greenhouses, cold frames, unheated garages and basements as places to overwinter potted Sarracenia, and warns that old leaves kept out of the weather can harbor pests or reduce air circulation, inviting fungus. That is why it gives long-pitcher types stored this way a haircut in fall, and why plants wintered under a deep pine-needle mulch may need some leaves thinned out ^[3].

A refrigerator does not stop it. UC IPM notes that refrigeration near 0 °C (32 °F) slows gray mold but does not completely stop it, that decay can proceed rapidly once infected tissue warms, and that old foliage in refrigerators should not be overlooked as a spore source ^[2]. For Venus flytraps grown indoors under strong lights, the ICPS considers refrigerator dormancy a waste in any case ^[7].

The troubleshooter lists dead leaves in still, cool air as the usual cause of winter fuzz.

How to check

Look closely at dead leaves, the base of the plant and the mulch around it. In high humidity, Botrytis sporulates on infected tissue as masses of gray or brownish spores ^[2]. On leaves it causes irregular dead areas, sometimes in a bull’s-eye pattern ^[1]. Then check the living parts: are the growing point and the newest leaf bases firm and clean?

What to do

1. Remove diseased and dead tissue promptly, from the plant and from the soil surface ^[1]. On stored Sarracenia, the ICPS’s winter haircut applies; on plants outdoors, it removes only dead leaves and brown leaf ends ^[3].
2. Disinfect your scissors between cuts. The University of Wisconsin plant clinic suggests at least 30 seconds in 70% alcohol, which it prefers because it is less corrosive, or in a 10% bleach solution ^[1].
3. Improve air movement and stop condensation. Space plants out so leaves dry faster and avoid water condensing on plant surfaces ^[2]; splashing water also spreads the spores ^[1].
4. Keep the soil moist but not waterlogged. Dormant plants still must not dry out ^[4]; the ICPS calls freeze-drying the biggest winter danger for outdoor flytraps ^[7].
5. Avoid wounds and excess fertilizer, which give the fungus entry points ^[1].

On fungicides, extension guidelines list several preventive products for ornamentals and point out that they must be applied before infection and that some can damage plants ^[2]. None of the sources reviewed here tested them on carnivorous plants, and the ICPS warns that any fungicide, even cinnamon, is deadly to Pinguicula ^[8]. Cleanup and airflow are the safer first steps. If you do use a product, follow its label.

Other molds and look-alikes

• Moldy food on sundews. Food left on sundew leaves can grow mold. The ICPS says to use less next time, and that a dab of 70% isopropanol from a small paintbrush will kill the fungus ^[9].
• Mildew on Cephalotus. The ICPS describes Cephalotus as highly susceptible to mildew, reports a case where plants were saved by quick fungicide treatment, and notes that whole collections have been lost to fungus attacks ^[10]. See the Albany pitcher plant profile.
• White fluff that is not mold. White, cottony material in leaf bases is often produced by mealybugs ^[11]; see aphids, mealybugs and scale.

Preventing gray mold in dormancy

• Before storing Sarracenia under cover, cut back long-pitcher types and remove dead leaves ^[3].
• Keep storage cool to cold; the ICPS warns that storage that is too warm starts plants growing too early ^[3]. NC State Extension suggests at least 10 weeks at about 2 to 10 °C (35 to 50 °F) for flytraps given a dormancy ^[4].
• Give stored plants air movement and avoid condensation ^[2].
• Remove dead and dying tissue through the winter ^[1].
• Consider growing indoor flytraps under lights all year, which the ICPS says needs no dormancy at all ^[7].

The dormancy guide covers storage methods in detail, and browning pitchers in autumn are discussed in Sarracenia pitchers turning brown.