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 Didymo or Rock Snot (Didymosphenia geminata) in Vermont and the Northeast

didymo growth in Batten Kill   microscopic view of didymo

Didymo growth in the Batten Kill

(NY section)


Didymo cell

(400X magnification)


Felt-soled wading boots prohibited in Vermont waters as of April 1, 2011

In an effort to curb the spread of aquatic invasive species in Vermont rivers, the Vermont legislature enacted, and the Governor signed into law, a ban on use of felt-soled wading boots in Vermont waters effective April 1, 2011.  Felt soled wading boots have been strongly implicated in the spread of didymo and several invasive species including New Zealand mudsnail and Myxobolus cerebralis, the parasite that causes whirling disease in trout.

For information about the role of felt-soled waders in the spread of invasive species, click here

Text of the law

§ 4616. Felt-soled boots and waders; use prohibited

It is unlawful to use external felt-soled boots or external felt-soled waders in the waters of Vermont, except that a state or federal employee or emergency personnel, including fire, law enforcement, and EMT personnel, may use external felt-soled boots or external felt-soled waders in the discharge of official duties. (Added 2009, No. 130 (Adj. Sess.), § 1, eff. April 1, 2011.)


Didymosphenia geminata, also known as ‘didymo’ and ‘Rock Snot’ is a type of freshwater alga belonging to the group commonly known as diatoms.  Individual cells can’t be seen without a microscope but they produce fibrous stalks that can form extensive thick mats on the stream bottom.  Didymo is believed to be native to northern latitudes of Europe, Asia, and North America. Rarely reported in the past, it is now present in many rivers in the western U.S. and Canada, some tailwater rivers in the southern US, and locations throughout northeastern North America, including New Hampshire, New York, Quebec and New Brunswick. Didymo has also been found in the  southern hemisphere, where it is considered an invasive species.  It was first reported in Vermont during the summers of 2006 and 2007 in the Batten Kill (NY/VT), in 2007 in the Connecticut (NH/VT) and the White Rivers (VT), in 2008 in the Mad River (VT), and 2010 in the Gihon and Passumpsic Rivers (VT).


Click here to see the known distribution of Didymosphenia geminata in Vermont (pdf, 212 KB)

Click on the following shortcuts or scroll down for more information about Didymo:

Things you should know about didymo in Vermont

What does didymo look and feel like?

How is didymo a threat?

What impacts will didymo have on our natural resources?

What is the Vermont Agency of Natural Resources doing about didymo?

What precautions should river users take to avoid spreading didymo?

How can I properly disinfect my recreational equipment?

What should I do if I think I’ve found didymo?

Where can I find more information?

Things you should know about didymo in Vermont:

  • Didymo can be present in a stream but not visible to the naked eye.  Current international research suggests that visible growth and nuisance mats form when phosphorus concentrations in the river fall below 2ppb. 
  • Spread prevention is the only effective management tool.  There is no way to eradicate didymo once established.  Do your part to minimize the spread of this nuisance alga to waters where it may not occur naturally by following best practices for cleaning and disinfecting your gear.
  • International research has yet to show conclusively that didymo has significant impacts to trout fisheries but the abundance and composition of aquatic macroinvertebrate communities does change in the presence of thick mats of didymo.  The appearance of a stream can also change dramatically as didymo mats grow and decay.  They tangle in gear and shoreline vegetation.  There may be economic and aesthetic impacts in areas where nuisance growth occurs regularly.
  • Disinfection and spread prevention practices are effective against didymo and invasive species.  They also help prevent the spread of pathogens such as whirling disease and VHS (viral hemorrhagic septicemia).  The Vermont Agency of Natural Resources collaborates with a number of federal, state, and local entities to reduce the spread of invasive species and pathogens in the Northeast.  Anyone active on rivers and lakes should ALWAYS follow the best practices for spread prevention (shown below) including proper disinfection before visiting ANY surface waters.

What does didymo look and feel like?

  • Light tan to brown in color
  • Clumps feel like wet wool; never slimy or slippery
  • Clumps resist being pulled apart, quite cohesive
  • Expanding colonies may attach to plant stems, forming rope-like strands
  • As stalks lengthen, they form ropy strands and may become white in color
  • Dead strands may dry on rocks and look like tissue, fiberglass, or toilet paper

Click here for a guide to identifying Didymosphenia geminata. (pdf, 51 KB)

Click here for procedures to follow if you think you have found didymo

Click here to download a didymo sample submission form. (pdf, 23 KB)


clump of didymo from White River VT   didymo on rock from Batten Kill   didymo on river rock
Didymo clump on rock from the White River, Vermont (click on photo to enlarge, pdf, 143 KB)
Didymo on rock from Batten Kill
Didymo on rock from Connecticut River


How is didymo a threat?

  • Didymo is not toxic.  The thick extensive mats that can form under certain environmental conditions are a nuisance and may affect fish and other organisms living in the stream.
    • Stalks are strong and not easily dislodged by normal flows.  The visual appearance of a stream may change significantly as mats developed and dried material washed onto the shore or clinging to exposed rock persists for many weeks.
    • Thick mats have been shown to provide more habitat for aquatic insects but may also make it more difficult for local fish to find and eat them.
    • Recurring didymo mats can have economic effects by
      • discouraging swimmers, fishermen and boaters from using a river
      • altering the scenic beauty of streams for Vermonters and visitors alike
      • affecting activities such as hydropower generation and fish hatcheries
  • Didymo can spread easily
    • It attaches to or tangles on fishing gear, clothing and equipment.
    • Anything coming in contact with live cells can carry them to a new location
      • Fishing gear
      • Boats and trailers, kayaks and canoes
      • Vehicles, construction and farming equipment
      • Tubing equipment
      • Footwear and clothing
      • Pets
  • By taking care to clean our gear before using it again, we not only reduce the chance of spreading didymo around Vermont, we also reduce the chance of spreading invasive species or disease that we can’t see.


scientist in New Zealand Didymo on a river rock

scientist in New Zealand Didymo on a river rock

"Worst case" didymo scenario - New Zealand

(photos courtesy of Biosecurity New Zealand) Click on right photo to enlarge (pdf, 63 KB)


What impacts will didymo have on our natural resources?

  • Didymo can alter aquatic insect and native algae populations.
  • Extensive mats can reduce or exclude mayflies, caddisflies, and stoneflies: important trout food.  Chironomids (midges) and worms – less-preferred trout food – are likely to become more abundant in their place.
  • Changes in aquatic insect communities could potentially impact fisheries but studies of didymo’s impact on fisheries are inconclusive so far.
  • Persistent stalk/mat material could reduce the availability of spawning habitat.
  • There are ongoing studies on fish impacts in the U.S., Canada, and Europe.thumbnail image of didymo poster

What is the Vermont Agency of Natural Resources doing about didymo?

  • ANR is keeping current on national and international research pertaining to the biology and management of didymo.
  • ANR provides information to the public about how to reduce the spread of didymo and aquatic invasive species.
  • ANR field staff have developed Best Management Practices and disinfection procedures for field activities to minimize the spread of both aquatic invasive species and pathogens in Vermont waters.
  • ANR evaluated and adjusted its hatchery stocking activities to address the threat of inadvertent spread of aquatic invasive species and fish and wildlife pathogens, including VHS.
  • ANR and its partners developed a poster (left) for placing at river access areas and in public locations to inform the public about didymo spread prevention. (Click on image at right to obtain poster full size, pdf, 300 KB)

What precautions should everyone take to avoid spreading didymo?

Anyone using Vermont waters has the potential to spread aquatic invasive species or wildlife disease from stream to stream and watershed to watershed.  As responsible stewards of state waters, we have an obligation to take precautions that will reduce the opportunity for organisms and disease to hitchhike with us.  Follow these “Best Practices” and “Disinfection Procedures” to keep your gear and possessions free of didymo, other invasive species and wildlife pathogens.

Best Practices for minimizing the spread of invasives or pathogens:

  • Inspect your boat, gear and other equipment before leaving a water body.  Remove all plants, plant fragments, animals, mud or other debris.  Discard them in the trash.
  • When possible, visit only a single water body in a single day rather than traveling to multiple locations.  If you do travel between water bodies and/or watersheds, take appropriate precautions to reduce the opportunity for an invasive organism or pathogen to travel with you.
    • Anglers – fish in a downstream direction when practical.  Most stream organisms move or are carried downstream with the flow rather than upstream.  If you fish at the mouth and then move upstream, you may be offering them a free ride to a new home.
  • Disinfect your gear, boat, and everything else that comes in contact with the water, before traveling to a new water body or watershed (see below).
    • Construct a portable disinfection kit to take on the road
    • Follow a routine disinfection procedure at home. 
    • Anglers – consider the use of easily disinfected waders and equipment.
    • Anglers, guides and outfitters – designate equipment that will be used in a single watershed, or have multiple sets available for same day travel.
    • Canoeists, kayakers, boaters and tubers – Don’t move water between water bodies and watersheds.  Drain any water prior to leaving the boat loading area.  If water is present as you unload, drain it onto the ground, not pavement, away from the shoreline.

How can I properly disinfect my recreational equipment?

DISINFECTION PROCEDURES – Disinfect prior to moving to another waterbody, watershed, or upstream site

The disinfection techniques noted below will kill most aquatic invasive species, fish/wildlife pathogens,and didymo.  Solutions made from dishwashing detergents or bleach offer the best combination as they are easily available and inexpensive as well as effective.  Choose the appropriate agent for the item you intend to disinfect – i.e. bleach will damage clothing.  Disinfected equipment should be rinsed on land, away from state waters.  If possible, used solutions should be discarded to a treatment facility, e.g. pour it down the drain.  Do NOT rinse items near storm drains or on pavement.  

Non-absorbent items – boats, canoes, kayaks, waders and other ‘hard’ objects

  • Dishwashing detergent – use a 5% solution (6.5 oz. mixed with water to make 1 gallon)
    • Soak and scrub for at least 1 minute.  ‘Green’ products are considered less effective and not recommended for disinfection.
  • Bleach – use a 2% solution (2.5 oz mixed with water to make 1 gallon)
    • Spray and soak all surfaces for at least 1 minute.
    • Bleach solutions lose their effectiveness over time and must be made fresh each day.
  • Hot water – Soaking time depends on how hot your water is
    • Soak at least 1 minute in very hot water (above 140°F, higher than most household water heaters)
    • Soak and keep hot for at least 20 minutes in 120° water (uncomfortably hot to the touch)
  • Drying – only for gear that can be left to dry for several days between uses
    • Items must be bone-dry everywhere.  Even a little moisture can keep didymo and other organisms alive for weeks. 
    • Works well for boats and items that can be left in bright strong sunshine for at least five days

Absorbent items – clothing, wetsuits, sandals with fabric straps, and other ‘soft’ items

These require longer soaking times for good penetration of the materials.  The thicker and denser the material, the longer required to get full disinfection.  Test your disinfection solution on a small area first as some solutions, particularly bleach, will damage some items. 

  • Hot water – use water above 140°F
    • Soak items for at least 40 minutes, keeping the water very hot.
  • Dishwashing detergent and hot water – use a 5% solution (6.5 oz. mixed with water to make 1 gallon)
    • Soak items for at least 40 minutes, keeping the water above 120°F.

A portable disinfection kit might include

  • Medium to large size trash can or plastic bin for soaking wading boots
  • Stiff bristle brush for scrubbing
  • Spray bottles or pump sprayer for disinfection with your choice of disinfection solution, and for rinsing


What should I do if I think I’ve found didymo?

  • Refer to the ANR Didymo Identification Guide (pdf, 51 KB) to validate your suspicions.
  • Collect a dime- to quarter-sized sample in a small container, zip-lock bag, or folded in a business card (a dry sample can be positively identified).
  • If possible, obtain GPS coordinates or provide a clearly marked map indicating where the sample was collected, or provide a detailed description so that Agency personnel can return to the site for follow-up.
  • Fill out a sample submission form (pdf, 23 KB).
  • Mail sample, map if available, and sample submission form, to the address provided on the form.  ANR will confirm receipt of your sample by mail.

Where can I get more information on didymo?



Updated: May 2013

VT DEC Watershed Management Division 1 National Life Drive, Main 2  Montpelier, VT  05620-3522  Tele: 802-828-1535   Fax: 802-828-1544

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