Early detection of didymo using DNA tool offers hope of control

6 August 2007
Scientists at the University of Waikato have developed a new genetic testing tool for early detection and surveillance of the invasive freshwater-borne pest, didymo, commonly known as "rock snot".

The highly sensitive DNA test, developed by Professor Craig Cary, Dr Brendan Hicks and colleagues in the School of Science and Engineering's Biological Sciences Department, enables early detection of didymo in waterways and lakes before new infestations are visible.

"Developments in molecular technology now allow rapid and specific low-level detection of algal species," Dr Cary told a recent MAF Biosecurity New Zealand didymo science seminar.

Didymo is a single-celled micro-organism that can spread from one river or lake to another by the movement of water, equipment, clothing, vehicles or other damp items.

It can form massive blooms on the bottom of streams and rivers, and sometimes lakes, attaching itself by stalks which can develop into a thick brown layer that smothers rocks, submerged plants and other materials.

Historically didymo has been found in the cool waters of northern Europe and northern North America, but since the mid 1980s, has become an invasive species in new areas outside its original habitat. It out-competes native algal species, forming unsightly biomass which clogs water intakes and reduces recreational enjoyment of freshwater ecosystems.

The newly developed diagnostic tool relies on gene amplification technologies that allow more rapid throughput of samples, and greater accuracy than traditional microscopy. It can detect didymo at extremely low levels (<1 cell/ml).

"If we can detect didymo early here in the North Island, we have a much better chance of preventing its spread and even possibly eradicating it using a chemical control tool currently being developed."

"The DNA test involves a highly sensitive quantitative protocol designed specifically for detecting and counting didymo cells in stream water. It's like looking for a needle in a haystack at this level of detection, and we just developed a tool that can quickly find the needle," Dr Cary said.

Dr Cary said field trials in New Zealand and other global sites had demonstrated the efficacy of the didymo DNA test.

NIWA has been contracted by MAF Biosecurity NZ, under the same science programme as Professor Cary's, to investigate potential didymo control tools.