CURWOOD: It’s Living on Earth. I’m Steve Curwood. Coastal wetlands help filter water and provide some of nature’s best protection from storms. But in parts of the Northeast coast of the U.S., something murky is going on in wetland salt marshes and scientists are perplexed. Once healthy green marsh grasses are now mottled with patches of brown stalks. Researchers call it Sudden Wetland Dieback and it’s happening from Maine to New York. Living on Earth’s Ashley Ahearn went to the wetlands to investigate.

AHEARN: The Wellfleet Salt marsh on Cape Cod Massachusetts stretches for acres. Green grasses sway gently above dark brown muck, tapering off into blue ocean. From a distance, it’s a painter’s paradise, the picture of wetland health.

AHEARN: But walk a hundred yards or so into the marsh, and the scene is a bit different.

AHEARN: Steve Smith is a plant ecologist for the Cape Cod National Seashore. He’s crouched down in the muck plucking at the dead stalks of what used to be green marsh grass.

SMITH: I mean this amazes me. Look at these plants just barely hanging on. There’s no soil left around their root mass. It’s just gone, all that is gone.

SMITH: That’s what’s perplexed us because this pattern of dieback does not correlate in any obvious way with the typical stressors on plant physiology and salt marsh plant vigor.

AHEARN: Stressors, like pollution, drought, flooding or ice damage, have been ruled out. But for every killer scientists cross off the list, another presents itself.

AHEARN: In a wetland a state away, plant pathologist Wade Elmer of the Connecticut Agricultural Experiment Station, takes some samples of sick plants at Hammonasset State Park.

ELMER: We’re the CSI of the plant world here.

AHEARN: Elmer specializes in a plant-eating family of fungus called Fusarium, which he thinks is causing lesions on the tips of the marshgrass.

ELMER: This is of interest to me because there’s a lot of death on there and if you look at how the sun shows through the leaf here you can see all the little lesions beginning to form there, and this is where I isolate a lot of Fusarium.

AHEARN: But this isn’t your day to day crop-devouring variety of the fungus. This is a brand new strain of Fusarium and Elmer’s got some good evidence that it might be part of what’s killing the Northeastern wetlands.

ELMER: What I find is that plants that are showing tip dieback, when I take them back to the lab and isolate them about 80% of the pieces that I put onto an auger plate will give rise to a Fusarium colony, suggesting that there’s a lot of Fusarium on this dying tip back.

AHEARN: Elmer’s colleague, Jim LaMondia, searches for clues in the marsh as well, but he’s working from the bottom up. In a metal pail, he rinses muck off a clod of marsh grass roots in search of nematodes.

LaMONDIA: A nematode is a plant parasitic round worm. They typically have stylettes that they stab the plant cells with and suck cell contents out and they cause the plants to be stunted and do poorly.

AHEARN: In the Hammonasset marsh, LaMondia has found a type of root-knot Nematode that only lives in wetlands. As he washes the mud away from the marsh grass sample, he looks for tiny white pustules, or galls, full of Nematode eggs.

LaMONDIA: There’s one right there! Ok what we have here is a sort of an egg shaped gall at the end of the root system which will have probably I’d guess maybe a dozen nematodes in it once we dissect this out under the microscope.

LaMONDIA: Exactly, it’s a feeding site for them and they develop and produce hundreds of eggs each and it spreads from there.

AHEARN: LaMondia has found high concentrations of nematodes in these grasses. He suspects that the parasitic root-eating worms may be teaming up with Wade Elmer’s leaf-eating fungus, to kill the marsh grass from root to tip.

He and Elmer aren’t the only ones following up on their hunches. Another scientist in Rhode Island plans to study nocturnal crabs that might also see marsh grass as a tasty treat.

Steve Smith, of the Cape Cod National Seashore, says scientists are just starting to piece together what little evidence they have to solve this intricate ecological puzzle.

SMITH: It’s this big complex web of interactions among environmental, physical, chemical, hydrological factors with biological and perhaps pathogenic factors that is difficult to untangle.

AHEARN: And with hurricane frequency and intensity predicted to increase in the Northeast, scientists here know there’s much to be lost if more isn’t learned about what’s causing sudden wetland dieback.

For Living on Earth, I’m Ashley Ahearn in Wellfleet, MA.