The Urban Pantheist

For the last two years this weird thing has been happening in a few patches of the lawn. From eye level it looks like someone spilled a dark gray milkshake on the grass ("grass" ha ha our yard is mostly weeds). Get down to where I like to look at things and you can see the structure of it. Rather than a fungus, which lives in what feeds on, this looks like something just resting on the surface of the leaves. Some of the affected plants look a little weird and dried-out but not like they're being destroyed or fed on. Our best guess is that this is the final stage of a slime mold, but we haven't seen it before we started seeing it in our yard.

EDITED TO ADD: well I posted this about an hour ago and since then have identified this as the turfgrass slime mold Physarum cinereum. There isn't much information out there, except that its habitat seems to be mowed lawns, and it is very widely distributed. Wikipedia's dumb entry describes it as a "pathogen" while everyone else points out that it doesn't harm the plants upon which it grows. Like all slime molds, the organism crawls across the surface of stuff, consuming bacteria and other little edible morsels.



A close-up of the growth on a leaf of narrowleaf plantain.

EDITED TO ADD: This pdf from Texas A&M has more information, including the assertion that the slime mold may cause damage to plants simply by blocking the amount of light that reaches the leaves. That's probably why the plants in these photos look a little stressed or withered.