Yes, it looks like a fungus.

But why are you calling it a pathogen? What plants, if any, are sick and/or dying?

Beyond that, it could be due to the large quantity of compost you mixed into the bed.

Well I guess I'm using the term a bit generically. I called it a pathogen because the bed had a progressive crop of lettuce last year but didn't do so well as years before - and this the year this fungus arrived, and the first year I gave it compost.

Also the soil is sort of glued together by it, about 10 inches deep over a quarter of the bed - can't be very good for what's growing up above. And this is after most of the compost has pretty much disappeared (although it could be a remnant of what was growing when the compost was fresh). In fact, it sort of has a dry look like it's not really alive anymore.

Maybe you're right though.
I have used this compost several times before, in similar quantities, with no problems, but it could have come in with it. Especially if there was too much water.

I'm just wondering if it's anything to worry about because although I'm a pretty experienced vegetable gardener I've never seen this type of thing before.
Are there such things as aggressive fungal invasions that are hard to get rid of?

Isn't this the beneficial mycorrhizal fungi?

That is most likely a fungus but whether it is a pathogenic or beneficial fungus is difficult to tell although the beneficial forms (just like insects) are motre prevelant than the pathogenic forms. That lettuce died means little because that could be from a number of causes.
Waht did a soil test say about this plot?

mmlm, not mycorrhizae which form associative relationships within or surrounding specific plants' roots.

Soil test? I didn't go that far I'm afraid. If it would be easy I would take a sample to a pathologist and have it analyzed. But that would make my lettuce pretty expensive. It's a bit more than I'm up for I think.

Again, the thing that's weird is that the amount of this beast is so prevalent that there are football (volleyball?) sized clods that you can dig up with a shovel - all glued together with the fungus.

There are a great many soil fungi that are decomposers - feeding off of soil organic matter. They are essential for cycling soil nutrients. If the fungus is growing in the soil, it likely is feeding off of something there rather than attacking plants. Once the food source is gone it will "disappear".

If your lettuce did poorly, I am sure it is not this fungus and we should focus on what is going on with the lettuce.

The soil test kimmsr refers to is a soil chemistry test which tells you the pH and several plant nutrient levels. It may be helpful, but I would like to hear about the lettuce symptoms first. Have you looked at the roots? Lettuce is susceptible to root-knot nematode which is microscopic, but causes the roots to become distorted and form big, gnarly knots.

Ok, thanks botanybob. The lettuce isn't around anymore so I can't tell you too much about it, but I think it may have suffered from two factors.

As I wrote, I am in a community garden that I've heard was developed over an old gravel parking lot many years ago. I think that when I deep dug it I probably drudged up some of the parking lot, or something not so savory at any rate. I remember thinking it was a bit mucky clay-like at the bottom. It was probably a mistake to go that deep.

The other thing is that it appeared that some of the lettuce got some kind of a damp-off type infection - it may have been from growing them too often in the same place, likewise if it was a nematode. I'll have to check on the roots and see what I can figure out. I'm glad you mentioned that as I didn't know about that one.

Do people typically rotate lettuce crops for this reason?

As for the fungus, after thinking about it for a while and absorbing the comments above, I think it probably has had it's feeding frenzy and moved on, so to speak - except that I just dumped more compost in there because last season's load had more or less decomposed into oblivion. But what's in the photos above is pretty dry, so I think it's just the remnant integument of its better days.

You wrote:
"Also the soil is sort of glued together by it, about 10 inches deep over a quarter of the bed - can't be very good for what's growing up above."

First a question: Is the fungal layer 10 inches deep, as in from top to bottom? Or is the layer of stuff 10 inches below the soil surface?

It's true the layer isn't good for any plants, this because the large amount of fungus is hydrophobic -- water won't penetrate unless you either remove the fungal mat or break it up.

The fungal layer is from the surface to 10" deep, from top to bottom.
Yes, I think I'll have to break it up. At first, when I went to add the new compost this season, and not knowing what I was dealing with, I pulled out most of the clumped fungus clods and put them to the side. But now that I have new compost in there I'll try to monitor it. I'm sure if this organism likes compost it'll probably come to life again.

As a test, I've actually taken a small clump and put it in a cup with fresh potting mix to see what would happen, but nothing seems to be going on so far.

Thanks for the info.

You wrote:
"As a test, I've actually taken a small clump and put it in a cup with fresh potting mix to see what would happen, but nothing seems to be going on so far."

And it probably won't until the required conditions of temperature and amount of moisture are just right for the fungus.

Because you already set the clumps aside, it might be a good idea to compost it right there. Add some green to the pile, mix it up, see what happens, then tell us how well it went.

I'm quite curious about your situation because I recently saw a similar soil situation. In this case, near where a very large maple had been cut down years ago, a major fungal growth occurred shortly after a client planted a young tree about a year ago.

I don't know for certain, but perhaps the ready supply of dead organic matter (the tree roots), plus aeration from digging and later, plus water given to the tree, triggered the growth. If so, I suspect the same thing happened in your soil.

The fungal mat in this case is currently about 1 1/2 inches thick over 50 square feet. As a result, the tree died. The client says the fungus killed it. But I suspect that occurred indirectly; the tree died not from a disease but because "all the water ran off." In other words, water can't penetrate the dense hydrophobic fungal mat until it breaks down.

I'm reading in my "Golden Gate Gardening" book by Pam Pierce . . .

"A fairly serious disease, lettuce drop, causes plants to collapse into a slimy mess. If this happens, check the base for white fungal mats and pea-sized, black resting bodies. Remove affected plants and as much of the debris as you can to halt spread of the disease. Soil solarization will control this disease."

I didn't know to check for the black resting bodies, but it seems that this might be what I have as I've only grown lettuce in this bed, and a lot of it. And now that I think back, I had a few heads collapse like that, and now two in another raised bed. I'll have to try the solarization.

It sounds like the book was talking about Sclerotium. So I have my doubts about the effectiveness of solarization.

Oops. I need to correct that fungus name. It should be Sclerotinia.

There actually is some evidence that solarization works for Sclerotium. See the link at the bottom of this post. However I doubt it would work in San Francisco - I just don't think I could get the soil hot enough.

But it turns out that there is a German company that has developed a biologial control using the fungus Coniothyrium minitans, which parasitizes and kills the sclerotia in the soil. It's sold as "Contans WG" in the U.S. and "Prophyta Biologischer Pflanzenschutz GmbH" in Germany. I contacted Sylvan Bioproducts in the U.S. and arranged to buy a pound of the stuff.

I can't say definitively that the picture I posted is Sclerotinia - it's possible that there are two fungi sp. in this bed - but I'm pretty sure I have Sclerotinia judging by the specific lettuce drop problems I've been having. I don't know which species I have, whether it's Sclerotinia sclerotiorum and Sclerotinia Minor, but the Contans works on both. I did have one instance of apothecia - the mushroom like structure which the Sclerotinia sclerotiorum sp produces - so perhaps that's the one I have.

This'll be an interesting little experiment.

Sylvan's url is http://www.sylvanbio.com/

Here is a link that might be useful: THE ROLE OF TEMPERATURE REGARDING SCLEROTINIA SCLEROTIORUM IN THE SOIL SOLARIZATION METHOD

I'm reporting back with a follow up.

I bought the Contans from Sylvan Bioproducts and applied it as a water mixture using a garden sprayer.

It seems to have worked. I've grown maybe forty/fifty heads of lettuce in the same raised bed, and only lost one or two. I think I'll have to reapply it for a while, or periodically until the Sclerotinia is gone, if that's actually an achievable goal. I don't think I'll necessarily be able to kill the schlerotia deep in the soil.

But for now I feel the problem is solved.

Here is a link that might be useful: Sylvan Bioproducts