Everything I mulched with straw died

I planted some tomatoes and peppers and then piled straw around them.  One row I only got half way through.  Everything that has straw has died but the rest of the row without straw is fine.  Any idea why?  The plants look like they got burnt, they are black and shrivelled.  The onions that I mulched are fine though.  I wanted to do the peas tomorrow but now I'm not sure.

How much time elapsed between mulching and shrivelling ?
And: Where did you get the straw from ?

I planted and mulched a week ago and when I checked 2 days ago they were fine. 

The straw came from a nearby farmer.

It was rainy last week but this week has been quite hot.

how much of the straw is in direct contact with the stems. move some out of the way and take a peek at the plants. pull one up gently to look at the roots if you have to.

Sounds like your farmer source is using Round Up, or other such herbicide.  The rains probably washed it into the soil.  Ouch!

It almost sounds like you had a freeze at some point.  For some reason, we had a similar episode here.  We planted tomatoes, and put straw around them, almost over them but not quite, and they all shriveled after it got down to 31 degrees F one night, the funny thing is, one that was totally uncovered was fine.
Tomatoes when frozen shrivel up and blacken.

Not that I am saying there was no possibility of some sort of contamination, but it sure sounds like they got hit with a freeze.

I think that is it.  As I'm looking outside I can see a light dusting of frost on the grass in places.  Wierd, I would have thought the straw would insulate the plant.

Mulching can prevent the soil from heating up. Generally there are two times when you don't want to mulch. When you want seeds to germinate, since they often want some light and warmth and in early spring when you want the soil to heat up. What you could do is mulch with something dark. I saw a video of a garden somewhere and in the fall they mulched half with something light (straw) and the other half with something dark (don't remember what it was) and when spring came the dark half melted the snow significantly faster.

Also you can think of it as when the soil is warm (as in summer) you are insulating to keep the warmth in, and when the soil is cold the effect is the opposite. Just wanted to clarify.

i'm thinking that it's possible that the area that you mulched with the straw may have been too wet... you may of had a bacterial or fungal infection of your plants... also.. if the mulch was too thick it's possible to create an anaerobic condition within the soil where the soils organisms can't breath... this isn't good for the plants either.. i usually keep the mulch away from the plants until they are big enough.. and even then I try to keep the mulch from directly touching the stem of the plant... if you want some good resources on mulch gardening try reading ruth stouts books or lasagna gardening or something..

Riki wrote:
I think that is it.  As I'm looking outside I can see a light dusting of frost on the grass in places.  Wierd, I would have thought the straw would insulate the plant.

Yes it does, and that can sometimes cause a problem with frosts.

Imagine that the soil is a big heat sink. Now pile it with straw so that the heat cannot protect the TOP of the plant from the cold night air: the top dies but the base lives.

With the unmulched plants the heat radiates upwards from the warmer soil. This might protect the plants from a light frost.

I wonder if the part of your tomato stems that are buried in the straw are still alive, since the cold air would have trouble getting into the insulating straw? You might check the tomatos again in a couple of days to see if the part of the stem buried in the straw is showing signs of life.

Here in Kansas we got a frost also, and I cannot tell yet if my tomatos were nipped back or killed: time will tell.

I have found a mulch usefull when it is hot out, because the mulch protects the cooler soil from the hot sun. But, very early in the spring the bare soil is sometimes better.

could have been frost, or possibly the roundup, but if it was too wet it could have been mold, disease or wilt

An update on my own frost-nipped tomatos: they DID survive! A few of the other vegetables are dead and I will replant them.