Mulching tomato plants is very beneficial and often discussed over on the Growing Tomatoes forum here. Straw is one of most frequently recommended mulches to use.

Mulching can help prevent many common problems with tomato plants but unfortunately Late Blight isn't one of them as the fungus that causes the disease is air borne.

Still by encouraging over all better health in the plant, it does improve its resistance. In your zone you'll want to be careful about applying the mulch only after the soil temperatures have risen well into the 70 degree range.

Search 'mulching' here for many discussions on its benefits and suggested material to use.

Dave

Yes. I use mulch. It helps to prevent soil borne diseases from splashing up onto the plants. I also remove the lower branches on all of my tomatoes so that there aren't any leaves/branches that touch or come really close to the soil. Again, just to help prevent soil borne disease from getting onto the foliage. If for nothing else, mulch is great to help maintain the moisture in the soil.

Here is a link that might be useful: My Garden

Thanks for the info guys. Dave, thanks for the reminder about the tomato forum, I had forgotten about it. I'm heading over there now. Thanks. Marg