Need help to ID mysterious foliar disease (pics attached)

[Eric02476]

I'm familiar with most tomato diseases, but am really stumped by this problem that's spreading on some of my young tomatos. I really want to know what I'm dealing with so I can take appropriate action.

The only symptom is uniform browning and necrosis between the veins of the leaves. It starts off as several small patches but they grow and eventually the leaf dies. The brown/dead tissue always stays neatly within the veins.

It spreads from older leaves to younger leaves fairly quickly, within 2 or 3 days. The damage tends to be all over the leaf, and its systematic pattern makes me doubt a pest is responsible, at least nothing that's feeding.

Of the 3 pics attached, the single leaf photo is fairly high resolution and allows for close inspection. The other two show branches at intermediate and advanced symptom stages.

I'm in the Northeast (Boston area).

I'd really appreciate any help - this doesn't look like any fungal or bacterial disease I know of, and I don't think it's a nutritional problem.

Eric