The season's barely begun, yet some of my tomato plants are already
showing signs of disease. I'm wondering if there's anything that can
be done about it, such as cutting off the diseased limbs or applying a
fungicide or something.

A photo of some diseased Sun Gold tomato leaves (with a backdrop of
nasturtiums) can be seen at
http://www.inhouzemusic.com/zzz/tomato.html

Thanks.