We planted several heirloom tomatoes that are growing like gangbusters - almost 6 feet tall already! But 2 of the plants in particular started to brown around the edges as if they'd been burnt with a match..then the leaves turned yellow, and then the entire branch of leaves... There are some long (1/2-1 inch) oblong spots on the stems, so I'm thinking it might be some type of speck? But there are no real spots anywhere else - just burned edges, and then dead branches. The fruit seems unaffected. I'm in Los Angeles, so it's hot and dry, and we've been watering every 3 days or so... I'm a gardening newbie, so any help is appreciated.