Q: I have had a rain barrel for the last several years and have also had blight on my tomatoes. Do you think there is a correlation between these two things? This year I bought all new soil and potting bags and still haven't had tomatoes without disease.
A: I don't think the collected rainwater is your source of trouble. Both early blight and Septoria leaf spot are becoming so common that tomatoes anywhere and everywhere are at risk of getting one or the other of these fungal diseases (or both).
The main ways these diseases spread is by wind and on the wings of birds and insects moving from infected plants to uninfected ones. You might also get blight from transplants and/or seeds that are already infected.
Once you've got Septoria or early blight, the spores overwinter in the soil -- mainly from left-over bits of tomatoes, weeds and other nightshade-family plants that can host these diseases (i.e. potatoes, peppers and eggplant).
The diseases really thrive in wet weather like we had this spring. What usually happens is that rain or hose water splashes overwintered, soil-borne disease spores onto the stems and lower leaves. The diseases then work their way up the plants.
If you're growing in containers, changing the soil and disinfecting the containers each year can help head off trouble. Otherwise, spraying a fungicide such as copper, Serenade or the chemical chlorothalonil (Daconil) every 7-10 days is the prime way to overcome tomato blights.
Also helpful is cleaning out as much of any infected plant refuse as possible at the end of the season. And a fresh coating of straw mulch after planting each spring can also slow down the splash effect.
Keep using your rainwater. It's actually better than treated public water and cold water from wells.