Answer: I'm not sure what the problem might be since it sounds as though they get off to a good start. In general, Tomatoes like a nice warm area in full sun, and need at least 8 hours of sunlight a day, or they get spindly and produce little mature fruit. Choose cool season or short season varieties (your summertime weather is cool compared to the rest of the country!) and choose varieties with the most resistance to diseases. To tell what diseases a hybrid can withstand, look at the letters after its name on the plant tag. For example, VFFNTA means the plant is resistant to Verticillium wilt, Fusarium races 1 and 2, root knot Nematodes, Tobacco mosaic virus, and Alternaria stem canker. A plant marked VFFNTA would be a good choice for you to try if diseases have killed your tomatoes in the past.
When watering, always keep the water towards the base of the plant, and try and keep the leaves dry. Tomatoes can become infected when airborne spores land on wet plants, so never use an overhead sprinkler. Obviously if it rains you can't do anything about it, but try not to unnecessarily get the plants wet.
Water regularly but allow the soil to dry a bit between waterings. You want tomatoes to have a regular available water source without keeping them soggy. And, use a mulch over the soil to help regulate water evaporation and keep fungal spores from splashing up onto the leaves.
I hope this information helps you grow healthy, productive tomato plants!
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