This could be septoria leaf spot but it could also be Early Blight. Both are fungal. Early blight typically starts at the bottom of the plant and works up. Both are fungal. Septoria leaf spot usually begins in damp, cool weather and one strategy to beat it is to plant your tomatoes LATER in the season when it’s warmer. Even if you solorize your soil you won’t get rid of either of these. You must learn to cope with them as follows.
The way we “dance with tomato fungi” is this: When planting, remove any leaves that touch the soil so that the only place the plant makes contact with dirt is at the stem. Mulch around the plants immediately with the mulching material of your choice. That is immediately after planting so that any spores in the soil won’t be able to get splashed on your plants when you water or it rains. Spray weekly with one of the bacteria-based, organic fungicides. Ask at your local garden center to see which ones they carry. Use these every week according to directions. Pick off the worst of the spotted leaves as they happen and throw away. Don’t let any dropped foliage or fruit remain in the garden. Water only in the morning and deeply less often – watering every four or five days deeply will allow the foliage to be dry most of the time.
The most early blight resistant varieties are Mountain Magic and Mountain Merit. Heirloom tomatoes always get diseased before hybrids, so look for hybrid plants.
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