What is wrong with this tomato plant? The yellow with black spots has been spreading through the plant. I am hoping someone knows exactly what this is and how I can treat it or prevent it.
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If this is early in the growing season, it's a fungal disease called Early Blight. Often happens after cool rainfall.
Remove all affected leaves and dispose of in the trash (usually starts in lower leaves and moves up the plant). Wash yourself down so you don't spread the fungus spores. If this has been let go too far, remove the complete plant so it doesn't spread.
Spray plants with copper or other fungicidal spray. Treat as soon as it happens, the fungus can quickly become resistant.
For future prevention, always completely remove tomato plant waste at the end of the growing season. Burn or remove from property. Use crop rotation to grow fungal resistant crops two or three years where tomatoes have been planted, plant tomatoes in clean soil.
Plastic film mulch can be used to keep weeds down, promote plant growth (red) and also seals the fungus down in the soil so it cannot reach the leaves and stems through air movement or rain splash.
Also water with drip or hose, do not use any sort of spray irrigation that gets the leaves wet or splashes soil.