The key to battling any fungal disease is prevention; plants need to be treated with a fungicide before any signs of damage. As an organic gardener, I reach for two products. The first is a copper-based fungicide, which is what I sprayed on my tomatoes at first but reticently. Even though copper is organic, it can negatively affect the environment, most notably by killing aquatic life. Keep it away from streams and ponds and follow the label instructions.
The other product is Bacillus subtillis, a beneficial bacterium that is sold under the brand name Serenade. There are other strains under different brand names. It's completely safe for the environment, it doesn't affect beneficial insects and it lets tomatoes be harvested the same day of spraying. But it needs to be applied often, every four to seven days.
Bacillus subtillis attacks the fungal disease pathogens, prevents spores from germinating and fights off late blight, hopefully, but the jury is still out on its effectiveness. That's one of the dilemmas of being an organic gardener -- trying to save the tomatoes and the environment.
Chemical gardeners have a host of products that will fend off blight. I would advise using the one that is the least toxic. Be sure to follow all safety precautions when applying and harvesting tomatoes. Whatever you use, it's more cost-effective and easier to buy a concentrate that's mixed and applied with a sprayer, especially if you have more than a couple of plants.
Peace
