Growing home grown tomatoes is not without it’s troubles. In the hot humid south diseases can be very difficult to overcome The best action is to prevent disease before it starts. Plant in full sun with lots of air circulation around your plants. Tomato plants need to be 3 to 4 feet apart from each other and away from walls and other plants. Air circulation will help to keep the leaves drier and cut down on the spread of diseases such a fungus. A fungus blight can not spread when the leaves are dry. Of course proper fertilization is important because too much nitrogen will cause excessive growth and lead to more disease. Mulching around the base of the plant with dried leaves or wood chips will keep water from splashing soil particles on the leaves that can carry disease. Always water tomato plants at the base of the plant and avoid getting the leaves wet. Pruning out some of the side shoots or suckers will cut down on the thick growth that keeps interior leaves wet longer there by increasing air circulation around leaves to dry them quickly after a rain or heavy dew.
Even after you have observed all these cultural practices you can still have disease problems if we are having periods of rain and heavy dews that keep your leaves wet. During these times when the environmental conditions are in favor of the fungus spreading you can use some organic sprays to help prevent the problem.
The things that I have found that work best for the home gardener are.
Organic copper soap
Potassium bicarbonate ( also sold as Green cure )
Serenade ( a biological control)
I recommend alternating these organic remedies every 7 days when diseases are present.
Pruning off leaves with black spots when they first appear is also helpful.
Check your plants daily and act at the first sign of disease