PlantLife Online

Answer

Unfortunately there is no cure all soil application for blight once a garden is infected. There are two common blights when talking about tomatoes "early" and "late". The term late blight in general refers to a Phytophthora species which is natural occurring fungal-like pathogen that survives in soil or on plant tissue. In the case of tomatoes and other Solanaceae late blight often survives on living tissue so the best defense is to properly clean and dispose of blight infected plants from the previous season. This includes the under ground portions of the plant, potatoes that survived the winter are often the culprit. Early blight of tomato is caused by the fungus Alternaria solani and is common in over crowded beds with poor circulation. If you take the steps to get rid of plant waste and be sure your compost is fully composted each season you should be able to avoid blight out breaks. I would not recommend composting infected plants. The following tips will help prevent the spread of blight as well as many other common crop diseases. 

1. 1. Use only clean seed saved from disease free plants.
2. 2. Use blight resistant or tolerant varieties.
3. 3. Remove and destroy crop residue at the end of the season.
4. 4. Practice crop rotation to non-susceptible crops (3 years). Be sure to control volunteers and susceptible weeds.
5. 5. Promote good air circulation by proper spacing of plants.
6. 6. Irrigate early in the day to promote rapid drying of foliage.
7. 7. Healthy plants with adequate nutrition are less susceptible to the disease.
8. 8. Minimize plant injury and the spread of spores by controlling insect feeding.
9. 9. Hand picking diseased foliage may slow the rate of disease spread but should not be relied on for control. Do not work in a wet garden.
10.  
11. And last resort use preventative fungicide, there are many on the market they need to be sprayed often and consistently throughout the season and can be a lot of work. Always follow the label when using pesticides. Here are a few: Daconil, Bravo, Echo, Fungonil, Bordeaux Mixture, Kocide, Tenn-Cop,Liqui-cop,  Basicop, Camelot