To keep your tomatoes healthy, thriving, and fungus-free, you can apply fungicide before any symptoms appear, and then continue treatment throughout the season.
Tomato fungi (early blight, late blight, and Septoria leaf spot) overwinter in the soil. They spread easily onto plants as water splashes up on leaves and stems, distributing pathogens. While these diseases cannot be cured once they attack a plant, they can be controlled.
Better yet, you can prevent them. To give your tomato plants a greater chance of resisting fungi, set up an anti-fungal treatment program and implement it early in the season. You can continue to treat plants to continue to prevent later fungal infection and to minimize any damage caused by funguses that chance to take hold.
The three most common types of tomato blight in tomatoes – early blight, late blight, and Septoria leaf spot –can be treated and controlled similarly with fungicides and by practicing good preventative care. These are the most effective fungicide treatments for tomato plants.
A biological fungicide is made up of beneficial bacteria or fungi that combat a pathogen – in this case, tomato fungus. Of the biological fungicides embraced by tomato gardeners, Serenade makes the short list. Its active ingredient is a bacterium, Bacillus subtilis, that helps suppress fungi and other pathogens on tomato plants. One significant advantage of using Serenade is that it allows you to harvest and eat fruits and vegetables the same day they are treated. In addition, the product does not pose a risk to bees and beneficial insects.
Copper fungicides are available in a variety of formulations. They are effective in preventing and treating both fungi and bacterial pathogens, which makes these products very valuable to the the home gardener. As you consider various copper fungicide products, be sure to check the ingredients. A copper sulfate formulation, on its own, can be more damaging to plants than a Bordeaux mixture, which combines copper sulfate with lime, neutralizing the acidic copper sulfate and reducing plant damage. One word of caution: young plants are particularly sensitive to copper, so be sure to dilute copper fungicides as you apply them early in the season.
Active ingredient chlorothalonil is the most recommended chemical for us on tomato fungus. It can be applied until the day before you pick tomatoes, which is a clear indication of its low toxicity. Chlorothalonil can be used as soon as tomato plants are subjected to humid or rainy conditions that can cause blight. Find it as a premixed product or in concentrated form to be mixed with water. Look for chlorothalonil under brand names Bravo, Echo, and Daconil. (Here's a selection.)
Other chemical fungicides available for home use are mancozeb and maneb, both requiring five days’ wait after application before harvesting.
How do I know if I should use fungicide on my tomato plants?
If you know your garden (or your neighbor’s garden) has a history of early blight, late blight, or Septoria leaf spot, then your plants are vulnerable. Use a spray program.
When should I start applying fungicide?
Begin before symptoms appear, especially if your plants have had tomato fungus in the past or if you live in warm, humid areas where early blight, late blight, and Septoria leaf spot thrive. You can begin as soon as you set plants in the garden.
What parts of the plant should I spray?
Spray upper and lower leaf surfaces, stems, and flowers.
How much should I spray?
Thorough coverage is essential. Spray to the point of runoff.
How often should I apply fungicide?
Spray at 7 to 14 day intervals. Re-apply after rain.
More about Tomato Fungus and Tomato Blight