How to Choose and Apply Fungicide to Tomato Plants: The Basics (2024)

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By Kathy Widenhouse, award-winning author of 9 books, content writer, website publisher, and owner of Tomato Dirt, the web's leading online source of for growing tomatoes in the home garden.

To keep your tomatoes healthy, thriving, and fungus-free,you can apply fungicide before any symptoms appear, and then continue treatmentthroughout the season.

Tomato fungi (early blight, late blight, and Septoria leafspot) overwinter in the soil. They spread easily onto plants as water splashesup on leaves and stems, distributing pathogens. While these diseases cannot becured once they attack a plant, they can be controlled.

Better yet, you can prevent them. To give your tomato plantsa greater chance of resisting fungi, set up an anti-fungal treatment programand implement it early in the season. You can continue to treat plants tocontinue to prevent later fungal infection and to minimize any damage caused byfunguses that chance to take hold.

Choose a Tomato Fungicide

The three most common types of tomato blight in tomatoes –early blight, late blight, and Septoria leaf spot –can be treated andcontrolled similarly with fungicides and by practicing good preventative care. Theseare the most effective fungicide treatments for tomato plants.

Organic Fungicides

Biological Fungicide

A biological fungicide is made up of beneficial bacteria orfungi that combat a pathogen – in this case, tomato fungus. Of the biologicalfungicides embraced by tomato gardeners, Serenade makes the short list. Itsactive ingredient is a bacterium, Bacillussubtilis, that helps suppress fungi and other pathogens on tomato plants. Onesignificant advantage of using Serenade is that it allows you to harvest andeat fruits and vegetables the same day they are treated. In addition, theproduct does not pose a risk to bees and beneficial insects.

Copper Fungicides

Copper fungicides are available in a variety of formulations.They are effective in preventing and treating both fungi and bacterialpathogens, which makes these products very valuable to the the home gardener. Asyou consider various copper fungicide products, be sure to check theingredients. A copper sulfate formulation, on its own, can be more damaging toplants than a Bordeaux mixture, which combines copper sulfate with lime, neutralizingthe acidic copper sulfate and reducing plant damage. One word of caution: youngplants are particularly sensitive to copper, so be sure to dilute copperfungicides as you apply them early in the season.

Chemical Fungicides

Active ingredient chlorothalonil is the most recommended chemical for us on tomato fungus. It can be applied until the day before you picktomatoes, which is a clear indication of its low toxicity. Chlorothalonil canbe used as soon as tomato plants are subjected to humid or rainy conditions thatcan cause blight. Find it as a premixed product or in concentrated form to bemixed with water. Look for chlorothalonil under brand names Bravo, Echo, andDaconil. (Here's a selection.)

Other chemical fungicides available for home use aremancozeb and maneb, both requiring five days’ wait after application beforeharvesting.

Follow Basic Steps toApply Fungicide

When using all fungicides, follow product instructions. Differentproducts are applied in different ways: as dust powders, in liquid spray, or aswettable powders.
Always wear work gloves and protective eyewear as you apply fungicide.
The lowest labeled rate of protectant fungicide, when usedearly in the season and then adjusted until tomato plants are finishedproducing, will provide protection and thus prevent a tomato fungus epidemic.
Choose a garden sprayer and designate it for fungicide useonly.

Applying Fungicide:FAQs

How do I know if Ishould use fungicide on my tomato plants?

If you know your garden (or your neighbor’s garden) has ahistory of early blight, late blight, or Septoria leaf spot, then your plantsare vulnerable. Use a spray program.

When should I startapplying fungicide?

Begin before symptoms appear, especially if your plants havehad tomato fungus in the past or if you live in warm, humid areas where earlyblight, late blight, and Septoria leaf spot thrive. You can begin as soon asyou set plants in the garden.

What parts of theplant should I spray?

Spray upper and lower leaf surfaces, stems, and flowers.

How much should Ispray?

Thorough coverage is essential. Spray to the point ofrunoff.

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