How to Use Sulfur Fungicide on Plants

A good sulfur fungicide can be used for everything from spider mites to treating powdery mildew. In this complete guide, we discuss how it works, what it works on, and where you shouldn't use it.

A shot of a person in the garden spraying her plants with a mixture of sulfur fungicide

Contents

Fungicides are designed to stop certain pests and diseases from destroying your garden. Sulfur fungicide in particular can reduce or prevent Alternaria leaf spot, powdery mildew, rust, chiggers, and many other problems that might show up in the garden.

However, you need to know a few things before you douse your plants in sulfur fungicides. For starters, those who live in hot climates shouldn’t apply it when the temperature exceeds 85°F (29°C), or it will fry your plants! Using it a couple of weeks before or after applying oils such as neem oil will also lead to fried plants. There are also a few safety concerns to be aware of that we’ll get into. But, the good news: it’s generally believed to be non-toxic to honeybees!

While the downsides may seem like a lot to deal with, especially if your summers consist of triple-digit temperatures, it’s not all bad. When you use it as intended, it will do you a world of good.

What Is Sulfur Fungicide?

A shot of a yellow powder repellent for plants placed on top of a white surface with a white background.
The option is commonly available as a fine powder.

Sulfur has been used to treat diseases such as powdery mildew for years. It’s said that Homer, the Greek author, mentioned the use of sulfur as a pest control method as early as 1000 BCE. This isn’t just “Grandma’s old gardening tip” we’re talking about!

Today, sulfur is most commonly available as a fine powder. It is sprinkled directly on plants or mixed with water to form a spray. Sulfur fungicide is also available as a ready-to-use (RTU) spray, which is handy if you don’t want to deal with the powder yourself.

Sulfur is naturally occurring in the earth, so it’s considered an organic fungicide. But, if companies add other ingredients to their products that are chemically derived, it may or may not qualify as an organic option, depending on what was added.

Sulfur will increase the soil’s acidity, which is problematic for some plants. But that’s where lime sulfur comes in. Lime is added to sulfur to cut down the acidity so it won’t affect the soil as much. Lime sulfur treats the same diseases and pests as regular sulfur fungicides, so the only difference is how it affects soil pH.

How Does It Work?

A shot of a person wearing multi-colored garden gloves using a fungi repellent spray to spray plants in an outdoor area.
It can help stop the spread of fungi.

We don’t totally understand how sulfur stops fungal diseases. But it’s believed that it disrupts the metabolisms of fungi, which stops them from spreading. It can also kill spores on contact. It’s most effective when it’s applied before diseases show up.

Sulfur also kills insects when they touch it or consume it. It disrupts their body’s ability to produce energy and eventually kills them. When treating pests, you don’t have to use this as a preventative measure. You can apply it when you see pests as long as all other conditions are right for application.

Benefits

Close-up of a honey bee hovering over a zinnia flower in a garden against a blurred background. Its body is covered in fine hairs, giving it a fuzzy texture, and is a golden-brown color. The honey bee's wings are transparent and delicate. Zinnia is a colorful and vibrant flower with a daisy-like bloom, bright pink petals surrounding a central disk of bright yellow.
This substance is non-toxic to pollinators like the honeybee.

Many pesticides harm wildlife and beneficial pollinators. It can be tricky to find something safe to use in the garden.

Fortunately, sulfur is non-toxic to honeybees, birds, and fish! You don’t have to worry about harming pollinating visitors or backyard ponds with organic sulfur plant fungicide.

Since sulfur occurs naturally in nature, it will break down and rejoin the natural sulfur cycle of the earth. It doesn’t dissolve in water, so water runoff can be an issue. But we’ll talk more about that and safety concerns in the drawbacks section.

Pests and Diseases Treated By Sulfur

The leaf affected by rust fungus shows small, orange to rusty brown colored pustules on the underside.
Rust fungus is one of the several diseases it can prevent.

Sulfur fungicides treat several diseases on fruits, vegetables, and ornamental plants. Diseases include:

  • Alternaria leaf spot
  • Powdery mildew
  • Rust
  • Black rot
  • Brown rot
  • Scab
  • Blossom blight

If rust, scab, brown rot, or leaf spot damage is already present on the plant, it will not cure the damaged tissue. Prune off severely damaged foliage and dispose of it. Blossom blight will usually cause the blossom to drop no matter what you do to treat, but the plant may flower again.

It can also treat a wide array of pests, including:

Use sulfur fungicides on most plants, including some types of berry crops, roses, ornamentals, and most leafy or fruiting vegetables. There are exceptions in each of these groups because some plants are intolerant of sulfur and can be damaged or killed if they’re exposed to it.

Drawbacks

A focused shot of sunscald stricken red pepper showing the visible damage on its body.
It may burn plants if the weather is too hot.

There are some plants that can’t handle sulfur fungicides. In addition, it will burn plants if temperatures are too hot or if it’s mixed with oils. It’s also somewhat toxic to humans and pets.

You shouldn’t apply sulfur if the temperature is 85°F (29°C) or higher, or if the temperature is going to reach that high later in the day. This can certainly be annoying if your summers are always 85°F or above. But you can still use it in cooler months with fewer problems.

If you apply neem oil to your plants, wait at least two weeks before you apply sulfur. The two will combine to burn your plant’s leaves in a similar way to sunscald.

Sulfur might be safe for bees, fish, and birds, but it’s slightly toxic to humans and pets. It can irritate your eyes and skin if you come into contact with it and can irritate your respiratory system if you breathe it in. Always wear protection when you’re handling it and don’t use it around kids or pets.

Speaking of pets, don’t let your furry friends consume this fungicide by snacking on your plants. If they eat too much sulfur, it can cause severe health concerns. If your pets roam freely through the garden, this may not be an ideal solution for you. Something like neem oil will be a safer option.

Sulfur won’t impact the environment too much but can gradually acidify your soil. Additionally, if the water runs off into a field or flower bed of plants that can’t tolerate sulfur, you may have problems. Therefore, prevent runoff as much as possible.

Plants At Severe Risk From Sulfur

A branch of an apricot tree laden with ripe, orange apricots. Sunlight shines through the leaves, casting dappled shadows on the fruit. In the background, there is a patch of green grass and several other trees.
Apricots might not be able to tolerate this agent.

The plants that can’t tolerate sulfur include:

These plants are highly sensitive to sulfur and can be harmed if they’re exposed to it. Certain types of exotic citrus may also be impacted, although most citrus should be fine to treat.

You can still use sulfur in your garden, even if you have these plants. You’ll just need to be extra careful when applying it to avoid it blowing onto these species.

Never apply the dust or spray in the wind, especially if the wind is blowing toward sulfur-sensitive plants. If your yard is sloped and water runs off in a certain direction, avoid planting the intolerant plants where the water collects.

How To Use Sulfur Fungicide In The Garden

A gardener wearing gloves sprays a leafy plant with an orange bottle, the dark green leaves standing out against a wooden fence in a backyard garden, with droplets misting over the foliage.
Use it as a spray or sprinkle the powder directly.

Sulfur is super easy to use in the garden. If you don’t want to mix up your own spray, buy a ready-to-use product or apply the dust directly to the plant. 

If you’re going to create your own spray, follow the directions on the product label of a reputable brand. Different products might have specifications that could affect plants both positively or negatively. Generally, the ratio is about three tablespoons per gallon of water. Remember, the powder won’t fully dissolve in the water.

Spray this mixture onto fruit and leaf surfaces. The mixture will dry and leave the powder behind. This will prevent diseases and kill pests for a couple of weeks.

Ready-to-use sprays are used just like a DIY mixture. The only difference is that you don’t have to mix it yourself.

Powders are applied directly to plants in a similar manner, but it may be more difficult to reach the undersides of leaves and stick to stems. One thing to note about sulfur dust is that it’s better for treating tiny little mites. Sulfur is natural, so if it touches the soil while you’re trying to get those hard-to-reach areas, it’s not a big deal.

The best time of year to start applying sulfur is in late winter or early spring before foliage appears and diseases develop. Once leaves start growing, apply the fungicide early in the morning. This allows the sprays to dry so they won’t burn the leaves. Remember, hot temperatures can cause leaves and fruits to burn, so it’s best to avoid applying it during the hottest parts of summer.

Apply sulfur every 10-14 days as needed or after it rains. Rain will wash everything away, and you’ll have to start the treatment over. If you need to apply neem oil, wait a couple of weeks after applying sulfur.

Frequently Asked Questions

Is sulfur a good fungicide?

Sulfur is an excellent fungicide to use because it can treat a wide variety of pests and diseases. It can be a bit finicky to work with since certain plants are intolerant, and it will burn leaves in hot temperatures, but it’s a valuable and reliable addition to your repertoire of pest and disease control.

How often can I spray sulfur?

You can spray sulfur every 10-14 days. You’ll need to apply it sooner if it rains, and you’ll have to wait two weeks after applying neem or other oils since oil and sulfur together will burn plants.

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