Fighting Fungi the Organic Way

By Eric Hopper
Published: August 11, 2020 | Last updated: April 23, 2021 03:27:40
Key Takeaways

When fungi problems strike in the vegetable garden, the results can be catastrophic. Eric Hopper has the latest on organic solutions that allow growers the option of preventatively treating plants.

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There are many possible problems an indoor horticulturist may encounter throughout the life cycle of plants. Pathogenic fungi are not only one of the most prevalent detriments to plague an indoor garden, but they may also be one of the most destructive. Because of the way pathogenic fungi reproduce they are able to, in some cases, take out an entire crop in just a few days.


Since these destructive organisms can get out of control so quickly, it is imperative for the grower to identify the pathogen and immediately implement a proper treatment program.

There are many different treatments available for battling a fungi problem, although some are not suitable for consumable plants. A wide variety of effective chemical fungicides exist for roses and other ornamental plants, but these should never be used on any plant meant for consumption.


Always be sure to check the label carefully when purchasing a fungicide. Close inspection of a fungicide label will disclose the intended usage and application procedures. Food crop growers are starting to turn their heads toward organic fungicides, which are much safer for the grower, the plants and the end consumer.

Organic fungicides, just like chemical fungicides, come in a variety of formulations. Some are intended as a general fungicide while others are specific to a given pathogen. The increasing demand for organic products has the popularity of organic fungicides steadily rising.

Because of their growing popularity, manufacturers are producing more innovative products using some of the oldest known fungicidal substances. Breaking down some of the organic treatments into general categories is a good way for growers to better understand their options.


Sulfur-Based Fungicides

Sulfur is one of the longest-used fungicides. Historical data tracks its use back to more than 1,500 years ago when its fungicidal properties were found to battle pathogens on wheat. For the indoor horticulturist, who probably isn't growing wheat, sulfur can be used to battle a wide variety of fungi, including rose black spot and powdery mildew. Sulfur's effectiveness as a fungicide stems from its ability to prevent the fungal spores from germinating. Sulfur-based fungicides are available in liquid or powder form.

Sulfur burners are devices commonly used by greenhouse and indoor horticulturists to prevent or combat powdery mildew. Sulfur burners heat up powdered sulfur (usually in a small tablet form) until the sulfur reaches its evaporation point. As the evaporated sulfur mixes with the room atmosphere, a thin layer of sulfur forms on the entire surface of the plant (and everything else in the room).


This is the most effective way to get complete coverage with a sulfur treatment. Sulfur burners should not be used within four weeks of harvest with any consumable where the sulfur powder cannot be physically cleaned from the surface of fruits and vegetables. Sulfur burners are not safe to use when a person or animal is in the room. Always put a sulfur burner on a timer and never be in the room when a sulfur burner is active.

Read More: How to Detect and Treat Sulfur Deficiencies in Plants

Copper-based Fungicides

There are many different copper-based fungicides available to today's hobbyists and commercial horticulturists. Copper-based fungicides are generally mixed with some other component, such as lime, to help neutralize the pH. Without first being neutralized, the acidic properties of copper sulfate (the most commonly used form of copper) could cause as much damage to the plant as the pathogen.

Copper-based fungicides are available in powder and liquid form, although liquid is much more common for the indoor horticulturist. Some plant varieties are more sensitive to copper-based fungicides than others and growers should always conduct a sensitivity test before using one.

To conduct a sensitivity test, spray a few leaves on each variety of plant in the garden and wait 24 hours. If there are no signs of burning on the foliage, go ahead and treat the entire garden.

Neem-based Fungicides

Neem is one of nature's greatest gifts to the indoor horticulturist. Neem oil contains azadirachtin, a natural insecticide and fungicide. Highly concentrated neem oil kills powdery mildew spores on contact. Growers looking for neem oil with high fungicidal properties should look for cold-pressed neem with high concentrations of azadirachtin.

Neem-based fungicides commonly come in two forms: granular and liquid. The granular neem is designed to be amended into the soil, which helps reduce soil-borne insects and is also an effective systemic fungicide for some plant varieties. Liquid neem comes with a variety of options.

Pure neem oil can be cold pressed or chemically processed. The cold-pressed neem retains a higher amount of the active ingredients and is generally more effective as a fungicide. On the downside, cold-pressed neem is generally more expensive; sometimes up to three times the price of chemically extracted neem oil.

Other neem-based liquids are usually concentrates of the active ingredients derived from neem. These liquids should be compared based on the percentage of the active ingredients they contain.

Read More: Fighting Fungi - The Organic Way

Bicarbonate-Based Fungicides

Yes, baking soda (sodium bicarbonate) can be used as a fungicide. However, growers may want to think about using some of the other bicarbonate-based fungicides available, such as ammonium bicarbonate or potassium bicarbonate.

Unlike baking soda, these bicarbonates will leave behind usable plant nutrients instead of toxic salts and are the most commonly used bicarbonates in manufactured fungicides. Bicarbonate-based fungicides are effective against powdery mildew, botrytis and rusts.

Bicarbonate-based fungicides are available in powder and liquid form, but for the indoor grower, liquid application would be the better choice.

Read More: 5 Reasons to Add Calcium Carbonate to Your Garden

Botanical-Based Fungicides

Advancements in extraction methods have led to an increasing amount of botanical-based organic fungicides. These fungicides are derived from the essential oils of various plants with fungicidal properties. Because of the innovative extraction techniques, higher percentages of the active compounds are retained, which means more potent and effective products.

Oftentimes these plant-based extracts are combined with other organic fungicides to increase effectiveness. Garlic, clove, cinnamon, oregano, thyme and wintergreen are just some of the plants with essential oil extracts that are making their way into organic fungicides.

Biological Fungicides

Of all the different types of organic fungicides available, none have made such substantial strides as biological fungicides. As we understand more about plant physiology and the way micro-organisms interact with one another, we have discovered some incredibly effective organic biological fungicides.

The use of beneficial micro-organisms, usually bacteria, has made organic pest control not only more effective but also healthier for our planet and ourselves.

The most common biological fungicides used in indoor horticulture are strains of beneficial bacteria that feed on pathogens. Organic biological fungicides are generally pathogen specific, or designed to fight one particular disease. Beneficial bacteria strains that feed on powdery mildew spores are one of the most effective natural defenses against this problem in an indoor garden.

There are many different organic fungicides that contain fungi-eating bacteria. These products are always a good first choice for a grower because they are easy to use, relatively inexpensive and are even safer than some of the other organic options. Biological fungicides should be the first line of defense for any grower battling or trying to prevent a pathogenic fungal attack.

Another benefit to biological fungicides is that they are generally contained in an inert substance that doesn't have any chance of hindering the garden's performance. Once the applied beneficial micro-organisms run out of food (in this case the pathogenic fungi), they naturally die off. This means no residue, no toxic run-off and no possibility of contaminating the sprayed fruits or vegetables.

To maintain optimal growth and avoid a potentially catastrophic occurrence in the garden, horticulturists must be ready at all times to battle a pathogenic fungus. Growers of consumable plants should, at the very least, consider their options with organic fungicides.

Today's advancements in technology have produced innovative extraction methods that create biological fungicides and highly concentrated organic substances. Organic fungicides give the grower an ability to preventively treat their plants without the risk of toxicity. Organic fungicides can give growers a shield against opportunistic pathogenic fungi that could otherwise give the unsuspecting gardener an unwanted battle.

Read Next: Is it a Mold or Mildew Problem in Your Garden?


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Written by Eric Hopper | Writer, Consultant, Product Tester

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Eric Hopper’s past experiences within the indoor gardening industry include being a hydroponic retail store manager and owner. Currently, he works as a writer, consultant and product tester for various indoor horticulture companies. His inquisitive nature keeps him busy seeking new technologies and methods that could help maximize a garden’s performance.

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