Fighting plant pathogens with the biofungicide Bacillus subtilis

Bacillus subtilis is a type of bacteria that can survive in the soil for a long time. Many species are very effective at colonizing plant roots and controlling plant pathogens.

They can act directly against other microbes by producing a variety of antibiotics that attack fungi or bacteria. They can also work indirectly by stimulating the plant to activate its own defense mechanisms so that it can ward off attacking microbes.

This combination of activities results in a powerful biological control agent that is sold commercially to inhibit a number of pathogens on many different crops.

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In fact, in addition to its biological control activities, B. subtilis helps the plants grow more effectively.

We at Gardener’s Path will explain in detail how these bacteria work and describe how you can use them in your garden.

Bacillus survives harsh soil conditions

Many types of bacteria die out when the soil dries up and becomes unfavorable to their growth. Bacillus species, on the other hand, produce spores that can survive for a long time and tolerate stressful conditions.

As soon as conditions improve, the spores germinate through various environmental cues.

The bacteria’s ability to survive as spores makes it easier to produce formulations that can be used to treat plants. The bacteria are much more likely to survive for a long period of time if they are in the form of spores rather than free-living bacteria.

After germination, the bacteria have specific sensors to guide them to plants, which they can then colonize. Some strains of B. subtilis can not only live on the roots, but can also live inside the plants and are known as endophytes.

Colonization of the roots

The area in the soil around the roots can be a lush haven for microbes. The roots secrete sugars and other compounds that microbes feed on. In return, these microbes help the plant by providing nutrients that would otherwise be scarce.

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This area is called the rhizosphere. Bacillus species usually live on the plant roots and promote plant growth to such an extent that they are part of a group of bacteria known as plant growth-promoting rhizobacteria (PGPR). These bacteria are so important they have their own acronym!

The bacteria form a so-called biofilm on the plant roots. This is an entire community of bacteria embedded in a matrix made up of chemicals from the bacteria cells.

The location of the bacteria in the rhizosphere gives them a competitive advantage when it comes to warding off plant pathogens.

Production of antimicrobial compounds

Strains of B. subtilis produce a variety of antibiotics and other compounds that inhibit other microbes. It has been reported that more than 24 types of different antibiotics are produced by different strains.

These bacteria produce unusual types of antibiotics known as lipopeptides. You may recognize the term lipo as something related to fat. These antibiotics interact with fungi’s cell membranes (consisting of lipids) and create holes in them, killing the fungi in the process.

There are different types of lipopeptides, the most important group being the iturins. They are “considered an excellent biopesticide.”

Iturin A has been found to exhibit antifungal activity against a variety of fungi and to be involved in the control of several plant diseases.

Another group of antibiotics are the fencygins, which inhibit both bacteria and fungi.

These bacteria can even inhibit the germination of mold spores!

Induction of host resistance

In addition to attacking other microbes, B. subtilis can help prevent disease by activating the plants to respond to pathogens.

Plants have an active immune system and often respond to microbial threats by ramping up their defenses to fight off viruses, bacteria, fungi or nematodes.

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They can respond in different ways, but one common type is known as systemic acquired resistance or SAR. Systemic means that the resistance is active throughout the plant.

B. subtilis has been shown to activate this type of resistance in many different crops and thus inhibit pathogens.

Pathogens controlled by Bacillus subtilis

These bacteria directly combat soil fungal pathogens such as Rhizoctonia, Phytophthora, Fusarium and Sclerotinia. The SAR they induce in the plant can control foliar pathogens such as the fungal pathogens powdery mildew, Botrytis and Sclerotinia and the bacterial pathogens Erwinia and Xanthomonas.

Microbial collaboration that benefits plants

You are probably familiar with mychorrhiza – the fungi that live on plant roots and help them absorb nutrients. B. subtilis works synergistically with a certain kind of mycorrhizal – arbuscular mycorrhizal fungi (AM).

By doing this, these organisms help the plants obtain nutrients such as phosphorus that are often unavailable to plants. The soil can be full of phosphorus, but if it is not in a form that is easily absorbed by the plants, they can become deficient in this nutrient.

This ability means that B. subtilis helps plants even if it doesn’t help protect them from pathogens.

Individual strains have different properties

With biofungicides like B. subtilis, everything is strain dependent. Different species make different combinations of chemicals and vary in their ability to colonize plant roots and inhibit other microbes.

So you can’t expect to add different types and have them work in identical ways.

The formulations also affect how well the microbes survive after application and how they protect the plants.

It’s best to buy an established brand that consists of one variety that has been used in different crops and soils.

CEASE Biological Fungicide is a good option and is available from Arbico Organics.

Using Bacillus subtilis

You can use CEASE on plants ranging from fruit and vegetable trees to roses and houseplants.

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STOP biological fungicide (Bacillus subtilis) through ARBICO Organics

While this microbe isn’t toxic to humans or animals, it’s theoretically possible to develop allergies to some proteins if you’re exposed to large amounts all the time. Therefore, wear a dust/mist filter mask if you intend to apply large amounts of CEASE.

You should apply CEASE at the first sign of disease development on the leaves of your plants. Then apply it every seven days.

If there is a lot of illness, you can increase the frequency of spraying to three-day intervals. You should also water more often if your plants are very humid.

You can spray in full sun or high temperatures, but don’t use if you expect water or rain within four hours of application.

CEASE can be used with some pesticides, but you should check beforehand that they are compatible.

And if you have a large organic farm or one that uses good Integrated Pest Management (IPM) practices, Arbico Organics also carries a variety of Bacillus subtilis in different species, application methods and sizes.

This soil organism controls both root and foliar pathogens

Bacillus subtilis colonizes plant roots and directly attacks soil pathogens. It also stimulates the plants to activate their natural resistance, which can fight foliar pathogens.

The ability of these bacteria to form spores is a major reason for their success in soil, as they can simply go dormant when conditions are unfavorable.

Different species vary in their properties, but CEASE is widely available commercially to control disease on a wide variety of plants.

Have you used Bacillus subtilis in your garden? If so, let us know in the comments how it worked.

And read on for information on other biofungicides:

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