It’s enough to bring a grown man to tears. Spotting powdery mildew on your leaves during veg, or finding botrytis (flower rot) while you’re harvesting can be a real heart stopper. But once you’ve had a battle with botrytis or a mêlée with mildew (and hopefully come out on top) you invariably develop into a better, more confident, grower.

Savvy growers, instead of reaching for the chemicals, look to a cue from Mother Nature in their search for effective biological products that can prevent and treat certain fungal diseases. The most notable are a specific group of naturally occurring bacteria that can be used to prevent and control fungal infections. These bacteria are called Bacillus subtilis and Bacillus pumilis.

So what are these strange Latin names and, most importantly, how can we use these bacteria to help us grow healthier, happier, and more productive plants? We asked Emily Walter from Agraquest, a provider of biological and low-chemical pest management solutions, to give us the lowdown on our friendly neighborhood fungal disease controllers.

Fungal diseases are a common issue among gardeners. And some of the more common diseases that gardeners struggle to control are powdery mildew and botrytis. Mildew can cause significant damage on some plants. It’s a common, but rarely fatal disease which affects many different types of plants. Most gardeners resort to removing infected plants, but often, the best strategy combines control and treatment.

So What Exactly Is Powdery Mildew?

Powdery mildew is a fungal disease that causes patches of white to gray powder on leaves, stems, fruits or flowers of infected plants. These patches can grow to cover the entire surface on both sides of leaves. Different strains of fungi cause the disease on different plants, but they are all similar in appearance. On some types of plants, the mildew will cause leaves to yellow and prematurely drop, or can cause stunted or deformed plant growth, and eventual plant decline. Mildew thrives in cool, damp conditions with poor air circulation. Indoor and greenhouse growers listen up!

What about Botrytis?

The dreaded botrytis or ‘gray mold’ is a fungal disease that infects many annual and perennial plants. There are several species of the fungus botrytis which can cause significant plant damage; the most common is Botrytis cinerea. Botrytis infections are favored by cool and humid conditions, and are most common during rainy spring and summer weather when temperatures hover around 60°F (15°C). Gray mold can take hold and spread rapidly if your indoor garden sustains long periods of high relative humidity, or outdoors when rainy, drizzly weather continues over several days. Botrytis can affect leaves, stems, crowns, flowers, flower buds, seeds, seedlings, bulbs and just about any other part of a plant with the exception of the roots.

[Left] Scanning Electron Micrograph (SEM) showing a healthy fungal spore on the surface of a leaf. [Rigth] SEM showing a destroyed fungal spore after foliar application of the beneficial bacteria Bacillus subtilis strain QST713. The Bacillus bacteria are the small rod shaped organisms around the top of the picture.

Steps Towards Prevention

Good cultural practices can help prevent and control the spread of fungal issues. The first and most important step toward prevention is to select healthy plants in the first place; these will be more likely to resist fungal attacks.

Plant breeders often select specimens that show resistance to common fungal diseases. This has lead to many different varieties of fruits, flowers and vegetables that have an ever increasing resistance to fungal diseases.

If you have no choice but to use susceptible types of plants outdoors, make sure they are in full sun and will receive a minimum of six hours of sun each day. Isn’t UV radiation great? Indoors, however tempting as it may be, never overcrowd your plants! Allow plenty of fresh air to circulate around your plants, this will discourage disease. When growing indoors, it’s absolutely crucial to focus on maintaining adequate ventilation.

Carefully remove infected fruits, flowers or mildew covered leaves. It is best not to do any removal of diseased plant when they are wet with dew or rain since this could spread fungal spores during conditions which favor infections. Likewise, avoid overhead watering or misting plants especially if fungal disease has been troublesome in the past.

If you encounter a heavily infected leaf, fruit, or flower and are worried about spreading spores around your garden, carefully cover the moldy item with a plastic bag before attempting removal. This way, spores end up in the bag rather than all over your garden!

Always throw away infect plant debris instead of placing it on the compost pile. Spores can overwinter on diseased plant material. New spores can be carried by the wind, so destroying the infected plant parts are essential to help stop the spread of plant disease pathogens.
In an effort to keep fungal diseases at bay, it’s good general practice to keep your indoor garden as clean and tidy as possible. Avoid leaving yellowing or dead leaves hanging from plants, and never keep piles old leaves and trash bags in or near you indoor garden.

Fungal Control Options

There are many chemical controls on the market but they do have some drawbacks. Some controls have temperature and timing restrictions impacting applications or harvest of your crops. Some diseases become resistant to certain chemicals over time as well. If you are using IPM (Integrated Pest Management) in the garden, you will want to preserve beneficial insects, which can be impacted by some chemical controls. Additionally, not all chemical fungicide treatments are acceptable for consumable plants. Some chemicals can only be used when the plant is dormant or cannot be used when the plant is close to harvest time. Carefully read fungicide labels to discover which is right for your particular need.

Beneficial Bacteria – Bacillus as a Fungicide

Another way to control plant diseases is to use products based on beneficial bacterial, specifically Bacillis subtilis or Bacillis pumilis. Both of these bacteria are common found in soil and have been used in horticulture and agriculture for many years.

Bacillus subtilis are naturally-occurring soil borne bacteria, fist characterized in 1835. Over the years varying strains of B. subtilis have been widely used for industrial processes (like detergents or waste water treatments). Bacillus subtilis strains produce extremely photo – and temperature – stable bacterial spores, making them ideal for gardening applications. B. subtilis is also Generally Regarded As Safe (GRAS) by the EPA.

Some strains of Bacillus subtilis are active ingredients in highly effective, broad-spectrum contact fungicides and bactericides. The Bacillus subtilis in these products produce lipopeptides, which are anti-fungal metabolites and anti-bacterial compounds. These lipopeptide compounds that Bacillus subtilis produce kill fungal spores and they are highly stable, resistant to elevated temp and pH extremes.

Lipopeptides are small peptide rings with a lipid (fat) attached. One end of the lipopeptide is negatively charged, the other is “greasy”. A fungal cell membrane can be compared to a sandwich – with hydrophilic (water-loving) surface and a lipophilic (fat-liking) core. The lipopeptides insert into those fungal cell membranes and create small holes in a fungal spore. As, they puncture the cell membrane, cell contents leak out and the fungus is killed.

Some B. subtilis products rely on prolonged wet periods on the leaf surface for the bacteria to become active, produce lipopeptides and then out-compete the fungal spores for leaf surface area. This is not the case with all products – the specific Bacillus subtilis strain QST 713 is unique in that it does not require time to activate, since the bacteria have already done their job producing the lipopeptide metabolites during production. Information about the active ingredient and how it works should be found on the container label of the product you decide to use.

Some Bacillus subtilis strains also illicit plant health and growth promotion in treated plants.  When applied, these strains can trigger the plants’ internal defenses and physiological responses. The effect is systemic – responses are triggered throughout the plant even when a small area is treated.

Products based on Bacillus subtilis are widely used by gardeners and commercial growers since they offer broad spectrum control, have little potential for resistance, have no temperature or time restrictions for application, are non-toxic to beneficial insects (including honey bees) and can be used up to and including the day of harvest. Some products based on B. subtilis are approved for organic production. Since Bacillus subtilis are non-toxic to beneficial insects you can use predatory insects for pest control and beneficial bacteria for disease prevention.

Products based on Bacillus pumilis strains are useful for gardeners for similar reasons. While products based on B. subtilis destroy fungal cell membranes, products based on B. pumilis instead focus on fungal cell walls. The compounds produced by B. pumilis compete with fungal diseases for amino sugars needed to build cell walls, effectively making it impossible for fungal cells to build and grow. B. pumilis does not control bacterial diseases. Instead, it is strongest against rust and mildews. B. pumilis is typically used by a gardener when targeting a specific type of fungal infection that is better controlled by this specific bacteria over the more broad spectrum approach of B. subtilis. Also, B. pumilis strains, like those of B. subtilis, have been shown to trigger plant’s natural defenses.

Both of these beneficial bacteria are best used when applied to plants in a preventative disease control program or at the very first sign of disease. Beneficial bacteria can be used in conjunction with other gardening products. Bacillus subtilis and Bacillus pumilis can be applied every seven days up to the day of harvest if needed. Beneficial bacteria can be applied more often if needed such as every four days during heavy disease pressure. When applying either type of bacteria as a foliar spray one should spay the leaves, shoots and new growth until the plant is dripping wet. Run-off spray will not affect beneficial soil fungi like mycorrhiza. When a gardener is planning to use beneficial bacteria or an organic gardening product to prevent or control fungal and bacterial diseases they should scout the garden often to look for any signs of disease. Strains of these beneficial bacterial can also be found in some compost teas since it can promote plant health and growth promotion.

Practical Tips:

When Do I Use It?

Most Bacillus subtilis or pumilus products can be sprayed as a preventative measure or be used as a curative control. They can be applied early on in the plant’s lifecycle on established cuttings or seedlings, and as late as the day of harvest on mature plants. Most growers freak out at the mere idea of spraying mature flowers or ripe fruit, but these natural Bacillus products are safe for human consumption and actively kill fungal growth.

Spray timing

When using outdoors, it’s best to spray in early morning or late afternoon when light intensity is not too strong. Sunlight contains a natural broad spectrum microbe inhibitor, Ultra Violet light. If applied during strong sunlight, the UV may prevent some Bacillus products from working effectively.
When spraying indoors, it’s also good practice to spray in low light. This may mean raising your grow lights up high before spraying, or spray just before the lights come on or go off.

Spraying the plants

The best fungal control is achieved when the plants are thoroughly wet, and run-off spray is dripping from the leaves. It’s a good idea to use a wetting agent for increased coverage. Avoid adding other foliar additives or nutrients as this may interact negatively with the beneficial bacteria. Spray the underside and top side of the leaves as well as any exposed stems. Sprays can be repeated every 3-4 days if plants are heavily infected, or every 7-10 days as a preventative.

What products contain Bacillus subtilis or pumilus for foliar fungal disease control?

Most good quality compost teas will contain some Bacillus subtilis and pumilus strains, so regular spraying can help with disease prevention. For a more targeted fungal control, the patented Bacillus subtilis strain QST 713 can be found in the commercial product ‘Serenade.’

Other Bacillus subtilis strains are used for root disease control, these include strains GB 03 found in the microbial inoculant ‘Companion’ and strain MBI 600 found pre-mixed into the substrate ‘Pro Mix MX with BioFungicide’.