If using foliar sprays to combat mold issues sounds a bit far out of whack, then you should definitely read on!
The appearance of powdery mildew on your leaves during the vegetative stage or finding botrytis (flower rot) as you’re harvesting is something every grower dreads. However, after a successful battle with botrytis or mildew you invariably become a more skilled and confident, grower.
It’s all too easy to panic and reach for the chemicals but there are many effective biological products on the market for both prevention and cure of plant fungal diseases. Bacillus subtilis and Bacillus pumilis are a specific group of naturally-occurring bacteria that can be used to prevent and control fungal infections.
What Is Powdery Mildew?
Powdery Mildew is a fungal disease that is easy to spot. Look
for what/grayish powder on leaves (either side), stems, fruits or
flowers. Itlooks like powdered sugar was sprinkled on the plant.
Some plants are more susceptible to PM than others. When plants are
early in their vegetative stage (when leaves are expanding) this
can be a time of high risk. Plants exposed to high levels of
nitrogen in the vegetative state are at great risk of PM infection
as well. PM thrives in cool, damp, stagnant conditions making it
the bane of many greenhouse and indoor gardeners who have not
invested in adequate atmospheric solutions.
What is Botrytis?
Botrytis (aka 'gray mold') is a fungal
disease that can devastate annuals and perennials alike. Botrytis
cinerea is the most common strain. Cool and humid conditions (such
as rainy springtime seasons and early summer weather - around 60
F/15 C) are perfect for gray mold to take hold. This can happen if
your indoor garden suffers from long periods of high relative
humidity as well. Botrytis can affect any part of the plant above
It sounds obvious, but always select healthy plants in the first
place - fungus preys on the weak! Full sun (or grow lights with a
ultraviolet (UV) component) is nature's natural antifungal agent.
If growing indoors, be sure to allow adequate space between plants
and plenty of oscillating fans to provide air movement in your grow
space. Maintain a high level of cleanliness in your garden and
never keep trash bags full of dead plant material in or near your
garden. Avoid long periods of high humidity. Also, avoid large
temperature swings in the garden. Run lights at night through the
use of thermostatically controlled block heaters and/or
dehumidifiers when your grow lights are off.
Remove diseased leaves and throw away in garbage, not the
compost. Also remove any yellowing or dead leaves hanging on the
bottom of the plant. Carefully cover the moldy item with a plastic
bag and gloves to help prevent the spreading of spores. Never keep
piles of old leaves and trash bags in or near your indoor garden.
This can be a breading ground for fungal and bacterial
Fungal Control Options
Photo courtesy of Agra Quest inc.
Take care if choosing a chemical control product. Some chemical
controls are not to be used on edible crops. Read label directions
for specific application and harvest times. Bear in mind that some
diseases can become resistant to certain chemicals over time.
If you prefer a more natural route, try beneficial bacterial,
specifically Bacillus subtilis or Bacillus pumilus -both are
endemic in soil and are well established horticultural techniques
and GRAS (Generally Recognized as Safe) by the EPA (United States
Environmental Protection Agency)
Some strains of B. subtilis are the active ingredient in
broad-spectrum contact fungicides. B. subtills QST713 produces
Lipopeptides that kills fungal spores by puncturing the cell
membrane. Lipopeptides are highly stable, offering resistance to
elevated temperatures and even pH extremes.
What Type of Bacillus subtilis?
Check the exact name of the active ingredient in your chosen B.
subtilis product. Keep an eye out for B. subtilis QST 713 or
MB1600. Some B. subtilis strains also elicit plant health and
growth promotion in treated plants. When applied, these strains can
trigger your plant's internal defenses and physiological responses.
The effect is systemic - this means that responses are triggered
throughout the plant even when just a small area is treated.
Bacillus subtilis Products
- Broad spectrum control of fungal diseases
- Little potential for infection to create resistance to
- No temperature restrictions for use
- Non-toxic to beneficial insects (including honey bees)
- Safe to use up to and including the day of harvest
- Some products based on B. subtilis are approved for organic
- Safe to use with predatory insects.
Bacillus pumilis Products
Products based on B. pumilus instead focus on fungal cell walls
rather than membranes. The compounds produced by B. pumilus compete
with fungal diseases for amino sugars needed to build cell walls,
effectively making it impossible for fungal cells to build and
B. pumilus does not control bacterial diseases. Instead, it's
strongest against rust and mildews. B. pumilus 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
When Do I Use It?
Most B. subtilis and B. pumilis products should 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 retard fungal growth.
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.
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 or surfactant for increased
coverage and a decrease of surface tension in the foliar
Avoid adding other foliar additives or nutrients as this may
interact negatively with the beneficial bacteria. Spray the
underside and top of the leaves as well as any exposed stems.
Applications can be repeated every 3-4 days if plants are heavily
infected, or every 7-10 days as a preventative measure. Always read
and follow label directions of any products used in the garden.
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
mycorrhizae. 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.
What products contain Bacillus subtilis or pumilus for foliar
fungal disease control?
Most good quality compost teas will contain some B. subtilus and
B. pumilus strains, so regular spraying can help with disease
prevention. For a more targeted fungal control, there are patented
strains of Bacillus subtilis that have specific MOA (Modes of
Action). Bacillus subtilis strain QST 713 can be found in the
commercial product SERENADE ASO. Bacillus subtilis strain MB1600
can be found in the commercial product Subtilex NG.
Some B. subtilus and B. pumilus strains can also be found in
substrates, beneficial bacteria packs and soils.
WORDS: Emily Walter. A Girl
and a Garden. Visit Emily's blog!