Late blight wiped out your tomatoes last year. You want to prevent it this season without chemicals. Bacillus subtilis is the active ingredient in commercial biofungicides like Serenade and Cease. It suppresses fungal diseases by colonizing leaf and root surfaces before pathogens can establish. Wild hay naturally contains Bacillus subtilis, the same bacteria in expensive commercial biocontrol products. A single application of commercial B. subtilis product costs $15 to $30. Or you can make a living culture from a handful of hay and a jar of warm water. When you soak hay in warm water, the spores germinate and multiply within days. Three days of soaking gives you a living culture ready to spray on your garden.
TL;DR: Stuff a 1-quart (1-liter) jar with natural hay (not straw). Fill with warm water. Add a pinch of any granular fertilizer (NPK). Cover loosely. Keep in a warm spot (75-85 °F / 24-30 °C) for 3 days. A thin white or grayish film forms on the surface. That film is your B. subtilis culture. For 1-gallon final volume, use 1.5 cups of culture per gallon of water. Drench soil or spray foliage every 10 to 14 days.
The recipe
You need
- Natural hay: enough to loosely fill a 1-quart (1-liter) jar. Must be real hay (dried grass), not straw (dried grain stalks). Buy at a garden center, farm supply store, or pet store (small animal hay). A small bale costs $5 to $8.
- Warm water: 1 quart (1 liter), not boiled, not chlorinated. Let tap water sit uncovered for 24 hours to off-gas chlorine, or use well water.
- A pinch of any granular NPK fertilizer (10-10-10 or similar). This feeds the bacteria during multiplication.
- A 1-quart glass jar or container
- A loose-fitting lid, cloth, or paper towel secured with a rubber band
Do it
- Pack a 1-quart (1-liter) jar loosely with dry hay. Push it down gently but do not compress it tightly.
- Fill the jar with warm water (75-85 °F / 24-30 °C). The hay should be fully submerged.
- Add a small pinch (about 1/8 teaspoon) of granular fertilizer. Stir once.
- Cover loosely. The bacteria need air exchange. Do not seal the jar airtight.
- Place in a warm spot (75-85 °F / 24-30 °C). A kitchen counter, top of a water heater, or warm closet works.
- Wait 3 days. A thin white, grayish, or slightly iridescent film will form on the water surface. The water will smell earthy or slightly musty, not rotten. That film is a colony of Bacillus subtilis.
- Strain the liquid through cheesecloth or a fine strainer. Discard the hay (compost it).
- Dilute: For 1 gallon of final spray volume, use 1.5 cups of culture mixed with 0.5 gallons of water. For larger batches, maintain the 1:10 dilution ratio in a sprayer or watering can.
Use within 3 days of straining. The bacteria are alive and most active when fresh. Store in a cool, dark place if you cannot use immediately, but potency drops after 3 days.
How to use it
Soil drench (primary method):
Pour about 1 cup (250 ml) of the diluted solution at the base of each plant. The B. subtilis colonizes root surfaces and the surrounding soil, competing with pathogenic fungi for space and nutrients. This is called competitive exclusion. The bacteria physically occupy the niches that root rot fungi need. Apply every 10 to 14 days during the growing season.
Foliar spray (secondary method):
Spray the diluted solution on both sides of leaves in the morning. B. subtilis colonizes the leaf surface and produces lipopeptide antibiotics (surfactin, iturin, fengycin) that disrupt fungal cell membranes on contact. Spray before disease appears for best results. Once a fungal infection is established inside leaf tissue, B. subtilis cannot penetrate and cure it.
Best timing:
Start applications 1 to 2 weeks after transplanting and continue every 10 to 14 days through the season. Apply after rain (rain washes bacteria off leaves) and after any fungicide application (copper and sulfur kill B. subtilis on contact). Wait at least 5 days after applying Bordeaux mixture or copper before spraying hay extract.
Which plants benefit most
B. subtilis protects any plant that faces fungal disease pressure, but some crops show dramatic improvement.
Best response: Tomatoes. Late blight, early blight, septoria leaf spot, and bacterial speck all colonize tomato leaves. B. subtilis on the leaf surface competes directly with these pathogens. Studies on tomato show 40 to 60% reduction in foliar disease severity with regular B. subtilis application. Combine with iodine-milk spray during active blight season for layered protection.
Best response: Cucumbers. Downy mildew and powdery mildew are the two biggest problems in cucumber production. B. subtilis suppresses both, especially when applied preventively before symptoms appear. Stack with milk spray for active mildew.
Strong response: Peppers and eggplant. Bacterial leaf spot and Phytophthora root rot respond to regular B. subtilis drenching. The bacteria colonize root surfaces and reduce pathogen access.
Strong response: Squash and melons. Powdery mildew control on squash leaves. Soil drenching also helps with Fusarium wilt, a common problem in melon patches that have grown the same crop repeatedly.
Good response: Strawberries. Gray mold (Botrytis) on fruit and crown rot in the soil both respond to B. subtilis. Apply as a soil drench around the crown and as a foliar spray on developing fruit.
Moderate response: Leafy greens and root crops. These have fewer fungal issues but benefit from the soil biology improvement that B. subtilis brings.
Not useful for: Pest control. B. subtilis is a fungal disease suppressor, not an insecticide. For pest problems, use other tools (garlic spray, hot pepper spray).
Why it works
Bacillus subtilis is one of the most studied beneficial bacteria in agriculture. It suppresses plant diseases through three mechanisms:
Competitive exclusion: B. subtilis is a fast colonizer. When it arrives on a root or leaf surface first, it occupies the physical space and consumes the nutrients that pathogenic fungi need to establish. This is a numbers game. A surface already covered with B. subtilis gives incoming fungal spores nowhere to land and nothing to eat.
Antibiotic production: B. subtilis produces cyclic lipopeptides, specifically surfactin, iturin, and fengycin. These molecules punch holes in fungal cell membranes. They are effective against a wide range of plant pathogens including Botrytis, Fusarium, Phytophthora, and Alternaria. The antibiotics are produced continuously as long as the bacteria are alive and metabolically active on the plant surface.
Induced systemic resistance (ISR): When B. subtilis colonizes root surfaces, it triggers the plant's own immune system. The plant upregulates its production of defense enzymes (peroxidase, polyphenol oxidase, phenylalanine ammonia-lyase) throughout its tissues, not just at the root. This means a soil drench with B. subtilis can improve disease resistance in the leaves even though the bacteria never touch the foliage.
Why hay specifically? Hay is dried grass harvested from meadows. The drying process kills most microorganisms, but B. subtilis survives because it forms endospores. These spores are extremely resistant to heat, drying, and UV. When you rehydrate the hay, the spores germinate and the bacteria resume growth within hours. Straw (dried cereal grain stalks) has far fewer B. subtilis spores because the grain harvest process and storage conditions select for different microorganisms.
What NOT to do
Do not use straw. Straw is the dried stalks left after grain harvest. It has a different microbial profile than hay and carries far fewer B. subtilis spores. You may end up culturing the wrong bacteria. Use natural hay from meadow grass.
Do not seal the jar airtight. B. subtilis is an aerobic bacterium. It needs oxygen to grow. An airtight seal creates anaerobic conditions that favor Clostridium and other undesirable bacteria instead. Use a loose lid, cloth, or paper towel.
Do not use the culture if it smells putrid. A successful hay infusion smells earthy, musty, or slightly like wet grass. If it smells like rotten eggs or sewage, the culture has gone anaerobic and is dominated by harmful bacteria. Discard it and start over with better air circulation.
Do not boil the water first. Boiling sterilizes the water and kills any residual B. subtilis spores that might be present. Use warm (not hot) dechlorinated water. If your tap water is heavily chlorinated, let it sit uncovered for 24 hours or use well water.
Do not apply within 5 days of copper or sulfur sprays. Copper and sulfur are biocides that kill B. subtilis on contact. Apply the hay extract at least 5 days after any copper-based treatment to give the bacteria a chance to colonize without being killed immediately.
Do not store the diluted solution for more than 3 days. The bacteria are alive and most effective when the culture is fresh. After 3 days, cell counts drop and competing microorganisms may take over. Make a new batch every time.
FAQ
How do I know the culture worked?
Look for a thin film on the water surface after 3 days. It may be white, grayish, or slightly iridescent. The water underneath should be cloudy and smell earthy. If you see a thick, slimy, brightly colored film (pink, orange, green), or if the smell is strongly putrid, discard the batch and start over with cleaner hay and better ventilation.
Can I use hay from a pet store?
Yes. Timothy hay and orchard grass hay sold for rabbits and guinea pigs work well. These are real dried meadow grasses and carry B. subtilis spores. Avoid flavored or scented pet hay products.
Is this the same as commercial Serenade or Cease?
Same species, different strain. Commercial products use specific patented strains of B. subtilis (like QST 713 in Serenade) that have been selected for high antibiotic production. The wild strains in hay are effective but may be less potent per cell than the commercial strains. The advantage of the hay method is that it is free, renewable, and does not require purchasing a commercial product.
How does this compare to the turmeric-boosted B. subtilis recipe?
The Bacillus subtilis with turmeric recipe uses a commercial B. subtilis product as a starter and adds turmeric to boost bacterial multiplication 100x. That method gives a more concentrated, more predictable culture. The hay method gives a free starter culture from scratch. Use the hay method when commercial B. subtilis products are unavailable or when you want zero cost.
Can I keep the culture going by adding fresh hay every few days?
Yes, but it is not reliable. Each batch introduces slightly different microbial populations. After several rounds, competing organisms may outcompete the B. subtilis. It is cleaner to start a fresh batch from dry hay every time.
Will this work in a greenhouse?
It works especially well in a greenhouse. Greenhouse conditions (warmth, humidity) favor both fungal disease and B. subtilis growth. Apply as a soil drench and foliar spray every 10 days in enclosed growing spaces.
Is there a gardening app that schedules biological protection treatments?
Yes. The easyDacha garden planner app schedules both feeding and protection tasks by growth stage. Free 14-day trial at easydacha.com/download.
Free fungicide from a handful of hay
Commercial biofungicides charge $15 to $30 per application for the same bacteria that live on every blade of dried meadow grass. Stuff hay in a jar, add warm water and a pinch of fertilizer, and wait 3 days. The white film on the surface is your B. subtilis culture. Dilute and spray. The bacteria colonize your plant surfaces, produce antibiotics that kill fungal spores, and trigger your plants' own immune systems. Total cost: the price of a small hay bale.
The easyDacha gardening app schedules biological protection treatments by growth stage so you apply before disease arrives, not after.
Try easyDacha free for 14 days →. The garden planner app that plans your season in 60 seconds. Cancel anytime.
Related reading on easydacha.com
- How to Use Bacillus subtilis with Turmeric (100x Boost Recipe) — the turmeric-boosted method uses a commercial starter for a more concentrated culture. Use hay when you want zero cost.
- How to Make Iodine-Milk Spray Against Late Blight (Tomato and Potato Recipe) — iodine-milk kills blight zoospores on contact. Hay extract prevents recolonization between treatments.
- How to Use Milk Spray Against Powdery Mildew (It Actually Works) — milk spray disrupts active mildew. B. subtilis prevents new mildew from establishing on treated surfaces.
- How to Make Bordeaux Mixture at Home (Copper Fungicide Recipe) — copper kills fungi and bacteria. Wait 5 days after Bordeaux before applying hay extract so the bacteria survive.
- How to Grow Trichoderma at Home (2 Easy Methods) — Trichoderma is another beneficial fungus for soil health. B. subtilis and Trichoderma complement each other without conflict.
- How to Culture Clonostachys rosea at Home (Late Blight Fighter) — another biological agent for blight suppression. Layer multiple biologicals for the strongest protection.
