Bacillus biofungicides: species, lipopeptides and how to read the label

Formulations based on endospore forming Bacillus are by far the most widely used agents on the biopesticide market [1]. Commercial preparations worldwide contain strains of B. subtilis, B. amyloliquefaciens, B. velezensis, B. pumilus and B. licheniformis, among others [2]. FRAC, the Fungicide Resistance Action Committee, places Bacillus strains in Group BM02, microbial biologicals with multiple modes of action, with resistance not known [3]. This article covers the species and their names, the modes of action, supported uses, and the practical reading of a product specification.

1. Species used and how their names changed

Bacillus taxonomy has moved several times, and product documents do not always keep up. Until 2020, FRAC coded Bacillus amyloliquefaciens strains as Group 44 under cell membrane disruptors; they were then reclassified to BM02 [3], although some extension tables still show the code 44 [4]. FRAC also notes that B. subtilis and B. subtilis var. amyloliquefaciens are previous names for B. amyloliquefaciens [3].

Phylogenetic analysis has reclassified several Bacillus strains as B. velezensis; strain FZB42, first described as B. amyloliquefaciens, is one example [5]. Researchers have proposed an operational group made up of B. amyloliquefaciens, B. siamensis and B. velezensis, and B. velezensis is the main source of the bioformulations increasingly used in agriculture [1]. B. halotolerans has been identified by phylogenomics among endophytes active against Fusarium [6], and an isolate that reduced wheat crown rot produced iturin, surfactin and fengycin [7]. Because names move, compare products by species and strain code together, as FRAC lists them [3].

2. Modes of action

Bacillus inhibits pathogens through competition for nutrients and space, antibiotics, hydrolytic enzymes, siderophores and induced systemic resistance [2]. Among the effects FRAC lists for BM02 is membrane disruption by fungicidal lipopeptides [3].

Lipopeptides

Cyclic lipopeptides are the most thoroughly studied antibiotic compounds of Bacillus, and the most important belong to the surfactin, iturin and fengycin families [2]. Many strains produce more than one family; the crown rot isolate above produced all three [7].

Research examples include iturin and fengycin from B. subtilis acting against Pythium ultimum, and surfactin, iturin and fengycin from B. amyloliquefaciens acting against Sclerotinia sclerotiorum [2].

Competition and colonisation

The ability to form biofilms is essential for colonising root surfaces [1]. Siderophores compete for iron and reduce its availability to pathogens [2], and B. amyloliquefaciens colonises the rhizosphere, stimulating plant growth and suppressing competing pathogens [8].

Induced systemic resistance

Induced systemic resistance from rhizobacteria depends mostly on jasmonate and ethylene signalling [2]. It starts when the plant recognises elicitors from the bacteria, including cyclic lipopeptides, siderophores and flagellin, and volatiles such as acetoin and 2,3-butanediol can also stimulate it [5].

3. Foliar and root uses supported by sources

Many bacterial biofungicides are labelled mainly for soilborne diseases, but some are also labelled for foliar diseases such as leaf spots, powdery mildew and downy mildew [11]. B. subtilis is used against fungal leaf spots, and growers can alternate Bacillus with copper against bacterial diseases to reduce the phytotoxicity that repeated copper sprays may cause [8].

These are research and extension findings, not registered uses. The label registered in each country sets crops, targets and rates.

4. Conditions that affect results

Performance varies: humidity, temperature and competition with native soil and plant microbes can reduce efficacy [12]. Timing matters most. Biological fungicides are preventive and will not cure diseased plants [8], and on foliage they must be in place before infection and be reapplied to protect new growth [11]. In California vineyards, UC IPM advises dropping soft products, biologicals included, when disease pressure is high, because alone they will not give adequate control [4].

Check compatibility before mixing [8]. Biologicals can affect each other too: in a Brazilian laboratory test, a bionematicide based on B. amyloliquefaciens reduced germination of Trichoderma afroharzianum conidia to below 9% [13].

5. Reading a viable count

The content of a Bacillus product is given as colony forming units (CFU), per millilitre for liquids and per gram for powders and granules. In an EU assessment, for example, the liquid product was described by a CFU per mL range and the agent as manufactured by a minimum and maximum CFU per gram [14]. Bacillus forms endospores that survive heat and desiccation and can be formulated into stable dry powders with long shelf lives [5], and endospores survive all processing phases during production [2].

When comparing offers, check four points together: species and strain, the unit (per gram or per millilitre), the guaranteed minimum, and the expiry date with its storage condition [3][8][14]. A count per millilitre of a liquid and a count per gram of a granule are not directly comparable.

6. Formulation forms: SC, WG and WP

Most Bacillus products are stabilised liquid suspensions or dried formulations made from durable endospores [1]. The international coding system defines the common forms [15]:

• SC, suspension concentrate: a stable suspension of the active ingredient in water, diluted with water before use [15].
• WG, water dispersible granules: granules applied after disintegration and dispersion in water [15].
• WP, wettable powder: a powder applied as a suspension after dispersion in water [15].

Bacillus can be formulated into stable dry powders with long shelf lives [5]. Liquid forms such as SC carry their count per millilitre, and dry forms per gram [14].

7. Storage and shelf life

Biofungicides are living organisms with a limited shelf life. Do not stockpile them, watch the expiry date, and keep most products cool and dry [8]. Shelf lives listed for Bacillus based greenhouse products in one extension table range from one to three years [8]. Apply immediately after mixing with water [8].

8. Place in a disease programme

FRAC regards BM02 biologicals as low or very low resistance risk and sets no limit on number, timing or sequence of applications beyond the label and local rules [16]. Biologicals can be rotated with conventional fungicides to reduce resistance risk to single site products [8].

• Start before disease onset or while pressure is low [4].
• Use soil and seed applications for root diseases and preventive sprays for foliar diseases [11].
• Rely on other fungicide groups when disease pressure is high [4].
• Check compatibility with fungicides, copper and other biologicals before mixing [8][13].
• Store cool and dry, use within the expiry date and apply soon after mixing [8].

Frequently asked questions