Controlling Foliar Diseases With Microbial Fungicides

Lettuce Red, Powdery Mildew Disease Griffin Greenhouse Supplies
Powdery mildew on red lettuce

Lots of interesting terms surface when the discussion turns to “softer” approaches to pest control. Terms such as these can all be correctly applied to certain pesticides: microbial, beneficial, bio, biochemical, reduced risk and organic. The focus of this discussion will center on a group of microbial fungicides that have proven to be very useful for the greenhouse and nursery industries.

Although the products mentioned below are also labeled for certain root and crown-area diseases, foliar diseases will be our focus. They’re referred to as microbial fungicides because their active ingredients are biological organisms — in this case, different types of bacteria that utilize various modes of action to prevent and control foliar plant diseases. Cease and Companion contain different strains of Bacillus subtilis; Actinovate SP uses a different bacterium called Streptomyces lydicus.

To be sure, there are safe and effective traditional foliar fungicides available to greenhouse and nursery operators. Some of them even share one or two of the attributes of microbial fungicides that are listed below. So why might a grower consider a microbial fungicide? Two of the three products mentioned are OMRI-listed, meaning they appear on a list of products approved for use in the organic production of food and fiber. Many microbial fungicides have broad labels allowing them to be used on a wide range of ornamentals and many of the edible crops that continue to gain popularity. Resistance is possible with any pesticide but is probably less likely with the modes of action used by these microbial fungicides.

Primula acaulis, Botrytis, Disease, Griffin Greenhouse Supplies
Botrytis on primula

All of these products require short re-entry intervals (REIs) and modest amounts of personal protection equipment (PPE). As always, read and follow the entire pesticide label! There is general agreement that the buying public wants our industry to use the safest products available and these products should be viewed in that light.

Modes of action used by microbial fungicides are not always completely understood. Often, more than one mode of action is employed by the same organism, a helpful trait in terms of fending off pesticide resistance. Some of the more common modes of action include:

Antibiotic: Compounds are produced that have an inhibiting effect on plant pathogens. An example would be the creation of compounds that inhibit certain fungal and bacterial spores from germinating. This is probably the most important mode of action for foliar applications.

Competition: The beneficial organism depletes the treated area of “food” needed by the pathogen and thereby out-competes it for that location.

Exclusion: In this case, the beneficial organism grows in such a way that it populates the same area where the pathogen would try to become established. By getting there first, it physically excludes the pathogen.

Parasitism: The beneficial organism feeds directly on the pathogen, inhibiting it or killing it altogether.


Microbial FungicidesTrade NameShelf LifeOMRI Listed?
Primary Modes of Action
* primary mode of action utilized for foliar applications
Streptomyces lydicus, WYEC 108Actinovate SP T&O18 monthsYesExclusion, parasitism, *antibiotic
Bacillus subtilis, QST 713 strainCease2 yearsYes*Antibiotic, some competition
Bacillus subtilis, GB03Companion Hort Label2 yearsNo*Antibiotic, some competition


Other attributes these products hold in common:

• Based on modes of action, they should be considered primarily preventative fungicides.

• Tank mixing with a wide selection of other fungicides and insecticides is possible. Most bactericides should not be used; check with your supplier for compatibility information.

• Excellent plant and bloom safety, though it is always best to trial on a small number of plants before widespread use.

To ensure maximum success with these microbial fungicides, first compare labels to be sure they’re suitable for the diseases you wish to control. They are relatively short-lived on leaf surfaces so pay close attention to re-application intervals. As with other types of fungicides, managing the growing environment to reduce disease pressure is essential. Even the best fungicides will fail if growing conditions are favorable for disease development.


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