Optimizing Biological fungicides

By this time in the season, growers may have run out of their “top shelf” botryticide options, as they all come with restrictions around maximum applications/season to manage resistance-development.

In a previous article, we had charted out the botryticides typically used in strawberries in NZ (see https://berryworldnz.wordpress.com/2025/01/06/botrytis-problems-in-canterbury-strawberry-trial/. The article has been updated regarding the interpretation of the botrytis risk period model–there are TWO high risk periods for botrytis infections to develop, one at flowering and the other when the fruit is ripening. In our harvest with botrytis problems, ripe fruit had been rained on as flowers, and rained on again the week of ripening.

When the traditional fungicide options have run out, growers may look towards the biological botryticides available to fill the gap later in the season.

To be honest, we haven’t been very excited about biological fungicide efficacy on botrytis.  Botrytis is a particularly hard nut to crack because it’s ubiquitous, grows rapidly, and grows at lower temperatures than biologicals.  Cal Poly has a multi-year summary of fungicide efficacy trials in biologicals and conventional fungicides available online at https://content-calpoly-edu.s3.amazonaws.com/strawberry/1/documents/NASGA%20Poster%202023%20Blauer.pdf  Note that this list doesn’t include a review of Trichoderma efficacy, which is a product we do have available in NZ and for some diseases, has worked quite well.

If you’re going to use a biological fungicide, there are some aspects about using them that are particularly important to keep in mind for the best chances of success.

• Mode of action: most biologicals work by colonizing the plant surface, taking up space and outcompeting other organisms for nutrients.  Some also exude chemicals that inhibit their competition.  Inherently, they are preventative surface colonizers, not systemic curatives, so they must be in place before the pathogen gets a foothold, and excellent coverage is critical.
• Storage:  some require refrigeration, and there will be an expiry date to keep in mind.
• Application: it’s best to avoid sunny and dry times of day—apply in evening or early morning.  Use an adjuvant for UV protection (such as NuFilm or Raingard).
• Temperatures:  Many of the biologicals need around 15 degrees minimum to grow actively.  This is not such a limiting factor in the summer, but can be quite limiting for spring and autumn.  Remember, we’re hoping that these organisms can outcompete botrytis, so we need to use them in the warm temperatures when they have a chance to do this.
• Compatibility:  This varies widely with biological fungicides.  After contacting the manufacturers, the following table outlines compatibility for a couple of the biological fungicides with label claims for botrytis in NZ.

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