Building a botrytis spray program

Building a sensible disease control program starts way before the spray stage.  Disease control encompasses and understanding of how the pathogen makes its living, meaning where it comes from, how it infects, and what it needs to thrive.

Let’s zoom in on botrytis, as one of the most consequential pathogens we deal with in strawberries.

Strawberries are a susceptible plant to botrytis.  We recognize that the majority of botrytised fruit at harvest was infected as an open flower.  Botrytis, as a generalist pathogen, is common in the environment and spores float in air.  For botrytis to thrive, it primarily needs high humidity/leaf wetness.  It’s flexible about temperature, but is quite happy at the ranges in which we grow strawberries, in both winter and in summer.  A mere 16 hours leaf wetness at 20°C is an infection period for botrytis, for example.  We have little control over humidity/leaf wetness outdoors, but some control under cover.

Considering an understanding of the pathogen and host, we can see why chemical controls focused on blossoms are really important for us.

In strawberries, blossoms are present for MONTHS.  September-December for growers of short day varieties, and September-March for day neutrals.  6 months is a very LONG time, when you compare it to the three week bloom period for a crop like apples.

How can we develop a sensible approach to season-long botrytis control?

Specifically, how can we craft a botrytis management program for the long haul, focusing not just on a the 6-month-long-spray program, but on preserving the longevity of our spray materials for years to come?  Managing resistance becomes quite challenging, and the not-too-distant history is littered with examples of failures in this regard.

A recent forum on resistance-management, hosted by the Lighter Touch program, is reassuring in that the basic principles of crafting a responsible spray program still remain the same.  Applying them in practice remains as challenging as ever.

The fungicides we use fall into three main camps.

1. Single-site fungicides act on a single target molecule, usually an enzyme that’s needed for some critical job in the fungus.  Think fludioxonil, one of the active ingredients in Switch.  They tend to be highly effective but very susceptible to resistance.
2. Multi-site fungicides make life hard for the pathogen in many ways, and often in ways we don’t fully understand. Think thiram, captan….they tend to be older and “broad spectrum” meaning that they work to some degree on many different fungi, not just botrytis.  They tend to be medium in terms of efficacy, but have a low chance of pathogens becoming resistant.
3. Biologicals work by colonizing plant material and competing for resources, spitting out toxins to block their competitors while they’re taking up the same space.

Development of resistance is an evolutionary process.  Evolution selects for individuals (pathogen spores, in this case) that don’t die when hardship (chemical exposure) ensues.  Evolution is largely a numbers game—the more pathogen spores are exposed to the same chemical, the higher the chance that one of them has a mutation that gives it resistance to the action of the chemical.  Crafting a spray program is managing the numbers game.

In practice, managing the numbers game for the single-site fungicides looks like:

1. Optimize the non-fungicide methods of disease control (especially humidity management and inoculum reduction), so there are fewer spores getting exposed to chemical.  
2. Limit the number of applications of the chemical.  This is why the labels of some chemicals give maximum uses per season (indicated in the chart below).
3. Mix a target-site fungicide with a multi-site fungicide.  Example: tank mix the likes of Thiram with the likes of Teldor. Similarly, alternate the use of a single-site product with other products (don’t use Switch three times in row).
4. Act early to prevent an epidemic. Don’t wait to use your arsenal until botrytis is out-of-hand.
5. Optimize the spray coverage, so when you do use a chemical it has the best chance of working because it has reached the target.  Multi-site fungicides are usually not systemic, and coverage is critical.  This season, Berryworld will be testing several wetters in the quest for flower spray coverage in strawberries.

In the SGNZ spray list, we now have the resistance-group for each fungicide.  Fungicides in the same FRAC group (“Fungicide Resistance Action Committee”) act on the same target (roughly), so alternate between FRAC groups.

Table 1 Note the crossed out fungicides that no longer work on botrytis due to resistance

Note that while Teldor and Prolectus are different chemicals, they are in the SAME FRAC group.  Alternating Teldor with Prolectus (both FRAC 17) is NOT a rotation.

Already lost through resistance:

Resistance to the cyprodinil ingredient in Switch is developing in NZ.  Botrytis has become resistant, globally, to Topsin (thiophanate-methyl) as well as to carbendazim.  Notice that Pristine (boscalid + pyroclostrobin) is also absent from the list, because of resistance.  Boysenberry growers did resistance-testing of botrytis to Rovral years ago and have subsequently stopped using it because it doesn’t work. Interestingly, resistance to Rovral tends to die away in a botrytis population after some years of non-exposure to FRAC group 2 (which includes Sumisclex), but we suspect we’ve never given it that break in NZ, and that resistance continues to make those two chemicals useless on botrytis.

Strawberry flowers are most susceptible to botrytis two days after opening, as flower parts start to die.  Ideally, to protect every flower, we might like to have a good botryticide going on every other day!  However, spraying every other day is not practical.  

For a botrytis rotation, a grower might choose to start with Prolectus (mixed with Thiram or Captan) as it has the longest WHP.  Then alternate between Esteem and Switch with another Teldor slotted in there somewhere (again, with a multi-site fungicide mixed in each time).  However, it quickly becomes apparent that strawberry growers run out of options after 9 applications if they use the “top shelf” single site options at every spray.  We therefore recommend saving those options for rainy weather or for big flushes of blossoms, and using the multisite fungicides alone in between.

In Summary:

• Plan a smart season-long botrytis spray program with longevity in mind
• Focus on good control early in the season
• Target wet weather periods with the best botryticides we have, filling in with broad spectrum products in between
• Success still depends on luck with weather during bloom

‍