Botrytis in Strawberries

written by Molly Shaw, Berryworld

Just the mention of fuzzy berries is enough to put a strawberry growers’ blood pressure up, but it’s worth a deep dive into the damp world of this ubiquitous mold to strategize better controls.

Botrytis is an issue in many crops.  This fungus is happy to eat just about anything dead, dying, or weak.  Its spores need surface moisture to infect, but it only takes a couple hours at 18-23 C (its optimal temperature range) to infect.  An overnight dew in summer will do it, let alone a rainy day. 

Blossom infections are the most insidious, as infections can be invisible for 3-4 weeks.  The fungus starts up again as fruits soften during ripening, and we get that classic calyx-end botrytis fruit rot.  Any part of the strawberry plant can become infected and the fruit can become infected at any stage of development, but most moldy fruit are the result of infected flowers. 

For infections to take place, you need three things:

  1. Fungus present
  2. The right environmental conditions
  3. A susceptible plant

Interfere with any of those factors, and infections become less likely. 

Fungus: Botrytis is ubiquitous, so the airborne spores are always present in production fields at some level.  However, having a lot of infected plants around poofing spores into the environment definitely makes matters worse.  There’s a bit of debate about how cost effective removing infected material is in preventing botrytis in a commercial setting.  Researchers in North Carolina figure that the time to do it is just before bloom starts, and that it shows the most benefit in systems that aren’t using fungicides (organic).   Any plant handling to remove infections should definitely be done separately from harvesting.

Environment:  You can’t change the weather, but you can reduce the microclimate humidity by orienting plant rows parallel to the prevailing wind and keeping good control on weeds.  Also, it’s important that beds are shaped so that water runs off the bed rather than puddling on the plastic.  Consider increasing plant spacing, especially with vigorous-leafed cultivars.

Susceptible Plant:  Strawberry flowers and fruit are naturally susceptible to botrytis, but there are definitely management factors that make them much more likely to get attacked. 

  • Overwatering plants, especially in cloudy weather, leads to softer cell walls and more botrytis
  • Excess N fertilization makes softer plants. Nitrogen form plays a part as well; ammonium in many crops promotes softer more “vegetative” growth, which in strawberries makes them more susceptible to both botrytis and anthracnose.  When liquid feeding strawberries, the nitrate form of N should make up the vast majority of the N fertilization. (Note: Within soils, nitrogen changes from form to form depending upon microbial activity and climatic/environmental factors. Urea is changed by soil organisms to ammonium, and if when there’s plenty of oxygen the nitrifying bacteria change it again into nitrate.)
  • Calcium is an important component of strong plant cell walls, and as such plants that are low in calcium make fruit with weaker cell walls that are more susceptible to both bruising and botrytis. This is true in both soil culture and tabletops. 

If you’re not too sure whether your fertilizer practices are delivering ideal N or Ca levels, leaf testing can give you a read on that.  Be sure to also take a soil test, to check on the pH as well as various other nutrient levels that can hinder Ca uptake if they’re too high.  Hill Labs has instructions on how to leaf sample at https://www.hill-laboratories.com/assets/Uploads/Strawberry.pdf

Fungicides to control Botrytis
It rains.  Strawberry flowers are inherently susceptible to botrytis, which is ubiquitous.  Of course we also use fungicides to fight botrytis.  Interestingly, trials in California found the best fungicides to be just over 60% better than no sprays.  Even the best fungicides can’t compare to dry sunny weather for botrytis control!

The following materials are listed in the approved list for strawberries being sold in NZ, in order of efficacy (highest at top).  See the full Strawberry Withholding Periods list for details of withholding periods for various export markets.

Pesticide

Example trade name

Resistance code

NZ withholding (days)

Efficacy 3

cyprodinil/fludioxonil

Switch1

9/12

3

++++

fenpyrazamine

Prolectus

12

7

A bit better than fenhexamid

procymidone

Sumisclex

2

1

As good as Prolectus or nearly useless, depending on resistance status of pathogen

fenhexamid

Teldor

12

1

+++

thiram

Thiram

M3

2

++

polyoxin D zinc

Esteem

19

1

++

Ulocladium oudemansii

Botryzen

Biological

 

We have found it similar to Thiram in strawberry efficacy work in Canterbury

chlorothalonil

Bravo

M5

1

A little less than thiram

captan

Captan

M42

1

+ (resistance)

  1. For strawberries, Switch may be used only 2 times/season if 6 or fewer botrytis sprays are used, 3 times if more than 6 botrytis sprays are used. Many other brands are available.
  2. “M” resistance codes mean “multisite.” They have a low chance for fungicide resistance to develop, typically offer broad spectrum control for many different pathogens, but often aren’t the strongest on specific pathogens (botrytis in this case).
  3. Efficacy determined by California Poly Strawberry Centre botrytis trials (9 trials over 5 year period, reported 2019). Materials without efficacy ratings weren’t in California trials, but are slotted in order of efficacy from Geoff Langford’s experience (Berryworld).

The best material currently available to us is a cyprodinil/fludioxonil mix (such as Switch), but it has a limit on the number of times it may be used in a season.  Strategically this could be used to protect blossoms when fruit will be worth the most in the market—early season, or late season. 

There are various other fungicides sold in NZ, labeled (and effective!) for botrytis on strawberries in other countries, but without an established NZ residue tolerance.  Merivon and Pristine come to mind.  We can’t use these, however, because the residue tales on these products are long, and likely will not come under the NZ default tolerance (when no official tolerance is set) of 0.1mg/kg.  Two options for the future are registration or establishing a residue tolerance without a label claim. 

References:
Louws, 2018.  Botrytis Fruit Rot/Gray Mold on Strawberry, North Carolina Extension.  https://content.ces.ncsu.edu/botrytis-cinerea-botrytis-fruit-rot-and-blight-on-strawberry

Blauer.K. and Holmes.G.  2019.  Comparison of fungicide efficacy generated from Cal Poly Strawberry Centre Botrytis fruit rot efficacy trials.

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Botrytis in Strawberries

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