November 2021 update
3 Base Pages
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:
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.
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) |
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.