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We harvest the Canterbury strawberry variety trial weekly. Last week, 27th December, is the first harvest where botrytis has been consequential.
We’re seeing classic calynx-end rot that would have started with a flower infection that sat there latent until the fruit started to ripen. We even have some where the fruit looks normal on the outside, but when you bite into it, there’s the beginning of the musty brown rot inside at the calynx end.
This spring we did some careful counts of the time between open flower and ripe fruit. In our greenhouse environment (12 degree nights, days in the high 20s), it took about 28 days for a king blossom to go from fully open to ripe fruit (450-525 growing degree day, base 4°C). It took a little longer for the secondary and tertiary blossoms to develop to ripe fruit, around 33 days.
We can look back at our weather data and look for the place where we had the initial botrytis infection period, around 16-21 November, when we had 6.3mm of rain and sprinkles on the days around that.
Outside in the Canterbury spring it takes longer to ripen fruit than it did in our greenhouse with summer weather; our recent harvest was 36 days past the botrytis infection period on 21st November.
Below is the botrytis infection model visualized by Tau Research through their dashboard, based on a leaf wetness sensor in our trial plot. We didn’t have the leaf wetness sensor installed properly until late November, so we can’t see how the mid Nov rain affected our botrytis model. However, the high risk periods Dec 16th and 20th, just prior to our “problem harvest,” show up clearly.
Monika Walter (Plant and Food Research) says that when a flower is infected with botrytis and it lies latent in the developing fruit, if wet weather coincides with fruit ripening, the brewing botrytis problem is compounded. Perhaps we had a perfect storm here–fruit with latent infections from their bloom period that were getting soft cell walls and higher sugars (ripening!) when it rained heaps again.
We should have applied a botryticide to blossoms open during early season infection periods. If we were using a protectant like thiram or captan, we would have wanted it on ahead of the weather, while we could go in just after the first rain if we were using something like Switch or Teldor.
One of the reasons we were paying close attention to flowering phenology in the spring was to work out a reasonable spray application frequency, if we were trying to get every flower protected while it is open. We figured there’s about a 5 day window on every flower when it is open (anther’s exposed) but before the green fruit starts developing and covering up the dried-out anthers. Once the green fruit is covering up the senesced anthers, we reckon spray coverage isn’t going to work.
Above we have labelled the open flower date for all the fruit on a truss.
Pictured in a chart for a single flower truss, here’s how the stages plot out:
In this graphic, yellow and orange are fresh and senescing anthers, respectively: the spray window. Red is ripe fruit. I’ve marked dates in blue as reasonable spray dates for this particular truss.
Not surprisingly, this works out to spraying about once a week (based on development with 12°C nights and 27°C days).
Of course, the elephant in the room is that if strawberry growers are to spray a botryticide once a week, we run out of good botrytis options while there is still more than half the season to go. See article from September, “Building a Botrytis Spray Program.”
There’s no botryticide as good as sunshine…. This is underscored by our experience with the Lincoln trial, where we’re really only running into serious botrytis at the end of December, while we’ve been harvesting for two months. When the weather is dry and sunny during the flowering phase of a fruit, it’s much less likely to get botrytis later, though of course we can still get secondary infections such as when the fruit sits on moist media.
Growers may look towards the biological botryticides available to fill the gap later in the season, when the traditional fungicide options have been used up. A subsequent article will outline things to keep in mind when using biological fungicides.