Bronzing strawberry fruit

The first thing that comes to mind when strawberry fruit get that “bronze” tinge is thrips damage (“Type I bronzing”), but this particular instance didn’t have the right pattern, nor were there thrips hiding out under the calynx. Large areas of the greenhouse had damaged fruit and they all started showing damage at exactly the same time.

The second thing that comes to mind is spray damage (‘Type II bronzing”), but again, the pattern wasn’t right. Not a single fruit had more damage on the exposed side, nor could we find any “spray shadow” outlined by a calynx leaf or flower petal. Fruit were uniformly bronzed shoulder to tip, undersides and top sides.

Turns out that there’s a “Type III bronzing,” and it’s caused by heat stress. The grower remembers that the greenhouse vents were closed to whitewash the roof, and the symptoms appeared after that. Bingo.

The heat apparently causes some epidermal cells to leak their contents, pectin among them, into the intercellular space. The damage isn’t deep, it’s only in the outer cell layers, but it’s enough to blemish the fruit. Cooling with sprinklers has been shown to reduce the problem, as does adequate irrigation. Good ventilation is, of course, a must.

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