Gold? Silver? Bronze?

This type of “bronzed” look is caused by stressful growing conditions, namely high heat and/or high irradiation, often associated with low humidity.  I most often see it when strawberries are grown under cover.  In this case, “high heat” can be a couple hours at 30°C.  One grower had a sudden appearance of the defect after the greenhouse roof was closed one afternoon to apply the shading.

It’s easy to mistake Type 3 bronzing for thrips damage, which can also make the fruit bronzed.

Type 3 bronzing happens evenly around the whole fruit, whereas bronzing caused by thrips or mite feeding (“Type 1” bronzing) is usually less even.  With a hand lens, thrips feeding near the achenes also shows up as whitish “empty” looking cells that have been sucked dry of their contents, (to me they look a little like Styrofoam).  Typically there are plenty of thrips are to be seen under the calyx.

California suggests a threshold of 10 thrips per flower (tapping-out flower methodology) before insecticide treatment, which suggests that it takes quite few thrips sucking out cells to get thrips-induced fruit bronzing.

Managing the Type 3 bronzing which is caused by heat and strong sun is not easy.  Research suggests that mist-cooling is helpful, particularly under low humidity conditions.  Interestingly, pesticide sprays before the heat episode can lessen the damage, because many pesticides are formulated with additives that protect the chemicals from solar radiation, and will help protect the fruit as well.

Read more about Type 3 bronzing in University of California’s article https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=2942

Mild frost can also cause Type 3 bronzing, https://strawberrycenterblog.com/2022/03/24/bronzing-caused-by-frost/

Given the mentions of Type 1 and Type 3 bronzing, I bet you’re wondering about Type 2 bronzing….that’s the one you can get from chemical burn, and would typically be more on the upper side of the fruit exposed to more spray.

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