Bronzing caused by frost

/ Gerald Holmes
Figure 1. Mild bronzing caused by frost on white fruit of cultivar Monterey. This level of bronzing will be harder to detect when the fruit turns red.

In my last post, I showed a bunch of photos of frost injury. Later I noticed another effect: Bronzing (Figs. 1, 2 & 3). Just like misshapen fruit can be caused by damage to the developing flower parts, bronzing can result from damage to the epidermal layers of developing fruit. I was surprised to learn that several prominent California strawberry researchers published three papers on bronzing (see references below). They classified the causes of bronzing into three types:
• Type 1 caused by arthropod feeding (e.g., thrips or mites);
• Type 2 caused by chemical burn (e.g., sulfur); and
• Type 3 caused by high temperatures and solar radiation.
Now we can add low temperatures to Type 3 bronzing and just call it “temperature extremes”.

Figure 2. Severe bronzing from frost in cultivar Monterey. Bronzing is more conspicuous on white fruit due to the contrast of the background color.

What I saw in our research plots at Cal Poly in ‘Monterey’ and ‘Albion’ was also present in Santa Maria fields (east side) in ‘Fronteras’ and ‘Monterey’. The bronzing looks the same regardless of the variety involved. It looks like the epidermal layers of the fruit are damaged and become darkened and rigid. If further expansion of the fruit occurs, the fruit will crack, especially on the shoulders near the calyx. The degree of bronzing varies a lot depending on the temperature and its duration which is influenced by the position of the fruit on the plant.

Although we can have frost each year, the temperature and duration is generally such that not much injury occurs. Fields nearest the coastline are safer and the degree of safety declines with the distance from the coast. Also, the effects are generally short-lived as the affected fruit are picked and discarded — followed by another flush of fruit. If we get a hard frost, all the fruit and blooms can be affected and this will eliminate any yield for about 25 to 30 days. This occurred a couple of years ago.

Something else we don’t think about as much for fruit production as we do for plant production in nurseries is that chilling of strawberry plants below 44 F increases plant vigor. So, growers will see increased plant vigor following frost. This doesn’t compensate for the loss of fruit, but we’ll take the small win anyway.

Figure 3. Bronzing on mature fruit showing cracking around the fruit shoulder in cultivar Monterey.

References

  • Koike, S. T., Zalom, F. G. and Larson, K. D. 2009. Bronzing of strawberry fruit as affected by production practices, environmental factors, and thrips. Hort. Science 44:1588-1593.
  • Larson, K. D., Koike, S. T. and Zalom, F. G. 2004. Bed mulch treatment affects strawberry fruit bronzing and yield performance. Hort. Science 40:72-75.
  • Polito, V. S., Larson, K. D. and Pinney, K. 2002. Anatomical and histochemical factors associated with bronzing development in strawberry fruit. J. Amer. Soc. Hort. Sci. 127:355-357.