Anatomical and Histochemical Factors Associated with Bronzing Development in Strawberry Fruit

Bronzing of strawberry (Fragaria ×ananassa Duchesne) fruit that is not the result of arthropod feeding or chemical spray application occurs frequently in California's central coast strawberry production region from late spring through midsummer, a period characterized by relatively high temperature, low relative humidity, and high solar irradiance. The cause of this phenomenon is not known, but in preliminary trials, intermittent, midday misting of plants and increased drip irrigation rate resulted in reduced incidence of fruit bronzing. To characterize the bronzing phenomenon and its development in strawberry fruit tissues, we conducted an anatomical and histochemical examination of bronzed fruit. Bronzed and nonbronzed fruit were sampled over a range of fruit maturities. Results show that bronzing derives from a lesion at the cortical surface early in the fruit's development. Epidermal cells become radially compressed and the cell contents coalesce into a densely staining mass. The cuticular layer becomes disrupted and discontinuous. As the fruit develops, densely staining materials, possibly phenolic precipitates, accumulate within subepidermal cells of bronzed fruit, subepidermal cell walls thicken, and intercellular spaces fill with pectic substances and other densely staining materials. Results are consistent with reports of sunscald injury from other fruit species, and raise the possibility that strawberry bronzing occurs in response to heat or solar radiation injury. to warm the soil and enhance plant growth, also reduced Type III bronzing (Larson and Sjulin, 2001). These observations suggest that environmental and plant physiological factors may be important in development of Type III fruit bronzing. Little is known about how Type III bronzing progresses in the strawberry fruit. In an attempt to better understand the phenomenon, we initiated the present research to characterize strawberry bronzing and its development. Bronzing of strawberry fruit often occurs in late spring and summer in the central coast strawberry production region of Califor- nia (lat. ≈36.5 to 37°N.), an area that produces ≈40% of the U.S. strawberry crop (Processing Strawberry Advisory Board of Califor- nia, 2001). Symptoms consist of discoloration, desiccation, and cracking of the fruit surface. Damaged fruit is typically tan, brown or yellow-brown in color, hence the term "bronzing." The disorder occurs annually to some extent, but timing of its occurrence and severity varies from year to year. In some years, bronzing results in serious economic losses due to production of large quantities of unmarketable fruit (Larson and Sjulin, 2001). Three distinct types of strawberry bronzing damage can be recognized (Larson and Sjulin, 2001). Type I bronzing results from feeding by western flower thrips (Frankiniella occidentalis Pergande) or cyclamen mites (Phytonemus pallidus Banks), and typically produces damage to the fruit (receptacle) tissue around the indenta- tions surrounding the achenes and beneath the calyx (Maas, 1998). Type II bronzing results from chemical spray burn, often as a result of application of sulfur products. In both cases, bronzing is observed in localized areas of the fruit surface that have been directly exposed to arthropod feeding or chemical contact. Type III, which typically occurs from May through July in California's central coast region, covers the entire fruit surface and is not associated with spray applications or arthropod feeding. The occurrence of Type I and Type II bronzing is readily correlated with arthropod activity or spray applications, and is localized and episodic in nature. In contrast, Type III bronzing often occurs during extended periods in late spring and summer, a period characterized by long days, high irradiance, high temperatures, and low relative humidities. Also, Type III bronzing is observed more frequently inland than in coastal areas where lower temperatures and higher relative humidities prevail. In preliminary studies, mid- day microjet misting of plants and increased drip-irrigation rates resulted in reduced Type III fruit bronzing (Larson and Sjulin, 2001). Use of clear polyethylene bed mulch, applied in early winter