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Strawberry quality affected by sun, rain

The California strawberry industry is renowned for the production of large amounts of high quality fruit over a long growing season. To produce excellent fruit, strawberry growers devise strategies to handle numerous production, fertility, irrigation, and pest management challenges.

The weather remains beyond the control of growers. The strawberry crop is subject to field and climatic conditions which can reduce strawberry quality.

Two specific field situations occurred in 2010 which provide illustrations of the impact of weather on strawberry production.

Phytophthora leather rot

California had a fairly wet spring this year with rain storms marching statewide into the month of May. The rain events provided conditions which allowed for the development of leather rot disease of strawberry fruit in the Watsonville-Salinas production area.

Caused by Phytophthora, leather rot disease resulted in significant fruit yield loss in some fields. Leather rot can affect immature green or pink fruit, plus fully ripened red fruit.

Symptoms consist of off-white, gray, or yellow-brown lesions. The lesions often begin as localized, circular-to-oval-shaped infection areas that later enlarge into irregularly-shaped patches which can affect much of the fruit. The infected area is very soft to the touch. Fruiting bodies or other fungal structures are not seen externally on the lesions.

Few control measures are available when the rain splashes and disperses the pathogen spores. Once the rains cease, drier conditions can prevent Phytophthora leather rot from further yield reduction.

Strawberry fruit bronzing

A second strawberry fruit problem triggered by the wet spring was seen in June and July. Strawberry fruit bronzing refers to the tan or bronzed discoloration that occurs on green or mature strawberry fruit. Bronzed fruit has dry, rough surfaces that render the fruit unmarketable.

The skin of the fruit can crack later. The bronzing can cover the entire surface of the fruit. There are three types of bronzing. The current concern is Type III bronzing which is associated with stressful environmental conditions caused by extreme solar radiation, high temperatures, and low relative humidity.

A series of field experiments documented the cause of Type III bronzing. Plant stress was reduced by overhead misting or sprinkling on the foliage and fruit, consistently resulting in reduced bronzing.

In contrast, in insecticide-treated and untreated field plots, bronzing was equally severe in all the plots despite reduced thrips populations in plots that received insecticide applications. This and other studies suggest that thrips are not involved with Type III bronzing.

A final component of the bronzing research was tied to observant growers and pest control advisors. They noted fields which received insecticide or fungicide sprays prior to a bronzing period. In many cases these fields had less bronzing compared to adjacent untreated fields.

Pesticides contain additives that protect active ingredients from solar and ultraviolet radiation. Pesticides might protect the strawberry surface from damaging radiation. When commercially-available pesticides were applied to field plots, bronzing was significantly reduced compared to the untreated controls. Bronzing was also reduced when lignin products were applied to the plots. Lignins are by-products of the paper industry often added to pesticide formulations to shield the active ingredients from solar and ultraviolet radiation. The lignin sprays protected the fruit in the trials and reduced bronzing damage.

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