Doug McInstosh, who submitted this item, writes, "If you look closely on the citrus truck door, the letter reads 'Tustin Hills Citrus Association.'"
Also note that the truck is not a flatbed. That isn't a big trailer on the back -- It's a perfect stack of field boxes -- full of fruit.
There's another 1930s photo of the truck (along with the rest of the fleet) on James Lancaster's "Historic Packing Houses and Other Industrial Structures in Southern California: Virtual Tour of Orange County" website.
Tustin Hills Citrus Association was a growers cooperative, organized in 1909. They had a packing house in Tustin, between Holt and Newport Ave. near Irvine Blvd. They later joined the Southern California Fruit Exchange, which eventually became Sunkist.
The aerial photo below comes from the Tustin Area Historical Society, and shows the Tustin Hills packing house around 1950.