The symbolic value of the desert represents the lack of hope in America at the time. When I see the desert and Dust Bowl conditions presented, I see areas that were once fertile and vibrant representing a sense of desolation and the embodiment of barrenness. On a more symbolic level, Steinbeck's use of the desert might also be exploring the idea of the promises of capitalism for many. The lure and assurance of wealth and material prosperity of capitalism was something that was barren and empty during the 1930s. The spirit of "striking it rich" that had preceded this time period for so long dried up, similar to the desert. Certainly, the physical setting for the Joads' journey is one use of the desert, but I see the concept as relating to much more about the economic and social condition of the Great Depression.
The harsh conditions of the desert symbolize the harsh conditions the Joads will face once they get deeper into California. In the desert there are "little starved bushes", which foreshadows the problems of hunger that will arise. The problems the Joads face will heighten just as the light of the sun reddens in the desert. The experience the Joads will face in California will be essentially as difficult as crossing the desert by walking.