Spray applications of plant growth regulators, particularly 4-chlorophenoxyacetic acid–4-CPA–are widely used in some parts of California as an aid in obtaining set of tomato fruit during periods when the environment is unfavorable for natural fruit production. This practice is followed to a considerable extent in the tomato producing areas of San Diego County during the spring and fall months when night temperatures drop below the level favorable for the growth of pollen tubes and normal fertilization of the ovary.