The measurement of rapid and microsize changes in fruit diameter can be used to understand how plants respond to diurnal variation in water content and long-term growth conditions. The most current techniques involve physical measurements. The contact of the physical sensor places a stress on fruit and affects normal fruit growth. To solve this problem, we present a noncontact optical method for measuring fruit diameter in crop fields accurately. A rough-to-fine strategy is considered, where a binary image is first obtained and used to find candidate fruit body edge points; then a Zernike moment operator is used to determine edges of the fruit body with subpixel accuracy. Finally, the fruit diameter is computed from the edge pixels of the fruit body. Measuring experiments performed during the bloom stage of grapes show high sensitivity of the proposed method. This allows for clear detection of diurnal patterns of grape diameter changes and precise monitoring of very slight variations in fruit growth rates. Experiments show that the developed system is robust, accurate, and effective. The proposed technique has proven to be an effective tool to better detect physiological disorders in plants.