Background: In the US alone, the citrus industry generates billions of dollars annually from sales of fresh fruits and juices. The presence of California stubborn disease (CSD) in arid environments, such as California, Arizona, and Mediterranean regions, has affected many of the major commercial citrus species. CSD, caused by the pathogen Spiroplasma citri, creates significant tree damage and loss of fruit production and quality. Diagnosing this disease is challenging and there are no practical cures. To ensure clean source trees for the industry, new and rapid diagnostic methods to detect CSD and similar diseases are urgently needed. Description: UCR researchers have developed a novel diagnostic method for detecting CSD. This method is able to detect the presence of a protein secreted from S. Citri that can be used as a marker for CSD. This invention opens the door for pathogen-secreted proteins to be used as indicators of other citrus bacterial diseases, such as HLB.