Xylella fastidiosa (Xj), a Gram-negative, xylem-limited bacterium, is the causal agent of several economically important plant diseases, including Pierce's disease (PD) and Citrus Variegated Chlorosis (CVC). Identified is a HccA-like hemagglutinin gene in Xylella fastidiosa involved in the virulence of the pathogen. In essence this protein is a "molecular glue" that specifically attaches to the surface of Xylella fastidiosa causing Xylella fastidiosa cells to form aggregrates. If this protein is expressed in trans-genic plants, this protein could cause greater aggregation of Xylella fastidiosa cells in planta, thus slowing down the movement of Xylella fastidiosa and decreasing disease symptoms. The protein can also be introduced into the plant by inoculation with a plant endophyte which expresses and secretes a HecA-like hemagglutinin. Thus plants containing increased levels of a Xylella fastidiosa HecA-like hemagglutinin protein could have an increased level of field resistance to disease caused by Xylella fastidiosa