Once inside the leaves, bacteria that come into contact with plant cells use type III secretions to inject pthA, a protein signal that essentially tricks the host cells into accelerated meiosis and programmed death, according to Gabriel. Exactly how pthA overrides or circumvents signals from the host cells is the subject of intense research, he says. Excessive division causes cells to burst through the plant's epidermal layer, creating the infection's namesake cankers and allowing the bacteria access to the plant surface where they can be picked up by wind and rain, continuing the infection cycle. The bacterium also exudes an oily residue that swells when wet, allowing further bacterial oozing onto tree surfaces during the most opportune conditions for spreading. While X. citri does not readily kill its host, it can lead to defoliation and early fruit drop in heavily infected trees.
Infected fruits are still edible, but are severely blemished by the brown, corky lesions.
Other methods provide mediocre control at best. For instance, disinfectants can kill bacteria that hitch rides on equipment and clothes, but they are not effective on infected trees because they cannot reach below the surface. Similarly, a copper sulfate spray can check bacteria on tree and fruit surfaces, but has no effect on bacteria hiding within the plant tissues. Moreover, widening infestation by the Asiatic leaf-miner, a tiny insect that tunnels through leaves, can lead to deeper entrenchment of the bacteria in tree tissues, further frustrating control efforts, Gabriel notes. Breeding citrus plants for resistance to X. citri would seem to be an obvious route for overcoming the current epidemic.