Recent Dieback in a Keystone Chaparral Species is Caused by a Fungal Pathogen, Botryosphaeria dothidea

Due to the recent drought in Southern California, the abundance of a keystone species within the Santa Monica Mountains (SMM) has been dwindling. This species, Malosma laurina, has an average root depth of 40 feet, so it has little difficulty tapping into the watershed. Recently, this species has undergone extensive dieback, which is potentially attributed to the unprecedented drought and the introduction of the fungus B. dothidea. While this fungi appears to be innocuous to M. laurina when the plant is healthy, it is more susceptible to invasion under water stress due to the dry heated conditions of the drought. Following Koch’s postulate to identify that a pathogen was the cause of disease, we set out to determine whether this fungus was the causative agent of the recent dieback. We analyzed fungal dieback in the field at two differently hydrated sites. We took samples of the fungi in twenty four plants located in an area of heavy dieback, grew them in pure culture, inoculated them in situ, and tested the growth to see if the fungi had invaded the xylem after thirty days of exposure. Then, at the two field sites, we inoculated 12 branches, one for each plant, and left 12 branches as controlled samples to compare. We found that both water stressed and hydrated plants were significantly affected by the fungus. We, however, were not able to replicate Koch’s postulate in full. It was concluded that the fungus B. dothidea was the major cause of the recent M. laurina dieback within the Santa Monica Mountains