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

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Research Poster

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Due to the recent drought in Southern California, the abundance of a keystone species, Malosma laurina, within the Santa Monica Mountains (SMM) has been dwindling. This species has a rooting depth that exceeds 13 m, so it typically has little difficulty tapping into the water table. Recently, this species has undergone extensive dieback, which might be attributed to the unprecedented drought in California and predisposition fungal pathogen Botryosphaeria. 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 and warmer 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 compared two areas of fungal dieback: those found in the field and those cultured in a laboratory growth chamber. We took samples of the fungi in twenty four plants located in an area of heavy dieback, grew them in pure culture, inoculated them into the living plants in the growth chamber or in the field, and tested the growth to see if the fungi had invaded the xylem after thirty days of exposure. In the field study we inoculated 12 branches, one from each plant, and sham inoculated 12 branches as a control. In the growth chamber the same procedure was followed but using 25 plants. Half watered every day and the other half every two days. We found that both water stressed and hydrated plants were affected by the fungus equally, and were able to successfully replicate Koch’s postulate in the field and in the lab (growth chamber). It was concluded that the fungus B. dothidea was the causative agent of recent M. laurina dieback within coastal exposures of the Santa Monica Mountains.

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