Prevalence of Airborne Spores Causing the Spread of Fungal Disease in Laurel Sumac, Malosma laurina, in the Santa Monica Mountains

At the trailing end of California’s historic drought in 2012-2016, we investigated the spread of spores from the fungal pathogen in the native chaparral plant laurel sumac, Malosma laurina, in the natural landscape of the Pepperdine University campus in Malibu, a coastal exposure of the Santa Monica Mountains. We predicted that the fungal disease would spread by way of spore dispersal from a previously studied dieback site above the Dresher Campus, to an intermediate site above the Law School Housing, and to a healthy control site receiving indirect water supplements from adjacent irrigated lawns. We hypothesized that that the average amount of spores would be higher in more fungal abundant locations (Dresher Campus and Law School site). We found that the fungal disease did indeed spread to an intermediate site where early stages of dieback were in progress and with similar percentage of stems (82 – 98%) containing the fungal pathogen, Botryosphaeria dothidea, cultured from xylem tissue. In contrast, healthy controls had an incidence of the fungal pathogen in only 12% of the sampled stem-xylem. The average amount of spores trapped at each sites per week did not support our original hypothesis with 105 spores per week at the healthy control site versus 8 spores per week at the dieback site. These results are consistent with the theory that it is not the presences of high spore counts that leads to severe dieback and whole plant death but the predisposition of plants to fungal infection and progression of disease in response to the recent 2012-2016 drought. The observed dieback in Malosma laurina appears to be a byproduct of erratic changes in climate now being experienced in southern California