Between 2012-2016, southern California experienced unprecedented drought that caused dieback in Malosma laurina, a keystone species of chaparral shrub communities. Dieback was especially severe in coastal exposures of the Santa Monica Mountains, leading to whole plant mortality exceeding 50% at some sites. We hypothesized that the endophytic fungus causing the dieback, Botryosphaeria dothidea, was successful in invading the xylem tissue of M. laurina because of protracted water stress, carbon starvation, or a combination of the two. We tested these possibilities in a controlled pot experiment by comparing three treatments, each inoculated with the fungus: (1) irrigated controls (2) non-irrigated (water stressed) plants and (3) carbon starved plants. We also compared the tissue dehydration limits of the chaparral host (M. laurina) to the fungal pathogen (B. dothedia) on culture media and living stem segments in the lab. In both field and controlled pot experiments, we monitored stem water potentials, leaf photosynthesis, and stem hydraulic conductance. We also estimated growth rate of fungi in the stem tissue, final branchlet dieback, and whole plant mortality. On living stems in the lab, we measured fungal growth rates under varied levels of dehydration.
Fungal elongation rates in xylem tissue of potted plants continued in all treatments over an eight week period but were over two fold greater in water stressed plants than irrigated controls, leading to increased incidence of whole branch dieback. Carbon starved plants showed an intermediate pattern relative to controls. Gas-exchange data indicated that potted plants undergoing water stress also experience significant carbon starvation due to stomatal closure and reduced quantum yield of photosystem II. Minimum water potentials at the time of whole plant death and vulnerability to water stress-induced embolism of xylem conduits indicated that the host plant, M. laurina, could not survive water potentials more negative than – 4 MPa. Stem segments in the lab at decreasing water potentials indicated continued rapid fugal elongation well below -4 MPa. Taken together, these results were consistent with field observations of fungal blockage in water transport of xylem leading to branch dieback and eventual whole plant mortality. Our results are also consistent with the hypothesis that both water stress and carbon starvation are contributors to dieback and mortality, facilitated by the dehydration tolerance of the fungal pathogen.