We tested the hypothesis that an opportunistic endophytic fungus Botryosphaeria dothidea that frequently infects and causes dieback in several different species of chaparral shrubs in the Santa Monica Mountains continues to elongate and grow in host tissues at dehydration levels that exceed host survival. This was done by collecting several large branches from the field from three dominant species of co-occurring chaparral shrubs, Malosma laurina, Ceanothus spinosus, and Ceanothus megacarpus. We allowed branches to dehydrate at increasing lengths of time, from a few days to one week, in an air-conditioned laboratory, then sealed in plastic bags to allow tissue-water equilibration, and measured water potential at different dehydration levels. Stem segments were then inoculated with the fungal pathogen B. dothidea, sealed into test tubes, and allowed to incubate for a total of six days. After six days of fungal growth, lengths of hyphal invasion into stem tissues was recorded and final water potentials were determined using a dew point hygrometer.
Results/Conclusions
In Malosma laurina (± 3.74 SE, n = 30), exceeding the survival limits of the host plant (100% cavitation of stem xylem at -4 MPa). In Ceanothus spinosus (± 2.91 SE, n = 30), approaching the survival limits of the host at -9 MPa. In Ceanothus megacarpus (± 3.34 SE, n = 30), also approaching the survival limits of the host at -13 MPa. This indicates that the invasion and growth of the fungal pathogen B. dothidea in the stem xylem of these co-dominant chaparral species can continue under severe drought conditions recently experienced in California and likely contribute to the whole plant mortality increasingly observed for these species.