Dieback and mortality of South African fynbos shrubs is likely driven by a novel pathogen and pathogeninduced hydraulic failure

We examined whether extensive dry season dieback and mortality in a South African fynbos community were due to drought or pathogen attack. Plant dieback and mortality have been reported elsewhere in similar plant communities suggesting potential for a widespread climatic or biotic threat to this community.We collected tissue samples from Brunia noduliflora, the dominant plant in the community, and cultured them for potential plant pathogens. We also measured dry season predawn and midday water potentials of healthy and stressed plants and constructed pressure-volume curves to assess turgor loss point. Plant stress and mortality were monitored over a 2-year study period. Both healthy plants and plants that displayed moderate signs of stress had dry season predawn water potentials well above their turgor loss point suggesting plants were not water stressed. However, plants displaying >60% crown dieback had much lower water potentials (as low as -12 MPa). A previously undescribed fungus (Pythium sp.) was isolated from the root vascular tissue of all stressed plants and was not present in healthy plants. The proximate cause of plant stress was likely pathogen-induced, while the ultimate cause of plant death appears to be extreme water stress.The present study suggests that Brunia (Bruniceae), Leucadendron (Proteaceae) and Erica (Ericaceae), all emblematic and dominant genera within the diverse fynbos community, may be susceptible to Pythium infection. This may pose a serious threat to communities already threatened by climate change.