Vertical disease transmission in the Cakile-Alternaria host-pathogen interaction

Cakile maritima (Brassicaceae) is an introduced herb growing on temperate beaches in Australia. In these situations it is attacked by the fungal pathogen Alternaria brassicicola (Dematiaceae), which forms necrotic lesions on all above-ground parts. Cakile plants produce dimorphic fruits, with half of the seeds produced by C. maritima being adapted for dispersal by water. This raises the possibility that pathogen movement among host populations may occur as a consequence of wave action and ocean currents. A field survey showed that 30% of the seeds were infected with A. brassicicola, with the frequency of infection positively correlated with lesion density on the fruit surface. Glasshouse inoculation trials demonstrated that infection of seeds could occur either through the flowers or, more readily, when spores were deposited on fruits. Seedlings grown from field-collected seeds were found to emerge with infections, indicating seed infection may lead to disease in emergent plants. Overall, the findings of this study suggest that vertical transmission is likely to be by far the most important component of disease transmission among separate host populations in this host–pathogen system. However, once disease is established in a host population, both vertical and horizontal transmission will contribute to epidemic development. A second species of Alternaria (A. alternata) was detected in many isolates from disease lesions. Simple pure culture and mixed competition trials between this fungus and A. brassicicola on C. maritimashowed both to be capable of establishing infections, although A. brassicicola was much more successful both on its own and when both fungi were applied simultaneously.