Gall disease in the alginophyte Lessonia berteroana: A pathogenic interaction linked with host adulthood in a seasonal-dependant manner

Lessonia berteroana is the most highly exploited seaweed in Chile and the Americas, based on the harvesting of wild stocks. It is frequently facing overexploitation problems, which is detrimental for coastal ecosystems in northern Chile. Concomitant to its abundance problems, L. berteroana stocks have widely been found to bear tumour-like outgrowths. These circumstances triggered us to characterize this gall disease from one population on the Atacama coast. Galls are plastic in size, colouration and morphology, and all are linked to the brown algal endophyte Laminariocolax aecidioides, identified by using three different DNA markers. Gall histology revealed the presence of both pigmented and colourless endophytic filaments. The pathogenic nature of this interaction was confirmed by electron microscopy (EM), which showed a marked hyperplasia with host cells having disrupted cell walls. EM also indicated cellular modifications of the Laminariocolax endophytic network in contrast to free-living isolates (e.g. organelle and cell wall reductions, larger area of plasmodesmata in markedly undulated contacting cell walls). A one-year epidemiological survey from the same locality, supported by a generalized linear mixed model, revealed that the gall disease is strongly correlated with the age structure of a given population, the season of the year and maturity of the hosts, although in summer months almost all the population gets infected. Resistant phenotypes are therefore rare in the population, which flags the question of how harvesting, restoration and breeding programs should be carried out for these susceptible populations.