Interspecific hybridization

The most likely impact of climate change on plant pathosystems would be shifts in the geographical distribution of hosts and pathogens. Plant disease epidemics following the dispersal of exotic pests or pathogens are not rare (Brown and Hovmoller, 2002), but the simultaneous occurrence of introduced and resident species in a given ecosystem could lead to the development of new pathogens. Interspecific hybridization between Phytophthora cambivora-like species and an unknown taxon similar to Phytophthora fragariae has led to the emergence of a new aggressive Phytophthora species pathogen on alder trees in Europe (Brasier et al., 1999; Brasier, 2001). Other natural hybrids of Phytophthora nicotianae and Phytophthora cactorum have demonstrated the evolutionary potential of this genus (Man in't Veldt et al., 1998).

Horizontal gene transfer due to nuclear or somatic recombination is a further source of new diseases and results from the simultaneous presence of different species in the same environment. The species Pyrenophora tritici-repentis, originally described as a saprophyte, became pathogenic by inducing a damaging disease of wheat called yellow spot or tan spot. Friesen et al. (2006) suggest that this change in virulence occurred after the trans fer of the ToxA gene coding for a protein-aceous toxin from Phaeosphaeria nodorum, the causative agent of blotch disease, into the P tritici-repentis genome. This example is evidence of a new disease emerging because of the interspecific transfer of a toxin gene that changed a previously benign microorganism into an important pathogen (Stukenbrock and McDonald, 2008).

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