New hybrid quarantine pests ?
Dr C. Brasier, pathologist for the British Forestry Commission, has recently high-lighted the possibilities for hybridization (or genetic introgression) between fungal pests as a factor in the establishment of introduced species or in the appearance of new pathogens. He cites particularly:
1);Ophiostoma ulmi, cause of the original Dutch elm disease epidemic, declined because of virus infection. Ophiostoma novo-ulmi, cause of the second epidemic, first appeared as a genetically homogeneous population with full vegetative compatibility, making it very susceptible to viruses. It acquired vegetative incompatibility genes from O. ulmi, which it replaced, and thus made itself more resistant to spread of viruses. Further hybridization between the American and European forms of O. novo-ulmi is occurring where they meet, and allows the possibility of appearance of other new forms. It may be noted that yet another species O. himal-ulmi exists in the western Himalayas. It is very aggressive to European elms, but has not spread.
2);New Melampsora rust hybrids have appeared in Pacific North America and New Zealand, when the parents were introduced there. The hybrids combine the host ranges of the two parents on different poplars.
3);The Phytophthora which has recently attacked alder in the EPPO region (formerly on the EPPO Alert List) is a hybrid between the introduced Phytophthora cambivora and the local Phytophthora fragariae.
4);A new Phytophthora in the Netherlands, on Primula and Spathiphyllum, is a hybrid between Phytophthora cactorum and the introduced Phytophthora nicotianae.
It is argued that the known potential of some fungal groups (Ophiostoma, Phytophthora…) for genetic exchange between native and introduced species is an element to be taken into account in PRA.
Sources
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Brasier, C.M. (2001) Rapid evolution of introduced plant pathogens via interspecific hybridisation.
BioScience, 51(2), 123-133.
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