POTATO LATE BLIGHT - The Epidemiology of Plant Diseases - page 445

of P. infestans . Interestingly, analyses of rDNA and mtDNA from herbarium
specimens collected during the years of the great famine suggest that late blight
epidemics were caused by isolates that had Ia mtDNA type, which is different than
the Ib mtDNA of the previously thought widespread lineage US-1 of P . infestans
(Ristaino et al ., 2001; May and Ristaino, 2004). This does not rule out Mexico as the
origin of the isolate(s) that migrated to Europe, since the Ia mtDNA type is
commonly found in this country (Gavino and Fry, 2002). Further studies based on
gene genealogy will certainly contribute to test these theories.
17.2.2 Recent migrations
Evidence for contemporary migrations was first signalled by the discovery of
isolates with the A2 mating type in Switzerland in 1981 (Hohl and Iselin, 1984).
Prior to the late 20 th century, there is no clear evidence of important migrations from
Mexico to the rest of the world. The report of isolates of an A2 mating type of P.
infestans was startling news for persons working with late blight because they had
assumed that individuals only of the A1 mating type were present in all worldwide
locations outside of Mexico. When other plant pathologists investigated their local
populations in response to the report of Hohl and Iselin (Hohl and Iselin, 1984), A2
mating types were discovered throughout Europe (Fry et al ., 1993). In northern
Europe, immigrant strains have largely displaced the previous indigenous strain
(Spielman et al ., 1991). The immigrating population contained both A1 and A2
mating types and, as would be predicted, sexual reproduction has now been detected
(Drenth et al ., 1995; Andersson et al ., 1998; Turkensteen et al ., 2000).
The pathway by which immigrant strains were introduced recently into Europe is
apparently via a large shipment of potatoes from Mexico to Europe in the winter of
1976/77 (Niederhauser, 1991). The importation was necessitated by an
underproduction of potatoes in Europe in the summer 1976, due to a drought.
Genetic evidence suggests that the 'new' population in Europe has again been
transported to other continents. There has been significant trade in seed tubers from
Europe to Africa and south American countries such as Brazil and Venezuela. The
linkage between Africa and South America to Europe seems especially tight,
because 'new' strains of P. infestans with the same Gpi alleles as in the European
population have been detected in Europe and in Africa and South America
(Goodwin et al ., 1994a; Forbes et al ., 1997; McLeod et al ., 2001; Reis
et al ., 2003).
An apparently separate migration has introduced a new clonal lineage to the Far
East (Mosa et al ., 1989; Koh et al ., 1994). The new clonal lineages in China, Japan,
Korea, the Philippines, and Taiwan appear to be displacing the previous clonal
lineage that had been distributed worldwide (Koh et al ., 1994; Jyan et al ., 2004).
However, sexual reproduction in Japan or Korea has not yet been detected, perhaps
because oospores produced from the A1 and A2 genotypes there fail to germinate
(Mosa et al ., 1991). Consequently, populations there remain very simple and strictly
clonal. In Russia there is also evidence of population displacement, but different