The Andean fruit crop, pear melon (Solanum muricatum), is a common host for A1 and A2 strains of Phytophthora infestans in Ecuador

The Andean highlands of Ecuador harbour a wide range of both wild and cultivated solanaceous hosts of the potato late blight pathogen Phytophthora infestans (Mont.) de Bary (Erselius et al., 1999). The astonishing diversity in host plants is associated with a diverse population structure of P. infestans sensu lato. Both the A1 and A2 mating types of P. infestans have been reported and several clonal lineages of the pathogen described (Oyarzún et al., 1998; Ordoñez et al., 2000). Despite the presence of both mating types, no evidence of sexual reproduction has yet been found and host specificity is thought to be an important barrier. Strains of the A1 mating type representing several distinct clonal lineages have been collected on wild and cultivated tuber bearing Solanum species as well as tomato (S. lycopersicon), tree tomato (S. betaceum) and pear melon (S. muricatum). Until recently, however, A2 strains had been found only on the wild non-tuber bearing species S. tetrapetalum and S. brevifolium (Ordoñez et al., 2000).

In the autumn of 2001, two isolates were collected from a severely blighted pear melon field near the town of Baños. During a subsequent visit in March 2002, the same field was visited and another isolate collected. The two isolates collected in 2001 were found to belong to the lineage EC-2, based on mating type, RFLP fingerprint and mitochondrial haplotype. EC-2 is A2 mating type. The isolate from 2002 was found to be US-1, which is an A1 lineage that in Ecuador is commonly found on pear melon, the related wild species S. caripense, and tomato. Since A2 strains had never been found on any cultivated solanaceous crop in Ecuador, these two isolates were re-inoculated on pear melon and found to be highly aggressive on that host. Therefore, this is the first report of A1 and A2 mating type strains of P. infestans causing disease on a common host plant species in the field in Ecuador. Breakdown of the existing pre-meiotic reproductive isolation between the A1 and A2 mating type strains may lead to sexual reproduction and the formation of oospores of P. infestans on pear melon. The presence of sexual reproduction could have a strong impact on both the population structure and epidemics of P. infestans in Ecuador, provided that the oospores are functional and the sexual offspring are pathogenic to one or more hosts.

The potential impact of the presence of both mating types of P. infestans on pear melon in the Andean highlands of Ecuador will be explored in inoculation experiments and the evolution of the population structure in P. infestans will be closely monitored.