Possible Reasons Affecting Different Phytophthora infestans Populations in Tomato and Potato Isolates in Thailand

Late blight, caused by the oomycete Phytophthora infestans, is one of the most important diseases affecting tomato and potato production worldwide. In Thailand, the disease is widespread in the north and northeast, especially in the Chiang-Mai and Tak provinces. The mating type, metalaxyl sensitivity, mitochondrial DNA (mtDNA) haplotype, RG57 fingerprinting, and microsatellite were used to characterize the P. infestans populations. The study revealed that the P. infestans of tomato isolates in Thailand are of the same lineage as those from 1994 until 2002. The clonal lineages that were found in the potato populations have changed since 1994. The changes in P. infestans isolates in the potato populations have likely been the result of the import of seed potatoes to Thailand. Furthermore, the P. infestans populations in potatoes show resistance to metalaxyl, whereas those from tomato isolates show sensitivity to fungicides. The reasons for the different responses can be attributed to (i) the use of metalaxyl, (ii) the host preferences of P. infestans, and (iii) the migration of new genotypes from infected potato seeds.