Host Specificity and Tomato-Related Race Composition of Phytophthora infestans Isolates in Taiwan During 2004 and 2005.

Late blight, caused by Phytophthora infestans, is a major disease of tomato in cool and wet environments. In this study, we report on the host specificity, race composition, and variation among races revealed by amplified fragment length polymorphism (AFLP) of P. infestans isolated from tomato production areas in Taiwan. In all, 177 P. infestans isolates were collected in Taiwan during 2004 and 2005. All were aggressive on both potato and tomato. Nine physiological races were identified based on disease response on a set of tomato differentials developed by the Asian Vegetable Research and Development Center-The World Vegetable Center. Eighty-seven polymorphic bands from 32 isolates of four races were detected by AFLP. No significant correlation between the polymorphism and the races was found using cluster analysis. This study revealed that a high variability of race composition among the asexual population of P. infestans isolates existed in Taiwan during 2004 and 2005. Breeding new tomato cultivars for resistance to P. infestans is an urgent and ongoing need because new races of the pathogen appeared continuously in Taiwan in past years. Further analysis of the genomic diversity is necessary to determine whether the high genetic variation of P. infestans is related to the complex race composition.