Virulence Patterns of Phytophthora infestans Isolates using R Differential Set of Solanum demissum: A Useful Tool to Identify Pathogen Races in Tunisia

Harbaoui, K., Harrabi, M., Vleeshouwers, V., and Hamada, W. 2011. Virulence patterns of Phytophthora infestans isolates using R differential set of Solanum demissum: A useful tool to identify pathogen races in Tunisia. Tunisian Journal of Plant Protection 6: 1-10. The current study uses a representative set of 31 Tunisian Phytophthora infestans isolates selected according to geographic diversity to analyze the virulence diversity of P. infestans via plant disease bioassays. Thirty-one isolates were tested onto detached leaves of a differential set of Solanum demissum having 11 late blight resistance (R) genes. Of these isolates, twenty-one races were characterized. P. infestans isolates showed different virulence patterns depending on sampling regions. While virulence patterns were found to be diverse in Korba and Bizerte, they seemed to be less diverse in Takelsa and North-West. Moreover, in all sampling regions, virulence patterns never followed the chronological level (year) of sampling. The most effective R genes recorded in Tunisia are R5, R9 and R8 that withstood more than 90% of tested isolates. These R genes could be useful in breeding programs. Furthermore, the presence of individuals with less than four virulence factors and that the 2 virulence factor races were the most frequent races in the population could be due to the possible presence of avirulent and heterozygous isolates. This implies that P. infestans population in Tunisia has not undergone high mutations in Avr genes contrarily to what was reported in other countries. The diagnostic of virulence in P. infestans needs a useful and high throughput technique reliable and linked to the bioassay screening that could replace neutral markers (allozymes, SSR markers, mtDNA). Combining phenotypic (disease bioassay) and genotypic techniques (virulence markers) could be a very interesting tool to identify the pathogen population structure and find a rapid solution for the break-down of resistance of potato cultivars.