Competition And Genetic Diversity In The Potato Late Blight Pathogen Phytophthora Infestans

Potato late blight, a destructive disease caused by Phytophthora infestans (Mont.) de Bary is a constraining factor of potato production. Although domestic and foreign scholars in this field have been carrying out a lot of research, our knowledge on the survival strategy of P. infestans is still limited. Being a largest food production and consuming country, China has initiated a strategic plan to grow potato for staple food. As a consequence, potato production is expected to grow substantially in the next decades and research related to late blight prevention and control will become more important. In this study, the impact of genetic similarity on competition and disease development were conducted to understand the survival strategy of P. infestans when multiple pathogen strains co-infected host. This thesis also investigated the genetic diversity of two genes each from nuclear and mitochondrial genome in the isolates collected from four major farming areas across the country to understand the population genetic structure of P. infestans with intention of providing scientific and effective information for the deployment of resistant varieties and application of chemicals.1 Results from the multiple-infection experiment suggest that lesion sizes on the fifth day, sixth day, seven day and the average of three days increased when host was simultaneously infected with more isolates. All correlation coefficients between lesion size and number of co-infecting isolates are greater than 0.900 and significant in all measurements except on the fifth day, suggesting that the more isolates infected the same sites, the larger lesion size is formed. In other words, multiple-infection will enhance the virulence of P. infestans.2 The effect of genetic similarity on the pathogen aggressiveness:lesion size on the fifth, sixth, seven day and the average of the three days was smaller when host was co-infected with genetically more distance isolates. The correlation coefficients between lesion size and genetic dissimilarity of co-infecting isolates are negative and significant on the fifth, sixth, seventh day and the average of the three days, suggesting that more disease will be formed when hosts were attacked by genetically closer isolates than genetically distance isolates.3 Pairwise competition experiment showed that only one genotype was recovered in four combinations, accounting for 2/3 of total tests. The mutual exclusion usually occurred when two genetically distanced isolates were paired, suggesting competition among isolates are more intensive between less related isolates than closely-related isolates.4 116 isolates collected from eight provinces were sequenced for two mitochondrial genes (Cox1, Nad9) and two nuclear genes (β-tubulin, EF1-α). Results show that nucleotide diversity (π) and haplotype diversity (Hd) of all but EF1-α gene were low in all populations, even no variation (π=0) in some populations, indicating that these genes are highly conserved. No genetic differentiation and negative selection were detected in Nad9 and β-tubulin genes. It is likely that Nad9 and β-tubulin genes are very critical and any mutations will reduce the fitness of pathogen.