Identification and Characterization of Isolates of Phytophthora infestans Using Fatty Acid Methyl Ester (FAME) Profiles

The recent resurgence of the late blight pathogen, Phytophthora infestans (Mont.) de Bary, as a major agricultural threat, as well as the associated changes in genetic complexity of this organism, have necessitated careful scrutiny and characterization of populations of P. infestans so that appropriate control methods can be implemented. Several techniques for molecular and genetic characterization of P. infestans, including allozyme analyses (5,7), mitochondrial DNA haplotyping (10), and DNA genetic fingerprinting (6,11), are available and routinely used to characterize different isolates of this organism. However, many isolates showing minor genetic differences or variant genotypes cannot be clearly distinguished with these tests. In addition, much variability apparently exists in the epidemiological characteristics of different isolates which cannot be accounted for by the present genetic tests. Additional tools for further characterization would be useful, particularly ones that may account for potential epidemiological differences among isolates. Analysis of cellular fatty acid composition routinely is used to characterize, differentiate, and identify genera, species, and strains of bacteria and yeasts (1,2,18,20, 28,29). Taxa are distinguishable by the types of fatty acids produced and the relative concentrations of individual fatty acids. In addition to genotype, fatty acid composition is strongly influenced by three primary environmental variables—growth substrate, incubation temperature, and incubation time—but results are highly reproducible, consistent, and conserved among different taxa when these factors are held constant (18,27). Although fungi and related taxa generally produce fewer types and lower quantities of fatty acids than bacteria, there is growing evidence that fatty acid profiles also may be useful for the identification and characterization of fungi (14,16,17,21,25). Fatty acid profiles have been used successfully to characterize and differentiate major groups and genera of fungi (16,17,21), and species and subspecific groups of Penicillium (14), Rhizoctonia (22–25), and mycorrhizae (9,15). Oomycetes, in the Kingdom Stramenopila, which include the genera Phytophthora and Pythium, may have good potential for differentiation by fatty acids, because they produce several fatty acids not generally produced by true fungi (16). However, there is little information on fatty acid profiles for this group of organisms. Fatty acid profiling represents a relatively rapid, cost-effective, and efficient approach that may be useful for characterization of P. infestans, as well as other oomycete pathogens. It represents a different approach that may reveal aspects of variability among isolates of P. infestans not readily detected by other means. The objective of this study was to investigate the potential of using whole cell fatty acid methyl ester (FAME) profiles for the identification, characterization, and differentiation of isolates of P. infestans. Factors that might affect fatty acid production and therefore interfere with consistent characterization—such as growth media, incubation temperature, and length of time isolates are maintained on culture plates— were also examined.