Genetic Analysis of Phytophthora nicotianae Populations From Different Hosts Using Microsatellite Markers

Two hundred and thirty-one isolates of P. nicotianae representing 14 populations from different host genera, including agricultural crops (Citrus, Nicotiana and Lycopersicon), potted ornamental species in nurseries (Lavandula, Convolvulus, Myrtus, Correa and Ruta) and other plant genera were characterized using SSR markers. A total of 99 multilocus genotypes (MLGs) were identified revealing a strong association between genetic grouping and host of recovery with most MLGs being associated with a single host genus. Significant differences in the structure of populations were revealed, but clonality prevailed in all populations. Isolates from Citrus were found to be genetically related regardless of their geographic origin and were characterized by high genetic uniformity and high inbreeding coefficients. Higher variability was observed for other populations and a significant geographical structuring was determined for isolates from Nicotiana. Detected differences were related to the propagation and cultivation systems of different crops. Isolates obtained from Citrus species are more likely to be distributed worldwide with infected plant material while Nicotiana and Lycopersicon are propagated by seed, which would not contribute to the spread of the pathogen and result in a greater chance for geographic isolation of lineages. With regards to ornamental species in nurseries, the high genetic variation is likely the result of the admixture of diverse pathogen genotypes through the trade of infected plant material from various geographic origins, the presence of several hosts in the same nursery and genetic recombination through sexual reproduction of this heterothallic species.