Phenotypic Characterization of Recent Clonal Lineages of Phytophthora infestans in the United States

Danies, G., Small, I. M., Myers, K., Childers, R., and Fry, W. E. 2013. Phenotypic characterization of recent clonal lineages of Phytophthora infestans in the United States. Plant Dis. 97:873-881. Phytophthora infestans, the causal agent of late blight disease, has been reported in the United States and Canada since the mid-nineteenth century. Due to the lack of or very limited sexual reproduction, the populations of P. infestans in the United States are primarily reproducing asexually and, thus, show a simple genetic structure. The emergence of new clonal lineages of P. infestans (US-22, US-23, and US24) responsible for the late blight epidemics in the northeastern region of the United States in the summers of 2009 and 2010 stimulated an investigation into phenotypic traits associated with these genotypes. Mating type, differences in sensitivity to mefenoxam, differences in pathogenicity on potato and tomato, and differences in rate of germination were studied for clonal lineages US-8, US-22, US-23, and US-24. Both A1 and A2 mating types were detected. Lineages US-22, US-23, and US-24 were generally sensitive to mefenoxam while US-8 was resistant. US-8 and US-24 were primarily pathogenic on potato while US-22 and US-23 were pathogenic on both potato and tomato. Indirect germination was favored at lower temperatures (5 and 10°C) whereas direct germination, though uncommon, was favored at higher temperatures (20 and 25°C). Sporangia of US-24 released zoospores more rapidly than did sporangia of US-22 and US-23. The association of characteristic phenotypic traits with genotype enables the prediction of phenotypic traits from rapid genotypic analyses for improved disease management. Phytophthora infestans, the causal agent of late blight disease, has been reported in the United States since the 1840s (26). Since its introduction, it is hypothesized that the pathogen population has been primarily reproducing clonally, and new clonal lineages have emerged mainly by mitotic recombination, mutation, or migration events (5). Due to the simple population structure of P. infestans in the United States, it has been possible to group individuals into clonal lineages based on neutral markers such as mating type, mitochondrial haplotype, nuclear DNA fingerprint patterns, and allozyme genotype (5). More recently, this approach has been complemented with the use of microsatellite markers (22,24). Isolates within lineages are recognized to be more similar to each other than to isolates in a different clonal lineage for characteristics such as metalaxyl or mefenoxam resistance and host preference (8,9,15,17,20). Several studies have characterized previously dominant clonal lineages and demonstrated the existence of considerable, predictable variation among different clonal lineages in epidemiologically important phenotypic traits. Some examples include phenylamide fungicide resistance in clonal lineages US-7 and US-8 (7) and the fact that US-8 is pathogenic mainly on potato whereas US-7, US-11, and US-17 are pathogenic on both potato and tomato (7,17). Environmental variables such as temperature and relative humidity have a significant impact on the development of late blight. Early studies conducted by Melhus (19) and Crosier (2) indicated that indirect germination was predominant at temperatures below 20°C. In a study by Mizubuti et al. (21), differential effects of temperature on sporangia germination were reported; the optimal temperature for indirect germination was lower for lineages US-7 and US-8 than it was for US-1. Clonal lineages US-7 and US-8 germinated indirectly at temperatures below 15°C. The differential effects of temperature on sporangia germination may differentially influence the efficacy of sporangia to initiate infection and, therefore, are important for disease epidemiology and disease management. The reemergence of late blight in northeastern United States and Canada in the summers of 2009 and 2010 revealed the existence of new clonal lineages of P. infestans (12). Both potato and tomato were infected by the new clonal lineages. Knowledge regarding the sensitivity of the new genotypes to the fungicide mefenoxam, their host preference, and other epidemiological characteristics could provide information important to disease management decisions. The objective of this study was to investigate epidemiologically important phenotypic characteristics of the recently emerged clonal lineages in the United States and Canada. Sensitivity to the systemic fungicide mefenoxam, pathogenicity on potato and tomato, and the effect of temperature on sporangium germination were investigated for clonal lineages US-8, US-22, US-23, and US-24. Materials and Methods Isolates and isolation. Isolates were obtained from throughout the United States and from select provinces in Canada during 2009, 2010, and 2011. Sampling of P. infestans isolates is limited to those places where the pathogen occurs and cannot be predicted from one season to another. For this reason a predetermined sampling strategy could not be employed. Isolates collected for this study were those reported by extension personnel and plant disease diagnostic labs in our network. Isolates were maintained on pea agar (13), rye B agar (1), and tomato or potato leaflets (depending on the isolate) at 15°C. When the source of the isolates was sporulating lesions, a small block of media (rye B and pea agar) with Corresponding author: W. Fry, E-mail: [email protected] *The e-Xtra logo stands for “electronic extra” and indicates that a supplementary table is available online. Accepted for publication 7 January 2013. http://dx.doi.org/10.1094 / PDIS-07-12-0682-RE © 2013 The American Phytopathological Society e-Xtra*