Lineage, Temperature, and Host Species have Interacting Effects on Lesion Development in Phytophthora ramorum

Eyre, C. A., Hayden, K. J., Kozanitas, M., Grünwald, N. J., and Garbelotto, M. 2014. Lineage, temperature, and host species have interacting effects on lesion development in Phytophthora ramorum. Plant Dis. 98:1717-1727. There are four recognized clonal lineages of the pathogen Phytophthora ramorum. The two major lineages present in North America are NA1 and NA2. With a few exceptions, NA1 is found in natural forest ecosystems and nurseries, and NA2 is generally restricted to nurseries. Isolates from the NA1 and NA2 lineages were used to infect rhododendron, camellia, and California bay laurel in detached leaf assays to study the effects of lineage, temperature, and host on pathogenicity and host susceptibility. Isolates within both lineages were highly variable in their ability to form lesions on each host. There was also a tendency toward reduced lesion size in successive trials, suggesting degeneration of isolates over time. Temperature had a significant effect on lesion size, with a response that varied depending on the host and isolate. Phenotypic differences between lineages appear to be heavily influenced by the representation of isolates used, host, and temperature. The importance of temperature, host, and lineage are discussed with respect to disease management, as well as future range expansions and migrations of the pathogen. Phytophthora ramorum is an emergent generalist Oomycete pathogen (Kingdom: Stramenopila) causing sudden oak death in the United States (62,76), sudden larch death in the United Kingdom (4,73), and ramorum blight in both the United States and Europe (5,12,34,63,76). The origins of P. ramorum are unknown but genetic evidence suggests it to be an exotic pathogen introduced to North America and Europe on multiple occasions via the commercial nursery industry (11,26–28,43,52,53), and potentially originating from Asia (8). P. ramorum is heterothallic, with two mating types (A1 and A2). Although sexual reproduction is possible, it is thought to be rare, and the two mating types are generally geographically separated. The A1 mating type is mostly restricted to Europe (37,60) and U.S. nurseries (29), while only the A2 mating type is found in North American forests (43,60). However, the A2 mating type has been found in three isolates from Belgian nurseries (70,74), one of which has since switched mating types from A2 to A1 (9). Expansion of the pathogen population is thought to be mostly via propagation of asexual clones (26,43,69). Four evolutionary clonal lineages are currently known within P. ramorum: EU1, EU2, NA1, and NA2 (32,43,67). NA1 and NA2 are restricted to North America, where the NA1 lineage is generally found in native forests and nurseries and NA2 isolates are mostly limited to nurseries. EU1 isolates are concentrated in Europe but have been found occasionally in U.S. nurseries (33,37). EU2 is currently limited to Northern Ireland and Western Scotland (67). Lineages can be discriminated using gene sequences, amplified fragment length polymorphism, and simple sequence repeat markers (26,43,44,59,71). Several studies have examined the phenotypic differences between lineages in terms of traits such as spore and colony morphology (7,43,75); colony growth rate (7,75); spore production (51,54,66,75); effect of fungicides (20,40,58); and pathogenicity, aggressiveness, or host susceptibility (7,15,20,36,39,40,51,66). Most have shown that there is substantial variability in phenotypic characteristics of individual isolates. However, assessments of the overall differences among lineages, if present, have been less consistent. The variety of growth media, inoculation techniques (agar versus zoospores, wounding versus nonwounding, and detached leaf versus sapling versus cut log), experimental temperatures, and isolates used in the disparate studies make it difficult to conclusively determine the existence and nature of phenotypic differences between lineages, or whether the differences seen are, in fact, an artifact of the isolates selected to represent each lineage. Finally, relationships and interactions among host susceptibility, temperature, and pathogen lineage have yet to be determined, reducing the power of predictive models. The majority of host susceptibility studies have focused on either a single host and multiple temperatures (20) or multiple hosts at a single temperature, often using a single or relatively few isolates representative of each lineage (1,15,51). Our objective was to conduct a more comprehensive study of the effect of lineage, temperature, and host on pathogenicity and host susceptibility. The NA2 lineage has been historically confined to nurseries and has been shown in some studies to be more aggressive (20,51). Providing more information on the virulence of NA2 and comparing it with NA1 is essential to determine whether its spread into native forests in North America would present an additional or different threat than that already posed by NA1. Indeed, a plant infected by an NA2 clone has recently been reported in California (23), as have some plants outside an infected nursery in Washington (10), highlighting the need for a better comparative assessment of virulence of these two lineages. Our overall objective was to assess variability among populations of NA1 and NA2 populations. Host susceptibility was first Corresponding author: M. Garbelotto; E-mail: [email protected] Accepted for publication 9 June 2014. http://dx.doi.org/10.1094/PDIS-02-14-0151-RE © 2014 The American Phytopathological Society