Gary Chastagner

Population genetic analysis infers migration pathways of Phytophthora ramorum in US nurseries.

Recently introduced, exotic plant pathogens may exhibit low genetic diversity and be limited to clonal reproduction. However, rapidly mutating molecular markers such as microsatellites can reveal genetic variation within these populations and be used to model putative migration patterns. Phytophthora ramorum is the exotic pathogen, discovered in the late 1990s, that is responsible for sudden oak death in California forests and ramorum blight of common ornamentals. The nursery trade has moved this pathogen from source populations on the West Coast to locations across the United States, thus risking introduction to other native forests. We examined the genetic diversity of P. ramorum in United States nurseries by microsatellite genotyping 279 isolates collected from 19 states between 2004 and 2007. Of the three known P. ramorum clonal lineages, the most common and genetically diverse lineage in the sample was NA1. Two eastward migration pathways were revealed in the clustering of NA1 isolates into two groups, one containing isolates from Connecticut, Oregon, and Washington and the other isolates from California and the remaining states. This finding is consistent with trace forward analyses conducted by the US Department of Agricultures Animal and Plant Health Inspection Service. At the same time, genetic diversities in several states equaled those observed in California, Oregon, and Washington and two-thirds of multilocus genotypes exhibited limited geographic distributions, indicating that mutation was common during or subsequent to migration. Together, these data suggest that migration, rapid mutation, and genetic drift all play a role in structuring the genetic diversity of P. ramorum in US nurseries. This work demonstrates that fast-evolving genetic markers can be used to examine the evolutionary processes acting on recently introduced pathogens and to infer their putative migration patterns, thus showing promise for the application of forensics to plant pathogens.

Standardizing the Nomenclature for Clonal Lineages of the Sudden Oak Death Pathogen, Phytophthora ramorum

Phytophthora ramorum, the causal agent of sudden oak death and ramorum blight, is known to exist as three distinct clonal lineages which can only be distinguished by performing molecular marker-based analyses. However, in the recent literature there exists no consensus on naming of these lineages. Here we propose a system for naming clonal lineages of P. ramorum based on a consensus established by the P. ramorum research community. Clonal lineages are named with a two letter identifier for the continent on which they were first found (e.g., NA = North America; EU = Europe) followed by a number indicating order of appearance. Clonal lineages known to date are designated NA1 (mating type: A2; distribution: North America; environment: forest and nurseries), NA2 (A2; North America; nurseries), and EU1 (predominantly A1, rarely A2; Europe and North America; nurseries and gardens). It is expected that novel lineages or new variants within the existing three clonal lineages could in time emerge.

First report of the Eurasian poplar leaf rust fungus, Melampsora larici-populina, in North America

Melampsora larici-populina, native to Eurasia, was found in October 1991 in hybrid poplar (Populus trichocarpa X P. deltoides) plantations along the Columbia River near Woodland, Washington, and Scapposse, Oregon. Clavate to broadly ellipsoid urediniospores measured 30--49 X 13--16 [mu]M and were echinulate except for an apical smooth patch. Telia were exclusively epiphyllous. Detached leaf inoculations were used to investigate the telial (poplar) host range of three different monouredinial isolates in the laboratory. Clones known to be susceptible in Europe and Australia to M. larici-populina (i.e., P. nigra var. italica and P. X euramericana cv. I-488) were susceptible in these tests, as were 20 clones of P. trichocarpa, the native black cottonwood of the Pacific Northwest. In general, the pattern of susceptibility among 50 clones representing many poplar taxa, including interspecific hybrid classes, was in accord with what is known of the host range of M. larici-populina in Europe and Australia. Pathogenic variation among the three isolates was not observed. 22 refs., 1 fig., 1 tab.

A leaf rust epidemic of hybrid poplar along the lower Columbia River caused by Melampsora medusae

Melampsora medusae caused an epidemic of leaf rust of hybrid poplar (Populus trichocarpa X P. deltoides) along the Columbia River in western Oregon and Washington in 1991. Although native to North America, M. medusae had not attacked hybrid poplars previously in the Pacific Northwest. The initial disease focus was a commercial plantation of 11 clones planted in monoclonal blocks of 5--50 ha near Scappoose, Oregon, By early fall, six hybrid clones infected by M. medusae were severely rusted and partially defoliated, four other clones were moderately rusted without leaf loss, and one clone was only lightly rusted. Ramets of the same clones were less severely rusted in other commercial plantations and a nursery within a 100-km radius of Scappoose. Two monouredinial isolates from Oregon and one from Kentucky of M. m. deltoidae were tested to determine host range on poplar. The isolated from Kentucky was distinguished from the isolates from Oregon on one clone of P. deltoides, three hybrid clones, and two clones of P. tremuloides. 13 refs., 1 fig., 3 tabs.

Recovery Frequency of Phytophthora ramorum and Other Phytophthora spp. in the Soil Profile of Ornamental Retail Nurseries

We tested the hypothesis that inoculum of the aboveground exotic plant pathogen Phytophthora ramorum would be limited to the organic layer (top layer of plant debris) of soils at infested retail nurseries located outside of the area where the pathogen has become established in the landscape. To test this hypothesis and compare inoculum levels of P. ramorum with levels of other Phytophthora spp. in the soil profile, soil cores were collected and sampled from three Washington State retail nurseries at which the soil had previously tested positive for P. ramorum. Phytophthora was isolated from soil using rhododendron leaves as bait, and pure cultures were obtained and stored on V8 juice agar. Isolates were identified to species using a combination of DNA sequencing of the internal transcribed spacer (ITS) region of rDNA, real-time polymerase chain reaction (PCR) diagnostic testing, and culture morphology. Recovery frequencies were tabulated and compared by species at the organic layer, 0 to 5 cm, 5 to 10 cm, and 10 to 15 cm depth classes. The three most common Phytophthora spp. recovered from the soil cores were P. citricola (32%), P. drechsleri (32%), and P. ramorum (27%). P. citricola and P. drechsleri were more evenly distributed throughout the soil profile, whereas P. ramorum was primarily recovered from the organic and 0 to 5 cm depth class (86% of recoveries). P. ramorum was not detected below 10 cm.

Genetic variation in postharvest needle retention among Nordmann fir families and grafted clones

Abstract Nordmann fir [Abies nordmanniana (Stev.) Spach.] is a widely used Christmas tree species in northern Europe. In recent years there has also been increasing interest in using Nordmann fir as a Christmas tree in North America. Postharvest quality is a central characteristic for high-quality Christmas trees. To examine the genetic variation in needle retention, a series of experiments was conducted using cut branches from a grafted seed orchard with 23 clones and a progeny trial comprising 19 open-pollinated clonal progenies of the orchard. For both clones and open-pollinated families strong genetic differences were seen in needle retention when branch samples were allowed to dry. Based on one progeny trial, single tree heritabilities were estimated in the range of 0.27–0.31 after 10–15 days of drying. A parent–offspring regression indicated a higher heritability. These results indicate that breeding efforts can be used to improve significantly the postharvest needle retention characteristics of Nordmann fir Christmas trees.

Clonal and sexual propagation in Botrytis elliptica

Genetic variation was found between isolates of Botrytis elliptica from the USA and Taiwan based on 22 polymorphic markers from five RAPD primers. Among these 69 isolates, there were 43 different genotypes. One clonal genotype was found spanning Oregon and Washington, USA, and in a population from Hsinshe, Taiwan, clonal genotypes were frequent, demonstrating the importance of asexual spread of this pathogen. However, when only unique genotypes were considered, gametic disequilibrium among putative RAPD loci of the genotypes from Taiwan (33) or Hsinshe (23) was not detected, suggesting an underlying sexual recombination system for this population of B. elliptica. A weak level of cultivar specialization was detected among B. elliptica isolates from the Hsinshe population based on analysis of genetic distances between isolates.

Screening ornamental lilies for resistance to Botrytis elliptica

Abstract The screening of ornamental lilies for resistance to fire, a disease caused by Botrytis elliptica (Berk.) Cooke, is discussed. Detached leaves from greenhouse-grown lilies were inoculated with spores of B. elliptica and incubated at 20°C and 100% relative humidity for 48 h. After incubation, the leaves were transferred to a growth-chamber. Percentage infection and lesion size, which were assessed 1 week after inoculation, varied with the fungal isolate used and with spore concentration. When 20 ornamental lily clones were tested for resistance, clones derived from Lilium longiflorum Thunb. (Easter Lilies) showed less variability in resistance than the more genetically diverse colored lily clones. A study carried out in the field generally confirmed results obtained with green-house-grown plants.

The influence of infection with Penicillium corymbiferum on the forcing performance of wounded or unwounded bulbs of iris cultivars ‘Ideal’ and ‘Blue Star’ treated with ethylene or ethephon

Abstract The incidence of infection with Penicillium corymbiferum Westling of bulbs of the Dutch iris ( Iris hollandica Hoog) cultivars ‘Ideal’ and ‘Blue Star’ was greatly increased by wounding. Nevertheless, wounding and the subsequent development of blue mold had no effect on the forcing performance of bulbs treated with ethylene or ethephon, as compared with untreated bulbs. With ‘Ideal’, ethylene or ethephon treatment did not influence incidence of infection. With ‘Blue Star’ an ethephon treatment resulted in an increased infection rate with wounded bulbs. Results presented here suggest that ethylene or ethephon treatment does not predispose iris bulbs to P. corymbiferum infections severe enough to influence early forcing performance.

First report of Phytophthora ramorum infecting grand fir in California.

Phytophthora ramorum was detected on grand fir in 2003 and 2005 in a Christmas tree plantation near Los Gatos, CA, in association with infected California bay laurel. Isolates derived from stem lesions were used to inoculate grand fir seedlings in two tests. Isolations from lesions on inoculated plants were positive for P. ramorum in both tests. This work provides the completion of Kochs postulates to establish grand fir as a host of P. ramorum. The potential for grand fir to be infected within its native range is unknown.