In planta detection used to define the distribution of the European lineages of Phytophthora ramorum on larch (Larix) in the UK

Two genetically distinct evolutionary lineages of the oomycete pathogen Phytophthora ramorum are responsible for the major epidemic on larch (Larix spp.) in the UK: EU1 (historically widespread) and the recently identified EU2 (reported only from Northern Ireland and a small area in southwest Scotland). Methods for lineage discrimination have required pure cultures of P. ramorum but, as the pathogen is challenging to isolate from infected larch tissue, only limited data have been available on the distribution of EU2. In this study a protocol was developed to determine the lineage of P. ramorum in infected larch tissue without the need for isolation. The protocol was applied to 134 UK samples collected during 2013–14. In addition, lineage testing was applied to over 300 P. ramorum isolates cultured from a wide range of hosts between 2002 and 2012. Combined data confirmed that EU2 is restricted to Northern Ireland and a small area of southwest Scotland where it is the dominant lineage. There was no evidence of EU2 spread into England and Wales where only EU1 was found. However, EU2 was more widely distributed in southern and eastern parts of Scotland than previously reported. Furthermore, EU1 and EU2 were detected <10 km apart in larch plantations. This study provides the first reports of natural EU2 infection on European larch (Larix decidua), hybrid larch (Larix × eurolepis), beech (Fagus sylvatica), noble fir (Abies procera) and western hemlock (Tsuga heterophylla).     

Managing Phytophthora ramorum in the Netherlands1

Phytophthora ramorum came to the Netherlands in 1993. Despite initially not seeming to pose a high risk, findings in California showed its potential destructive impact on ecosystems. A programme began in the Netherlands to eliminate P. ramorum from nurseries and surveys in the natural environment were held to obtain information to determine a strategy for dealing with the disease. About 1100 nurseries are inspected annually by NAKtuinbouw under the auspices of the Plant Protection Service and measures are taken according to EC directives. The percentage of infected nurseries decreased steadily during recent years, from 4% in 2002/2003 to 0.5% in 2004/2005. Surveys in the natural environment show that P. ramorum occurs on 2% of the sites with Rhododendron and therefore it was concluded that an elimination scenario is not realistic. A programme based on containment measures supported by an extension programme was put into place with its effects being monitored by the Plant Protection Service. 12 years of observing P. ramorum show that the risk for indigenous trees and shrubs in the Netherlands is very limited. Spread from infected rhododendrons to other potential hosts, even at heavily infected Rhododendron sites, hardly takes place although some infected Quercus rubra trees have been found. Recently several new Phytophthora species were found in natural environments in Europe and California, mainly as a result of intensive P. ramorum surveys. As well as P. ramorum, the Phytophthora spp. P. kernoviae, P. numerosa and P. pseudosyringae pose risks, indicating the need for a more general approach against Phytophthora diseases. As a result, a new protocol for detection and identification of Phytophthora spp. both as a group and individually is being developed and workers are asking whether these Phytophthora species could be managed together.     

Potential susceptibility of Australian native plant species to branch dieback and bole canker diseases caused by Phytophthora ramorum

Susceptibility to branch dieback caused by Phytophthora ramorum was tested using a detached branch assay for 66 Australian native plant species sourced from established gardens and arboreta in California. Six of these species were further tested for their susceptibility to bole cankers caused by P. ramorum using a sealed log assay. Isopogon formosus and Eucalyptus denticulata were identified as potentially highly susceptible Australian branch dieback hosts. Thirteen potentially tolerant Australian host species included Banksia attenuata, B. marginata, E. haemastoma, E. regnans, Pittosporum undulatum and Billardiera heterophylla. Eucalyptus regnans was identified as a potentially highly susceptible bole canker host, while E. diversicolor and E. viminalis were considered potentially tolerant species to bole cankers caused by P. ramorum. Phytophthora ramorum was able to infect all 66 species, as confirmed by reisolation. These results extend the known potential host range for P. ramorum, confirm it as a possible threat to Australian plant industries and ecosystems and highlight additional associated hosts that are important in the global horticultural trade, native forests and plantation forestry.     

Phytophthora ramorum: the situation in Scotland1

In Scotland inspections for Phytophthora ramorum on plants in the horticulture nursery trade started in July 2001 and are currently carried out four times per year. In addition, approximately 130 established gardens have been inspected for the disease during the years 2003 and 2004. Phytophthora ramorum has been found on Rhododendron, Viburnum, and lilac (Syringa vulgaris); the most important host plant is Viburnum tinus. The pathogen is confined to nurseries and garden centres with the exception of one private garden. The first finding was in April 2002 with 17 more outbreaks the same year. Since then the number of outbreaks per year has declined dramatically to 6 in 2003, 5 in 2004 and 3 until November 2005. Altogether, there have been 21 different outbreaks sites since the first finding, some with repeated occurrences of the disease.     

Phytophthora diversity and the population structure of Phytophthora ramorum in Swiss ornamental nurseries

Invasive oomycete pathogens have been causing significant damage to native ecosystems worldwide for over a century. A recent well‐known example is Phytophthora ramorum, the causal agent of sudden oak death, which emerged in the 1990s in Europe and North America. In Europe, this pathogen is mainly restricted to woody ornamentals in nurseries and public greens, while severe outbreaks in the wild have only been reported in the UK. This study presents the results of the P. ramorum survey conducted in Swiss nurseries between 2003 and 2011. In all 120 nurseries subjected to the plant passport system, the main P. ramorum hosts were visually checked for above ground infections. Phytophthora species were isolated from tissue showing symptoms and identified on the basis of the morphological features of the cultures and sequencing of the ribosomal ITS region. Phytophthora was detected on 125 plants (66 Viburnum, 58 Rhododendron and one Pieris). Phytophthora ramorum was the most frequent species (59·2% of the plants), followed by P. plurivora, P. cactorum, P. citrophthora, P. cinnamomi, P. cactorum/P. hedraiandra, P. multivora and P. taxon PgChlamydo. The highest incidence of P. ramorum was observed on Viburnum × bodnantense. Microsatellite genotyping showed that the Swiss P. ramorum population is highly clonal and consists of seven genotypes (five previously reported in Europe, two new), all belonging to the European EU1 clonal lineage. It can therefore be assumed that P. ramorum entered Switzerland through nursery trade. Despite sanitation measures, repeated P. ramorum infections have been recorded in seven nurseries, suggesting either reintroduction or unsuccessful eradication efforts.     

Surveys for Phytophthora ramorum in the Czech Republic1

The State Phytosanitary Administration (SPA) of the Czech Republic has been conducting an official survey for Phytophthora ramorum since 2003. A single outbreak of P. ramorum was detected by the SPA on guelder rose (Viburnum bodnantense) in autumn 2003 during a phytosanitary inspection in an ornamental nursery made on the request of owner of the nursery as he had noticed plants of Viburnum bodnantense withering. The infested lot consisted of 15 plants, which were imported from Italy into the Czech Republic at the end of April in 2003. A post‐entry inspection was carried out at the place of destination just after arrival and no visual symptoms were seen at that time. The lot was destroyed by the owner of the nursery immediately after the sampling. A survey was carried out by the SPA in all nurseries in which host plants coming from the same origin as the infested plants had been planted. No other outbreak was found. Phytosanitary inspections were carried out in ornamental nurseries and garden centres, public parks and forestry sites. No outbreak was detected in 2004 (451 visual inspections) and 2005 (522 visual inspections). The status of Phytophthora ramorum in the Czech Republic is: absent, one isolated outbreak linked to an imported consignment was eradicated.     

In vitro leaf inoculation studies as an indication of tree foliage susceptibility to Phytophthora ramorum in the UK

Leaves of 11 coniferous and 23 broad-leaved tree species important to UK forestry were tested for their susceptibility to the quarantine pathogen Phytophthora ramorum using a detached leaf assay. Two European and two USA isolates were used. Wounded and unwounded leaves were dipped in zoospore suspensions during summer; conifers were also tested in winter. Successful infection of tissue and amount of necrosis were assessed. Highly susceptible broad-leaved hosts included Aesculus hippocastanum , Fraxinus excelsior , Quercus ilex , Ulmus procera and, to a lesser extent, Castanea sativa , Q. cerris and Q. petraea , together with Umbellularia californica and rhododendrons. Acer pseudoplatanus , Alnus glutinosa , Carpinus betulus , Corylus avellana , Fagus sylvatica , Prunus avium , Q. robur , Q. rubra and Q. suber showed consistently low susceptibility. Conifer species including Abies procera , Picea abies , P. sitchensis , Pseudotsuga menziesii , Sequoia sempervirens and Tsuga heterophylla were also susceptible. Pseudotsuga menziesii and A. procera were severely affected. Pinus contorta , P. nigra var. maritima and P. sylvestris were virtually resistant, while Taxus baccata was only slightly affected. Increased necrosis was apparent on leaves that were wounded prior to inoculation. These results extend the known range of trees that P. ramorum is able to attack and confirm its relative host-nonspecificity.     

First report of Phytophthora blight of celery (Apium graveolens var. dulce) caused by Phytophthora tentaculata in Japan

Journal of General Plant Pathology - A new disease on celery discovered in Japan caused extreme stunting, dwarfed and yellowed leaves, black rot of the celery “heart”, and root rot. A...     

Fitness characteristics of the European lineages of Phytophthora ramorum

As an introduced pathogen, Phytophthora ramorum exists as four near-clonal evolutionary lineages, of which only EU1 and EU2 are established in the UK. EU1 has become widespread since the first findings in 2002 whereas EU2, detected in 2011, has a more limited distribution. Both lineages are epidemic in plantation-grown larch, sporulating asexually on needles, but also causing heavy dieback and mortality. To understand whether EU1 and EU2 pose different threats to forest health, we compared their growth characteristics on agar, pathogenicity on several hosts, and sporulation on Japanese larch needles. When pathogenicity was evaluated by measuring colonization at 20 °C in mature bark (phloem) of Japanese and European larch (Larix kaempferi and L. decidua), English oak (Quercus robur), and beech (Fagus sylvatica), Japanese larch was the most susceptible and oak the least susceptible. On average, EU2 isolates produced significantly larger lesions than EU1 isolates in Japanese larch and oak although not in the other hosts. With tests using young saplings of Japanese and European larch, damaging bark lesions formed at both 10 °C and 20 °C, but EU2 was significantly more pathogenic at 20 °C on both hosts compared with EU1. In contrast, both lineages caused similar amounts of necrosis on inoculated leaves of rhododendron (Rhododendron ponticum). Moreover, EU2 isolates usually sporulated less abundantly on larch needles compared with EU1 isolates, suggesting a trade-off in pathogenicity and sporulation between lineages. As EU2 tends to have smaller sporangia than EU1, this could also reduce the inoculum potential of EU2.