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New Plant Disease article posted Dec. 2017

First report of alder Phytophthora closely related to P. uniformis on Alnus glutinosa seedling in Finland. 

A. Poimala, S. Werres, T. Pennanen, and J. Hantula' Plant Disease 0 0:0, PDIS-03-17-0322-PDN
 
The allotriploid Phytophthora × alni (Brasier & S.A. Kirk) Husson, Ioos & Marcais and its progenitors, the diploid P. uniformis (Brasier & S.A. Kirk) Husson, Ioos & Aguayo and the allotetraploid P. × multiformis (Brasier & S.A. Kirk) Husson, Ioos & Frey are the causal agents of alder decline in Europe. In June 2015, a dark, ∼40 mm stem lesion was found on one out of 100 inspected Alnus glutinosa (L.) Gaertn seedlings in Mäntyharju, Finland. Surface wood pieces from the lesion edges were plated onto malt agar. Cultures on carrot piece agar (CPA) showed an appressed colony with woolly aerial mycelium. Oogonia (Ø mean 44.8 µm, n = 50) and oospores (Ø mean 40.0 µm, n = 50) developed after 7 days at 20°C in the dark. They were commonly smooth walled, and 49 out of 50 of the amphigynous antheridia were single celled. Sporangia (21.7 × 18.1 µm, n = 50) were produced on pea broth, with 2-day flooding with soil extract, and they were commonly unpapillate and obpyriform-ellipsoidal. DNA was extracted, and the internal transcribed spacer (ITS) rDNA was amplified (ITS6 and ITS4; Cooke et al. 2000). The PCR products were then cloned because of double peaks in the sequence. Among 33 clones, four alleles with five polymorphic bases were obtained (A1, 55%; A2, 6%; A3, 15%; and A4, 24%; these were deposited in GenBank as accession nos. MF356294, MF356295, MF356296, and MF356297, respectively). The closest match to the ITS allele sequences in GenBank was the very closely related species P. cambivora AF087479 (differences, 7 to 9 bases). All other sequenced regions showed single alleles. The mitochondrial cox spacer (MF356298; amplified by FMPhy8 and FMPhy10, Martin et al. 2004) matched 100% with P. cambivora P1431 (GU221955) and P. alni P16202 (GU221933) in GenBank. A partial beta tubulin gene (MF356299; amplified by 901F and 1401R, Bilodeau et al. 2007) matched 100% with P. uniformis ALN58 (KU899249). ASF-like, GPA1, RAS-Ypt, and TRP1 genes were amplified with primers by Ioos et al. (2006). GPA1 (MF356301) matched 100% with P. uniformis PAU84 (DQ092849), and RAS-Ypt (MF356302) had exact matches with four P. uniformis isolates in GenBank (e.g., PANM53 and EU371549). The TRP1 gene (MF356303) differed by 1 base from P. uniformis alleles PAU60 (DQ202480) and PAU89 (DQ202481). The ASF-like gene (MF356300) matched 100% with PAU84 (DQ092815). Nine 1-year-old seedlings of A. glutinosa and of Betula pendula Roth were inoculated with plugs from 10-day-old mycelial culture on CPA after making a bark incision. Nine control seedlings of each species received a sterile CPA plug. Inoculations were wrapped in damp cotton wool, and the plants were kept outdoors at 10 to 22°C. After 15 days, eight out of nine (89%) A. glutinosa seedlings had developed lesions (mean length 22 mm), as well as six out of nine (67%) B. pendula seedlings (mean length 4 mm). No lesions were observed in control seedlings. The pathogen was reisolated from two symptomatic seedlings of both hosts. This is the first report of an alder Phytophthora in Finland. The sequence data suggested the isolate to be closely related but dissimilar to P. uniformis. The morphology corresponded to that previously reported for P. uniformis (Brasier et al. 2004), except for the antheridia, which were almost all single celled. Multiple ITS alleles could also refer to the initially reported P. uniformis karyotype 2n+2 (Brasier et al. 2004). These findings add to our knowledge on the variation among the alder Phytophthora group. Furthermore, they demonstrate the risk that the pathogen could be transported in new hosts.
 
 
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New Phytopathology article December 2017

Morphological and Genetic Analyses of the Invasive Forest Pathogen Phytophthora austrocedri Reveal that Two Clonal Lineages Colonized Britain and Argentina from a Common Ancestral Population

Béatrice Henricot, Ana Pérez-Sierra, April C. Armstrong, Paul M. Sharp, and Sarah Green. Phytopathology 2017. 107:12, 1532-1540 https://apsjournals.apsnet.org/doi/abs/10.1094/PHYTO-03-17-0126-R
 
Phytophthora austrocedri is causing widespread mortality of Austrocedrus chilensis in Argentina and Juniperus communis in Britain. The pathogen has also been isolated from J. horizontalis in Germany. Isolates from Britain, Argentina, and Germany are homothallic, with no clear differences in the dimensions of sporangia, oogonia, or oospores. Argentinian and German isolates grew faster than British isolates across a range of media and had a higher temperature tolerance, although most isolates, regardless of origin, grew best at 15°C and all isolates were killed at 25°C. Argentinian and British isolates caused lesions when inoculated onto both A. chilensis and J. communis; however, the Argentinian isolate caused longer lesions on A. chilensis than on J. communis and vice versa for the British isolate. Genetic analyses of nuclear and mitochondrial loci showed that all British isolates are identical. Argentinian isolates and the German isolate are also identical but differ from the British isolates. Single-nucleotide polymorphisms are shared between the British and Argentinian isolates. We concluded that British isolates and Argentinian isolates conform to two distinct clonal lineages of P. austrocedri founded from the same as-yet-unidentified source population. These lineages should be recognized and treated as separate risks by international plant health legislation.
 
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Forest Phytophthoras volume 7 Special Issue: Sudden Oak Death Management

Forest Phytophthoras vol. 7 can be viewed and downloaded at:
http://journals.library.oregonstate.edu/ForestPhytophthora

This special issue of Forest Phytophthoras contains five peer-reviewed articles on management of sudden oak death in California and Oregon (USA) with a foreward by Susan J. Frankel, guest editor. It serves as part of the proceedings from the Sixth Sudden Oak Death Science Symposium held June 21 - 23, 2016 at Fort Mason Center in San Francisco, CA, USA.
Proceedings for the 8th IUFRO meeting in Viet Nam available

Phytophthora in Forests and Natural Ecosystems, Proceedings of the 8th Meeting of the International Union of Forest Research Organizations (IUFRO) Working Party S07-02-09 are available for download at http://forestphytophthoras.org/proceedings
Early view Phytopathology article July 2017

 Morphological and genetic analyses of the invasive forest pathogen Phytophthora austrocedri reveal two clonal lineages colonised Britain and Argentina from a common ancestral population.

Béatrice Henricot, Ana Pérez-Sierra, April Armstrong, Paul Sharp, and Sarah Green

Phytopathology, Vol. 0, No. ja http://apsjournals.apsnet.org/doi/abs/10.1094/PHYTO-03-17-0126-R

Phytophthora austrocedri is causing widespread mortality of Austrocedrus chilensis in Argentina and Juniperus communis in Britain. The pathogen has also been isolated from J. horizontalis in Germany. Isolates from Britain, Argentina and Germany are homothallic with no clear differences in the dimensions of sporangia, oogonia or oospores. Argentinian and German isolates grew faster than British isolates across a range of media and had a higher temperature tolerance although most isolates regardless of origin grew best at 15°C and all isolates were killed at 25°C. Argentinian and British isolates caused lesions on both hosts when inoculated onto A. chilensis and J. communis; however the Argentinian isolate caused longer lesions on A. chilensis than on J. communis and vice versa for the British isolate. Genetic analyses of nuclear and mitochondrial loci showed that all British isolates are identical. Argentinian isolates and the German isolate are also identical but differ from the British isolates. Single nucleotide polymorphisms are shared between the British and Argentinian isolates. It is concluded that British isolates and Argentinian isolates conform to two distinct clonal lineages of P. austrocedri founded from the same as-yet unidentified source population. These lineages should be recognised and treated as separate risks by international plant health legislation.
Early view Phytopathology article
Genotypic diversity of Phytophthora cinnamomi and P. plurivora in Maryland’s nurseries and Mid-Atlantic forests
Ms. Justine Beaulieu, Mr. Blain B Ford, and Dr. Yilmaz Balci. Phytopathology 0 0:ja
 
Genetic diversity of two Phytophthora species, P. cinnamomi (102 isolates) and P. plurivora (186), commonly encountered in Maryland nurseries and forests in the Mid-Atlantic United States was characterized using amplified fragment length polymorphism (AFLP). Expected heterozygosity and other indices suggested a lower level of diversity among P. cinnamomi than P. plurivora. Hierarchical clustering showed P. cinnamomi isolates separated into four clusters, and two of the largest clusters were closely related, containing 80% of the isolates. In contrast, P. plurivora isolates separated into six clusters, one of which included approximately 40% of the isolates. P. plurivora isolates recovered from the environment (e.g. soil, water) were genotypically more diverse than those found causing lesions. For both species, isolate origin (forest vs. nursery or among nurseries) was a significant factor of heterozygosity. Clonal groups existed within P. cinnamomi and P. plurivora and included isolates from both forest and nurseries, suggesting that a pathway from nurseries to forests or visa verse exists.
 
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January 2017 Plant Disease article
First Report of Phytophthora × multiformis on Alnus glutinosa in Spain
C. Pintos-Varela, C. Rial-Martínez, O. Aguín-Casal, and J. P. Mansilla-Vázquez. Plant Disease 2017 101:1, 261-261
Alder species are threatened by a lethal disease caused by the oomycete Phytophthora alni, one of the most important emergent pathogens of natural ecosystems in Europe during the last 20 years (Aguayo et al. 2014). Phytophthora alder decline has caused substantial economic losses and ecological damage from riparian alder populations. Initially, three different subspecies had been described, P. alni subsp. alni, P. alni subsp. uniformis, and P. alni subsp. multiformis. Recently, they have been raised to species status and renamed P. × alni, P. uniformis, and P. × multiformis, respectively (Husson et al. 2015). P. × alni was reported to be the most aggressive and pathogenic to alders. The other two species appear to be less aggressive, but are also considered pathogenic (Brasier and Kirk 2001). In Spain, P. × alni and P. uniformis has also been detected (Pintos Varela et al. 2012). In April 2014, crown dieback and mortality of Alnus glutinosa were noted across the riparian area along the Muiños River in Galicia (northwest Spain). Affected trees, showing abnormally small, yellow, and sparse leaves and necrotic lesions in the inner bark, were surveyed. Samples of bark including the cambium from active lesions, roots, and soil were collected. Phytophthora spp. were baited from saturated rhizosphere soil using carnation petals. Roots and tissue from fresh active inner bark lesions were plated onto selective medium V8-PARPH agar and incubated for 7 days at 22°C in the dark. A Phytophthora sp. isolated from root and bark was transferred to carrot agar (CA) and incubated in the dark. More....
 
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New Plant Pathology article Nov 2016

Two Phytophthora species causing decline of wild olive (Olea europaea subsp. europaea var. sylvestris). (2016) González, M., Pérez-Sierra, A., Serrano, M. S. and Sánchez, M. E., Plant Pathol. doi:10.1111/ppa.12649.

Since 2009, a severe decline leading to mortality has been observed affecting nearly 5 ha of a wild olive woodland of high ecological value in Seville, southern Spain. Phytophthora cryptogea and P. megasperma were consistently isolated from roots and rhizosphere of trees with symptoms sampled in 2009, 2011 and 2013. The isolates were identified on the basis of colony and reproductive structure morphology as well as temperature–growth relationships, and identification was further corroborated by their ITS and β-tubulin sequences. Koch's postulates were demonstrated for both species on 1-year-old wild olives. Pathogenicity tests showed that both Phytophthora spp. are highly aggressive pathogens, although temperature–growth requirements for each species were distinct. As a consequence, the two species may be active in different seasons and their epidemiology may be differently influenced by global climate change, and they may show their active periods in different climatic scenarios. The climate change models for the Mediterranean Basin forecast a global temperature increase that favours the more thermophilic P. cryptogea. The high susceptibility to phytophthora root rot should not be disregarded in olive breeding programmes where wild olive is used as a source of resistance to verticillium wilt.

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New Plant Pathology article

Sporulation potential, symptom expression and detection of Phytophthora ramorum on larch needles and other foliar hosts. Harris. A. R. and Webber, J. F. (2016). Plant Pathol, 65: 1441–1451. doi:10.1111/ppa.12538

Phytophthora ramorum has caused extensive dieback and mortality of commercially grown Japanese larch (Larix kaempferi) in many parts of the UK, as infected foliage generates spores that then cause bark lesions and girdling cankers on trees. Following inoculation, individual needles of Japanese, European (L. decidua) and hybrid (L. × eurolepis) larch infected with P. ramorum can produce thousands of sporangia. Mean numbers of sporangia ranged from 806 to 1778 per cm2 (hybrid larch and Japanese larch, respectively), surpassing mean sporulation levels on foliar hosts previously associated with P. ramorum outbreaks in Britain, namely Rhododendron ponticum, Castanea sativa and Vaccinium myrtillus. Sporulation on larch even exceeded that of California bay laurel (Umbellularia californica), which drives the sudden oak death epidemic in California. Inoculation of foliage selected at different times of year revealed that foliage age significantly affected sporulation levels, but this varied with host species. However, symptom development and sporulation were often not correlated. Symptoms on larch were frequently insignificant or even absent at certain times of year, with sometimes the only evidence of infection being the emergence of sporangia from needles, without any sign of discolouration or necrosis. Plating infected but symptomless needles onto Phytophthora selective medium also often failed to yield the pathogen. Symptomless infection of larch needles apparently occurs, but is only detectable with microscopy. More generally, it is suggested that diagnosis of Phytophthora infection in conifers is often underestimated due to isolation difficulties and delayed symptom expression.

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Journal of Phytopathology article

The Effect of Exposure to Decreasing Relative Humidity on the Viability of Phytophthora ramorum sporangia. Tooley, P. W. and Browning, M. (2016), J Phytopathol, 164: 874–881. doi:10.1111/jph.12506

Sporangia of three isolates of Phytophthora ramorum representing three different clonal lineages were subjected to relative humidity (RH) levels between 80 and 100% for exposure periods ranging from 1 to 24 h at 20°C in darkness. Plastic containers (21.5 × 14.5 × 5 cm) were used as humidity chambers with 130 ml of glycerine solution added to each container. Glycerine concentrations corresponded to 100, 95, 90, 85 and 80% RH based on refractive index measurements. Sporangia suspensions were pipeted onto nitrile mesh squares (1.5 × 1.5 cm, 15 micron pore size) which were placed in the humidity chambers and incubated at 20°C in darkness. Following exposure periods of 1, 2, 4, 8, 12 and 24 h, mesh squares were inverted onto Petri dishes of selective medium and sporangia germination assessed after 24 and 48 h. At 100% RH, we observed a mean value of 88% germination after 1 h exposure declining to 18% germination following 24 h incubation. At 95% RH, a steeper decline in germination was noted, with means ranging from 79% at 1 h to less than 1% at 24 h exposure. At 90% RH, no germination was noted after 8 or more h exposure, and values were 57%, 22% and 3% germination for the 1, 2 and 4 h exposures, respectively. Germination was only observed at 1 h exposure for both the 85% RH treatment (52% germination) and the 80% RH treatment (38% germination). The three isolates responded similarly over the range of RH values tested. The germination response of P. ramorum sporangia to RH values between 80% and 100% was comparable to that reported for other Phytophthora species. Knowledge of conditions that affect P. ramorum sporangia germination can shed light on pathogenesis and epidemic potential and lead to improved control recommendations.

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