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Leaf ‘bait’ uncovers dieback crossbreeds

ScienceNetwork Western Australia, Wednesday, 16 September 2015 article by Samille Mitchell

  • Dieback becomes pathogenic as it mixes with other species
  • South-west waterway ‘fishing’ reveals naturally occurring hybrid
  • Discovery raises questions about how species came into contact

MURDOCH University scientists have discovered the first known natural occurrence of hybrids of the plant pathogen Phytophthora, commonly known as the dieback, which has ravaged south-west bushland and forests.

But the Centre of Phytophthora Science and Management (CPSM) research also shows that Phytophthora is not always the ‘baddy’ of the plant world that it is perceived to be.

Murdoch University Associate Professor Treena Burgess says Phytophthora species naturally occur in many areas throughout WA, and not all become damaging pathogens.

She says Phytophthora species tend to become more pathogenic the more they hybridise.

But she says the newly discovered natural hybrids come from relatively low-pathogenic parent species and therefore do not pose a big risk to native vegetation in the short-term.

She cites example of naturally occurring endemic species of Phytophthora in the northern sandplains which are believed to survive on plant roots with little impact.

However, if a seedling from its host plant sprouts nearby, the Phytophthora will kill it, thus ensuring wider spread growth for the host species.

“Everything that’s naturally occurring is here for a reason,” she says.

“Everything is interconnected—we just don’t see what’s going on underneath the earth.”

Before this discovery Phytophthora hybrids had only been known from mixing in nurseries, rather than from the natural environment.



New Phytopathology article

Winter Conditions Correlate with Phytophthora alni Subspecies Distribution in Southern Sweden

Miguel A. Redondo, Johanna Boberg, Christer H. B. Olsson, and Jonàs Oliva

Phytopathology 2015 105:9, 1191-1197 http://dx.doi.org/10.1094/PHYTO-01-15-0020-R

During the last century, the number of forest pathogen invasions has increased substantially. Environmental variables can play a crucial role in determining the establishment of invasive species. The objective of the present work was to determine the correlation between winter climatic conditions and distribution of two subspecies of the invasive forest pathogen Phytophthora alni: P. alni subspp. alni and uniformis killing black alder (Alnus glutinosa) in southern Sweden. It is known from laboratory experiments that P. alni subsp. alni is more pathogenic than P. alni subsp. uniformis, and that P. alni subsp. alni is sensitive to low temperatures and long frost periods. By studying the distribution of these two subspecies at the northern limit of the host species, we could investigate whether winter conditions can affect the geographical distribution of P. alni subsp. alni spreading northward. Sixteen major river systems of southern Sweden were systematically surveyed and isolations were performed from active cankers. The distribution of the two studied subspecies was highly correlated with winter temperature and duration of periods with heavy frost. While P. alni subsp. uniformis covered the whole range of temperatures of the host, P. alni subsp. alni was recovered in areas subjected to milder winter temperatures and shorter frost periods. Our observations suggest that winter conditions can play an important role in limiting P. alni subsp. alni establishment in cold locations, thus affecting the distribution of the different subspecies of P. alni in boreal regions.


New Forest Pathology article

Action of fosetyl-al and metalaxyl against Phytophthora austrocedri

Silva, P. V., Vélez, M. L., Hernández Otaño, D., Nuñez, C., Greslebin, A. G. (2015). Forest Pathology. doi: 10.1111/efp.12216

Fosetyl-Al and metalaxyl, the most commonly used systemic fungicides against Phytophthora, were evaluated for their efficacy to control Phytophthora austrocedri, the pathogen that causes a serious disease at the Austrocedrus chilensis forests in Patagonia. The effect of the chemicals on pathogen development in vitro and in planta was analysed. Both chemicals were shown to protect plants from the pathogen. In vitro assays showed that asexual reproduction was sensitive to both chemicals. However, mycelial growth and sexual reproduction, which were clearly sensitive to metalaxyl, were sensitive only to high concentrations of fosetyl-Al. Fosetyl-Al and metalaxyl had almost the same efficacy when applied preventively by soil drench to seedlings. This difference between in vitro and in planta results can be attributed to the dual action of fosetyl-Al, not only inhibiting the pathogen but also stimulating host defence. In adult trees, preventive and curative treatments were tested, but only the fosetyl-Al preventive treatment was effective in the assayed conditions. Interestingly, seedlings pretreated with both fungicides were less susceptible to the effectors secreted by the pathogen. Our results indicate that fosetyl-Al and metalaxyl provide some resistance to the plant besides the fungistatic direct action on the pathogen. Further studies to elucidate a possible resistance-inducing activity of these chemicals and the mechanisms involved are underway.


New Plant Disease Note

First Report of Phytophthora occultans Causing Root and Collar Rot on Ceanothus, Boxwood, Rhododendron, and Other Hosts in Horticultural Nurseries in Oregon, USA

P. W.Reeser, W.Sutton, and E. M.Hansen, Department of Botany and Plant Pathology, Oregon State University, Corvallis OR 97331 USA; E. M.Goheen, USDA Forest Service, Forest Health Protection, Medford, OR USA; V. J.Fieland, Department of Botany and Plant Pathology, Oregon State University, Corvallis OR 97331 USA; and N. J.Grunwald, USDA-ARS, Horticultural Crops Research Lab, Corvallis OR 97331. http://dx.doi.org/10.1094/PDIS-02-15-0156-PDN

New Disease Reports

Phytophthora siskiyouensis causing stem lesions and cankers on Alnus incana

A. Perez-Sierra*, M. Kalantarzadeh, S. Sancisi-Frey and C.M. Brasier

New Disease Reports (2015) 31, 17. [http://dx.doi.org/10.5197/j.2044-0588.2015.031.017]

In late summer 2013, stem cankers and sparse foliage were reported on European grey alder (Alnus incana) growing on a 500 ha site recently-planted with broadleaf and coniferous trees in south-west England. A site visit showed that approximately 10% of  more than 1000 grey alders (thought to have been imported from Europe and planted in the late 1990s) had symptoms including bleeding stem lesions similar to those caused by Phytophthora alni (Gibbs et al., 2003). In November 2013, samples were collected from stem lesions (Fig. 1), roots (internal lesions tracking-down from stem lesions) and rhizosphere soil from symptom-bearing trees. Tissue from root and stem lesion margins was plated onto Phytophthora selective medium (SMA) (amended as per Brasier et al., 2005) and incubated at 20°C for 48 hrs. Green apples were used as baits for soil samples by inserting a few grams of soil under a flap cut in the side of the apple and incubating for 4-7 days at 20°C. Isolation from developing SMA mycelial cultures and incubated apple baits onto potato dextrose agar (PDA) and carrot agar (CA) was then undertaken.


Plant Disease - Disease Note

First Report of Phytophthora pluvialis Causing Needle Loss and Shoot Dieback on Douglas-fir in Oregon and New Zealand

E. M.Hansen, P.Reeser, and W.Sutton, Department of Botany and Plant Pathology, Oregon State University, Corvallis 97331; and J.Gardner and N.Williams, New Zealand Forest Research Institute (Scion), Private Bag 3020, Rotorua 3046, New Zealand.
May 2015, Volume 99, Number 5
Page 727

Early view Phytopathology article

Temporal Epidemiology of Sudden Oak Death in Oregon.

Peterson, E.; Hansen, E.; and Kanaskie, A. 2015. Phytopathology.  http://dx.doi.org/10.1094/PHYTO-12-14-0348-FI

An effort to eradicate Phytophthora ramorum , causal agent of sudden oak death, has been underway since its discovery in Oregon forests. Using an information-theoretical approach we sought to model yearly variation in the size of newly infested areas and dispersal distance. Maximum dispersal distances were best modeled by spring and winter precipitation two years before detection, and infestation size the year prior. Infestation size was best modeled by infestation size and spring precipitation the year prior. In our interpretation, there is a two year delay between the introduction of inoculum and onset of mortality for a majority of sites. The year-long gap in between allows ample time for the production of inoculum contributing to the spread of P. ramorum. This is supported by epidemic development following changes in eradication protocols precipitated by an outbreak in 2011, attributable to a 2009 treatment delay and an uncharacteristically wet spring in 2010. Post-eradication, we have observed an increase in the total area of new outbreaks and increased frequency in dispersal distances greater than 4 km. While the eradication program has not eliminated P. ramorum from Oregon forests it has likely moderated this epidemic, emphasizing the need for prompt treatment of future invasive forest pathogens.


Early view Forest Pathology article 13 May 2015

Variation in pathogenicity among the three subspecies of Phytophthora alni on detached leaves, twigs and branches of Alnus glutinosa

Haque, M. M. U., Martín-García, J. and Diez, J. J. (2015). Forest Pathology. doi: 10.1111/efp.12198

Pathogenicity tests were carried out on leaves, twigs and branches of Alnus glutinosa using several isolates of Phytophthora alni ssp. alni, P. alni ssp. multiformis and P. alni ssp. uniformis in vitro. Healthy fresh leaves were collected from disease-free areas and inoculated with mycelium on agar discs or by dipping in zoospore suspensions. In addition, twigs and branches were collected from both disease-free and disease-affected areas, inoculated with mycelium on agar discs and incubated at four temperatures (15, 20, 25, 30°C). All subspecies tested were pathogenic but with varied level of virulence. In inoculation tests on foliage, wounding was a key factor in causing infections: lesions on inoculated wounded leaves were larger than on non-wounded leaves. In the twig and branch inoculation tests, no differences in virulence were observed among the P. alni subspecies in terms of sampling locations, but lesions differed in size according to incubation temperature, with the largest lesions occurring on tissues incubated at 25°C. The work is the first to report foliar necrosis caused by P. alni on A. glutinosa. P. alni ssp. uniformis was the least virulent of the subspecies in branch inoculations. These findings demonstrate that various tissues of A. glutinosa could act as sources of pathogen inoculum and may disseminate alder Phytophthora in natural ecosystems.



New Plant Pathology article April 2015

Phytophthora austrocedrae emerges as a serious threat to juniper (Juniperus communis) in Britain.

Green, S., Elliot, M., Armstrong, A. and Hendry, S. J. (2015), Plant Pathology, 64: 456–466. doi: 10.1111/ppa.12253

From 2011 to 2013, Phytophthora austrocedrae was isolated from diseased Juniperus communis exhibiting dieback and mortality at eight geographically separate sites in Scotland and northern England. The pathogen was also confirmed present either by standard PCR of the ITS locus and sequencing or by real-time PCR on J. communis with symptoms at a further 11 sites in northern Britain. Out of 167 J. communis sampled across the 19 sites, 154 had foliage dieback over all or part of the crown as a result of basal lesions, which extended up the stem. Thirteen sampled trees had aerial branch lesions or discrete stem lesions with no apparent connection to the base of the tree. At 13 sites, dieback was concentrated in areas of poor drainage and/or alongside streams and other watercourses. In artificial inoculation experiments, P. austrocedrae caused rapidly extending stem and root lesions on J. communis and was reisolated from these lesions. Lesions also developed on Chamaecyparis lawsoniana and Chamaecyparis nootkatensis but the pathogen was not reisolated. All P. austrocedrae isolates obtained from J. communis in Britain shared 100% identity across the ITS locus but were distinct at one sequence position from P. austrocedrae isolates collected in Argentina from diseased Austrocedrus chilensis. This study provides clear evidence that P. austrocedrae is a primary pathogen of Jcommunis and now presents a significant threat to this species in Britain. Pathways for the emergence of P. austrocedrae in Britain, and possible ways in which the pathogen may have spread within the country, are discussed.


New European Journal of Plant Pathology article

Role of salicylic acid in phosphite-induced protection against Oomycetes; a Phytophthora cinnamomi - Lupinus augustifolius model system

Groves, E; Howard, K; Hardy, G; Burgess T Eur J Plant Pathol (2015) 141:559-569 DOI 10.1007/s10658-014-0562-y


Phosphite is used to control Oomycetes in a wide range of horticultural and native plant species worldwide. However, phosphite can be phytotoxic, and some pathogens have exhibited a reduction in the effectiveness of phosphite due to prolonged use. In this study, salicylic acid (SA) was investigated as an alter- native, or supplementary, treatment to be used to protect plant species. With the use of aeroponics chambers, foliar application of phosphite, SA, and phosphite/SA to Lupinus augustifolius was assessed in relation to root tip damage, in planta phosphite and SA concentration and lesion development. Both phosphite and SA were measurable at the root tip within 24 h of application, and all treatments significantly (P≤0.05) reduced the lesion length at 7 days. However, while phosphite and SA application increased the in planta SA concentration, phosphite caused significantly more damage to the root tip by reducing root cap layers and length than the SA, or phosphite/SA application. This study supports the notion that phosphite-induced sensitivity may be SA- dependent, as both phosphite and SA were found to control P. cinnamomi and stimulate SA accumulation. A combination of phosphite and SA may be more beneficial to plants if it can reduce phytotoxic effects and reduce the chance of pathogen sensitivity to phosphite.