Probing Historic Plant Collections. Is Hawthorn Powdery Mildew a threat to Australian Cherries? – ‘Plant Pathology’ Highlight

Cherry Powdery Mildew is a quarantined disease in Australia. This disease causes significant yield loss to farmers from the US to Japan through reduced photosynthesis in infected leaves, but Australian farmers have yet to record it. Here we discover the first study resulting from a set of intricate experiments at the Victorian Plant Pathogen Herbarium in Australia, where powdery mildew DNA sequences were extracted from historic plant tissue samples using the power of Next Generation Sequencing.

 

Podosphaera clandestina or Podosphaera cerasi?

Cherry powdery mildew was previously called Podosphaera clandestina, which had a wide host range including Crataegus sp. or hawthorn. A recent molecular revision showed that cherry powdery mildew or P. cerasi is a genetically different species from P. clandestina on hawthorn. In Australia, there have been no reports of cherry powdery mildew within the cherry industry; however, specimens of P. clandestina on hawthorn have been collected and preserved in the Victorian Plant Pathogen Herbarium (VPRI). Using DNA extraction and sequencing methods established for VPRI powdery mildew specimens, P. clandestina specimens on Crataegus, Amelanchieris and Prunus hosts from Australia, Europe and the U.S.A. were reanalysed to reidentify the powdery mildew species present in Australia.

Herbarium samples of Hawthorn – Crataegus monogyna from Australia (left) and Amelanchier laevis from Germany (right). Next Generation Sequencing classified the powdery mildew species to be:  Podosphaera clandestina on Hawthorn and P. amelancharis on Amelanchier. P. cerasi was not found on any Australian samples. [CLICK TO ENLARGE]

The study consisted of 19 specimens up to 130 years old which underwent DNA extraction and whole genome next generation sequencing (NGS) to produce sequence reads that were mapped to powdery mildew reference sequences of the internal transcribed spacer (ITS) region. This was the first study to reanalyse powdery mildew specimens using these techniques and provides a way to overcome the issue of fragmented DNA from preserved specimens. The short DNA lengths are mapped across the whole ITS, or any target gene region, to provide a complete sequence to be used in phylogenetic studies.

The phylogenetic analysis of the 19 VPRI P. clandestina sequences showed that all seven Australian specimens and one specimen from Switzerland were P. clandestina in the strict sense on Crataegus. Five VPRI sequences from the U.S.A. represented the newly described cherry powdery mildew, P. cerasi and one VPRI U.S.A. sequence was identified as P. prunicola, another Prunus infecting powdery mildew. One VPRI sequence from Germany was reidentified as P. amelanchieris and there were three VPRI sequences (two from U.S.A. and one from Finland) which failed to be molecularly identified to species and one specimen was excluded due to poor sequencing.

The results of this study firstly provided phylogenetic evidence that cherry powdery mildew P. cerasi is not present on Australian Crataegus specimens held in VPRI. This information will assist Australian biosecurity to prevent the entry of P. cerasi into Australia and protect the Australian cherry industry. Secondly, this study demonstrated the use of a new molecular approach to reanalyse VPRI powdery mildew specimens up to 130 years old for phylogenetic studies when morphology cannot determine species alone.

 

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Reannon L. Smith, Tom W. May, Jatinder Kaur, Tim Sawbridge, Ross Mann and Jacqueline Edwards, at the State Government of Victoria’s Agriculture Victoria and Royal Botanic Gardens Melbourne, published this study in Plant Pathology Journal:

Molecular data from up to 130‐year‐old herbarium specimens do not support the presence of cherry powdery mildew in Australia

Index Herbarium VPRI link: http://sweetgum.nybg.org/science/ih/herbarium-details/?irn=124840

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TITLE IMAGE: 130 year-old Herbarium specimen of Cherry (Prunus cerasus) from USA, infected with Podosphaera cerasi. Magnified view (inset top right) of Powdery mildew (P. cerasi) cleistothecia and mycelia on the leaf. Original label (top left) on specimen packet with initial label from 1889 naming the disease P. oxyacanthae. Credit: Reannon Smith © 2020 Commonwealth of Australia. All images used with permission of the author.