Pathogen Profile #3

Hello and welcome back to another week of Robigalia, delivering the latest in plant pathology directly to your inboxes.

Late last year, I was fortunate enough to travel to Tasmania for the first time. Although small in size, this island off mainland Australia is the world's largest producer of licit opioid alkaloids from poppies (Papaver somniferum), supplying nearly half of the world's demand. These alkaloids have been used for centuries to relieve pain, with pharmaceutical companies using them to synthesise opiate-based medicinal since the early 1800’s. While keeping the location of these crops secret stops theft, it’s not enough to protect these plants from microscopic invaders.

Downy mildew is one of the greatest threats to P. somniferum production worldwide. Caused by two oomycete pathogens, Peronospora somniferi and P. meconopsidis, these obligate biotrophs can spread through airborne sporangia, contaminated seeds, and contaminated soil.

Once in contact with the host plant, the sporangia germinate to produce a germ tube penetrating the plant's epidermal cells. The pathogens rapidly colonise the internal tissues of the host plant, using the plant's nutrients to proliferate. The pathogens spread internally, evading host detection, and continue to extract nutrients without killing the plant.

As colonisation progresses, disease symptoms begin to develop. This is where the two pathogens differ. P. somniferi causes systemic infections, resulting in stunted growth, chlorotic and distorted leaves, abundant sporulation leaf surfaces, and deformed stems. While P. meconopsidis induces localised, angular necrotic lesions, with minimal sporulation on the leaf surfaces. After sporulation, these sporangia can then be dispersed by wind or water to infect new plants.

As you can imagine, pathogens impacting a billion dollar global industry have huge economic impacts, mostly associated with implementing control measures and yield losses. As a result, there is increasing emphasis on preventative measures for downy mildew. This includes resistance breeding efforts, biological control, disease monitoring (spore trapping), and seed treatments. Seed treatments have been shown to highly effective at preventing downy mildew, reducing seed to seedling transmission entirely in some cases.

Peronospora somniferi and Peronospora meconopsidis papers

Effect of physio-chemical seed treatments on opium poppy downy mildews caused by Peronospora meconopsidis and P. somniferi

• This study investigated seed treatment methods to combat downy mildew in opium poppy. Using acidified electrolytic water and hypochlorite solution, the treatments significantly reduced the transmission of P. meconopsidis and P. somniferi, by up to 100%.

Molecular insights into enhanced resistance of Papaver somniferum against downy mildew by application of endophyte bacteria Microbacterium sp. SMR1

• This study investigated the application of the endophyte bacteria Microbacterium sp. SMR1 to increase resistance of P. somniferum to downy mildew caused by P. meconopsidis. Through molecular analyses, the study reveals SMR1 treatment activates the salicylic acid-dependent defense pathway in the plant, leading to defense against the pathogen.

Host range of Peronospora somniferi and Pe. meconopsidis within selected members of the Papaveraceae under controlled conditions

• This study explores the host range of the downy mildew pathogens Peronospora somniferi and P. meconopsidis within the Papaveraceae family. The research identifies new host species for these pathogens and demonstrates the potential for seed transmission, underscoring the need for vigilant disease management strategies in poppy cultivation.

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Until next week,

Alyssa