Pathogen Profile #19

Hello Robigalia Reader and welcome back to another week of Robigalia, delivering the latest in plant pathology directly to your inbox!

This week’s edition will be the last for 2024 while I take some well-needed time off after my PhD submission (hooray)! If you’re living in Australia, you’re probably preparing to indulge in some cherries over the summer holiday season. This week’s article is about Asiatic brown rot impacting cherries and other fruit crops. Scroll down to learn about this pathogen!

Monilinia polystroma, a fungal pathogen belonging to the Monilinia genus, is an emerging threat to fruit crops, particularly stone fruits like cherries and almonds. First described in 2002, when it was differentiated from M. fructigena, this pathogen has been steadily expanding its geographical range and host repertoire, causing significant concern in the horticultural industry.

The life cycle of M. polystroma follows typical brown rot fungal patterns, with the fungus able to infect fruits at various ripening stages. M. polystroma spreads through multiple pathways, with particular emphasis on its ability to establish both visible and latent infections. The pathogen primarily enters through the stomata of fruits, where it progressively colonises and spreads through the epidermal cells and subepidermal tissues.

On infected fruits, the pathogen produces buff to brownish-grey sporodochia with conidia, forming distinctive yellowish or buff-coloured concentric rings. These single-celled spores serve as the primary means of dispersal. In the case of visible infections, the disease manifests on the fruit surface within 24 hours, followed by the collapse of epidermal cells after 48 hours. The pathogen then colonises both the epidermis and mesocarp.

During latent infections, no visible signs appear on the fruit surface for extended periods. However, microscopic examination reveals that the pathogen colonises the stomata and gradually expands its presence, leading to the progressive collapse of epidermal cells. The ability to establish both visible and latent infections makes this pathogen particularly effective at spreading, as infected fruits may appear healthy during harvest and transport, only to develop symptoms later during storage or distribution.

Monilinia polystroma causes significant economic damage through its impact on fruit production and postharvest losses. In a documented case from Jilin Province, China, surveys revealed infection rates reaching approximately 25% of plants in an apple orchard. The impact is particularly severe in unprotected orchards, where infection rates can vary dramatically - from around 1% in protected cherry orchards to over 50% in almond trees.

The economic consequences are amplified by the pathogen's ability to affect fruits at various ripening stages and its capacity to cause postharvest decay. The disease manifests as brown lesions on fruits, leading to fruit shrivelling in cherries while almonds remain firm but unmarketable. This dual impact on both pre-harvest and postharvest stages makes M. polystroma a significant threat to fruit production economics.

The control of M. polystroma and other related species combines multiple management strategies. At its core, effective control begins with orchard sanitisation practices and proper pre-harvest management to minimise inoculum potential and reduce the risk of blossom and fruit infections. These cultural practices are typically supplemented with strategic fungicide applications, both before harvest and during post-harvest handling, though there is growing emphasis on reducing chemical dependence in alignment with sustainable agriculture principles.

Alternative control methods of Monilinia spp. have emerged as promising additions to the management toolkit. These include biological control using microbial antagonists, the application of natural products, and various physico-chemical methods. Integrating these approaches offers the potential for more sustainable and environmentally friendly disease management.

The management of M. polystroma faces both challenges and opportunities. While there is increasing pressure to reduce chemical fungicide usage and develop more sustainable practices, there are promising developments in cultivar breeding for improved disease tolerance and the advancement of integrated control strategies. The path forward will likely require a multidisciplinary approach that successfully combines traditional sanitary measures with innovative alternative strategies, particularly in post-harvest treatments that integrate natural antimicrobial substances with physical control methods.

If you want to read more about Monilinia polystroma, I have summarised a recent paper. Robigalia subscribers can scroll down to access my online database, which includes full summaries of these and all other featured Robigalia articles.

Monilinia polystroma paper

LAMP-based detection of Monilinia fructigena, Monilinia polystroma and Monilinia fructicola in latently infected apple fruit

• A LAMP-based method for detecting latent Monilinia infections in apple fruit has been developed by researchers at the National Institute of Horticultural Research in Poland. The technique can simultaneously detect M. fructigena, M. polystroma, and M. fructicola. Using primers targeting the hsp60 gene, the method demonstrates high specificity and sensitivity, capable of detecting as little as 4-6 fungal spores in a sample. The method was validated on artificially inoculated, asymptomatic apples after 120 days of cold storage. This rapid and sensitive technique could be valuable for predicting brown rot occurrence in stored apples, aiding in disease management strategies and phytosanitary control for fruit exports.

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Until next week,

Alyssa