Pathogen Profile #14

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

Even if you’re new to plant pathology, chances are you’ve heard of today’s feature pathogen. Just like The Terminator, it never gives up, leaving a trail of destruction in its wake. Scroll down to find out which pathogen we’re discussing today. If you’re a Robigalia subscriber you can also access my online database and today’s title song! Oh, and yes, I did write this edition just to include a Terminator reference.

Few plant pathogens have left such a mark on history as Phytophthora infestans. This oomycete is responsible for the devastating disease known as potato late blight. First recognised in the United States in 1843, it quickly spread to Europe by 1845 where it began wreaking havoc on potato crops across the continent. Phytophthora infestans caused the Irish Potato Famine of the 1840s which resulted in the deaths of approximately one million people and forced another million to emigrate. The strain responsible for this devastation, identified as HERB-1, was later replaced by the US-1 strain, which continued to dominate until the late 20th century. Today, there are several strains of concern with the pathogen continually evolving.

The life cycle of P. infestans on potatoes is a complex process involving both asexual and sexual reproduction phases. In its asexual cycle, the pathogen begins with hyphal growth, followed by sporulation where sporangia are produced. These sporangia either germinate directly or release zoospores, which can swim in water films on leaves or soil, causing additional infections. The sporangia and zoospores are primarily responsible for the rapid spread of the disease, as they can be dispersed by wind and water. Under ideal conditions, the pathogen can complete its life cycle in just five days, leading to large-scale epidemics. The sexual cycle occurs when two different mating types (A1 and A2) meet, resulting in oospore formation. Oospores serve as long-term survival structures, capable of persisting in the soil for several years and infecting crops in subsequent years.

Potato late blight causes global losses estimated to exceed $6.5 billion annually, sinificantly affecting yield and quality of potato crops. In the United States, the costs associated with fungicide application and lost revenue are substantial. Annually, fungicide expenses are estimated to reach $77.1 million, with additional revenue losses amounting to $210.7 million. Meanwhile, in Australia, strict biosecurity measures are in place to prevent the introduction of the more severe A2 mating type of Phytophthora infestans. While the A1 mating type, which affects tomatoes, is already present, the A2 type, which devastates potatoes, remains exotic to the continent.

The impact of the pathogen is especially severe in developing countries, where the high cost of chemical control measures often prevents effective management. In P. infestans’ centre of origin Latin America, Peru and Ecuador grapple with challenges posed by both A1 and A2 mating types. Similarly, in Asia, countries like India, Bangladesh, and Indonesia have experienced significant losses due to the introduction of new, potentially more aggressive pathogen populations. These regions have small-scale farming, where resource-poor farmers often lack access to effective disease management tools and training. Relying on fungicides, often applied without proper guidance, increases costs and health risks.

Effective control of Phytophthora infestans requires an integrated disease management approach that combines several strategies. These include using fungicides, planting resistant potato varieties, implementing cultural practices, and employing biological control methods. While fungicides play a central role in late blight management, their potential for resistance underscores the need for careful chemical rotation. Cultural practices—such as crop rotation, field sanitation, and proper drainage—are critical for reducing the initial inoculum and disease spread. By integrating these strategies into a comprehensive management plan, we can effectively mitigate the impacts of potato late blight.

If you’re interested in diving into the science of Phytophthora infestans, I’ve summarised a few articles below.

Phyophthora infestans papers

Evolution of Phytophthora infestans on its potato host since the Irish potato famine

• This 2024 study on Phytophthora infestans reveals significant evolutionary changes in its virulence genes and host resistance genes since the 19th century. Using targeted enrichment sequencing of historical herbarium specimens, researchers found that the pathogen's ability to overcome resistance genes has evolved, highlighting the ongoing challenge of breeding potatoes with durable resistance.

Comparative transcriptome profiling of potato cultivars infected by late blight pathogen Phytophthora infestans: Diversity of quantitative and qualitative responses

• This study investigates the genetic responses of resistant (Ando) and susceptible (Arielle) potato cultivars to P. infestans. Using RNASeq, the researchers identified upregulation of multiple signalling pathways, including salicylic acid (SA), jasmonic acid (JA), and ethylene (ET) pathways in the resistant Ando.

Sensitivity of dominant UK Phytophthora infestans genotypes to a range of fungicide active ingredients

• The study by Lynott et al. investigates the sensitivity of dominant Phytophthora infestans genotypes in the UK to various fungicides. It found that while the EU37 genotype is insensitive to fluazinam, other fungicides remain effective against the tested genotypes, highlighting the importance of ongoing monitoring to manage resistance and ensure effective late blight control.

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In Arnold’s words … I’ll be back,

Alyssa