Effective Management Strategies for Controlling Potato Late Blight Caused by Phytophthora Infestans
by Jorge Luis Alonso with ChatGPT
The review published in the Journal of Integrative Agriculture outlines the current understanding of the molecular interaction of Phytophthora infestans with host plants, discusses the integrated pest management strategy for the control of potato late blight, and explores the potential for sustainable control through genetic improvement of plants and new protection technologies. A summary of the contents is provided below.
Introduction
The devastating potato late blight (PLB) disease, caused by the plant pathogen Phytophthora infestans, has spread globally, affecting nearly all major potato-producing countries. This disease remains the most significant biotic constraint to potato production, resulting in yield losses and management costs of up to $3–10 billion annually.
In underdeveloped areas that rely heavily on potatoes as a primary food source, the disease is a major threat to global food security. Chemical control is often prohibitively expensive, resulting in yield losses of more than 60% in developing countries. In developed countries, the cost of pesticides to control PLB can account for 10 to 25% of the market value of the potato crop. This heavy reliance on pesticides has both environmental and economic costs associated with potato production.
Symptoms of PLB infection in the field typically first appear as dark gray to brown, water-soaked spots on leaf tissue surrounded by white, mold-like growth. Under high humidity and low temperatures, infection spreads rapidly within and between plants by systematic filamentous growth and dispersal of infectious asexual sporangia by air and water splashes. This leads to total necrosis of infected plants within 5–10 days and may spread to underground tubers, rendering them unfit for human or animal consumption.
To control this disease, growers and researchers have successfully implemented local epidemic forecasting systems and guided chemical pesticide management systems. Similarly, detailed molecular and genomic research has improved the understanding of the pathogenic interaction between P. infestans and host plants, leading to the development of next-generation, environmentally friendly management strategies for PLB. Integrated Pest Management (IPM), on the other hand, is an emerging approach that balances the immediate efficacy and long-term environmental and ecological costs of pest control in agricultural practices.
In this review, published in the Journal of Integrative Agriculture, the authors focus on the action-oriented potential of improved PLB management through the genetic enhancement of plant resistance and other emerging crop protection technologies.
Molecular interactions between P. infestans and hosts
P. infestans, a hemibiotrophic pathogen, begins by extracting important nutrients from living cells by using special structures to penetrate the surface of the plant. Once inside the plant, it forms small vesicles and hyphae that invade the cells and extract more nutrients. This helps the pathogen switch to a different lifestyle where it obtains nutrients from dead plant tissue. Finally, it develops reproductive structures on the underside of the leaves.
When P. infestans infects host plants, the plants respond with defense mechanisms. The outcome of the disease is determined by how the host defense and pathogenesis mechanisms interact. During infection, the penetration peg and pathogen elicitin molecules cause physical pressure and are recognized by the plant, respectively. This causes the plant to rearrange its microfilaments, deposit localized callose, and activate signaling pathways that produce antimicrobial small molecules and proteins through the regulation of reactive oxygen species, salicylic acid, and ethylene.
In addition, plant-produced peptidases and proteases inhibit the growth of P. infestans hyphae between plant cells. When the pathogen’s virulence molecules are recognized within plant cells by intracellular nucleotide-binding oligomerization domain-like receptors (NLRs) encoded by plant R genes, a process known as the plant hypersensitive response occurs. This leads to localized programmed cell death, which restricts further expansion of the pathogen during its early biotrophic phase.
P. infestans uses effector molecules to overcome host defense and invade successfully. These molecules help the pathogen take nutrients from the host plant, break down the plant’s cell walls, and interfere with the plant’s defense signaling. The genes for effector molecules are often found in parts of P. infestans’ genome that have many repeats. This means that they evolve quickly, which gives the pathogen more flexibility in its genome. P. infestans has a large number of effector molecules because it quickly evolves new strains that can evade the plant’s natural defenses. This makes it challenging to create crop plants that can resist P. infestans’ attacks over the long term.
Recommended agronomic practices for late blight management
Here are the main actions to take for the effective management of this disease:
- Prevent excess moisture and reduce initial inoculum load around the field.
- Improve soil aeration and drainage by tilling the soil and using other recommended practices.
- Discourage excessive irrigation and space plants properly while hilling to reduce shading and contact with infectious P. infestans sporangia.
- Eliminate infectious P. infestans spores and sporangia by selecting disease-free seed tubers, avoiding cutting and wounding them, and supplementing with pesticide treatment if necessary.
- Eliminate cull piles and volunteer plants from previous growing seasons as they can be a primary source of initial inoculum.
- Control weeds, especially those in the same nightshade family as potatoes and tomatoes, to help reduce the inoculum load.
- Eliminate aboveground tissues by either chemical desiccation or physical defoliation before tuber harvest since P. infestans infection is typically initiated on aerial organs.
- Take action and implement these practices for successful PLB management.
Late blight management with chemical pesticides
Chemical pesticides remain the most widely used option for effective late blight control in potato production worldwide. There are 36 fungicides and fungicidal mixtures registered for its control in Europe, and three types of commercial pesticides are commonly used.
To prevent P. infestans infection, protective pesticides must be present in or on plant tissues prior to the arrival and germination of P. infestans spores. Curative pesticides can stop the growth of P. infestans hyphae after initial penetration and local colonization of plant tissues. Anti-sporulants, another chemical pesticide, reduce the formation of reproductive sporangiophores and sporangia.
Pesticides can also be categorized based on their mode of action. Contact pesticides remain on the surface of plant tissues after application and are prone to leaching. Translaminar pesticides can be absorbed by plants and remain in locally treated tissues, while systemic pesticides are transported throughout the plant after initial application and absorption.
However, the widespread use of chemical pesticides poses economic and environmental risks. The misuse of pesticides has led to the emergence of several pesticide-resistant populations of P. infestans worldwide. Therefore, the mixed-use of different pesticides with different modes of action is encouraged to prevent the development of pesticide-resistant populations.
Concerted efforts to evaluate the efficacy of various pesticide combinations against PLB have resulted in publicly available data from multi-year collaborative field trials. Since the 1980s, decision support systems that integrate critical environmental attributes have been developed to predict the potential outbreak of local blight epidemics. These efforts have been a major advance in PLB management in recent decades.
Decision support system-guided control of potato late blight
To effectively control PLB, accurate prediction of potential epidemic outbreaks and timely application of protective pesticides are critical. This can be achieved through the use of decision support systems (DSS) that integrate multidimensional environmental data and quantitative detection methods of P. infestans from field-collected samples.
Using in-field sensors and sophisticated molecular techniques such as loop-mediated isothermal amplification and recombinase-polymerase amplification assays, the DSS can quantitatively assess the probability of PLB outbreak and calculate residual pesticide efficacy. In addition, by knowing the R genes present in the crop and the corresponding avirulence (Avr) genes present in the local P. infestans population, the probability of successful genetic resistance to PLB can be estimated.
By incorporating this high-dimensional data into sophisticated predictive models, the DSS can dramatically improve the efficiency of chemical control of PLB and promote the durability of genetic resistance in crops. Therefore, an integrated pest management strategy is necessary for successful and sustainable PLB control. Act on this information by using DSS to guide preventive pesticide treatment and optimize chemical control of PLB for maximum efficiency.
Genetic enhancement of potato resistance against late blight
Developing potato varieties with genetic resistance to this disease has been a priority for breeders since the Irish potato famine. Research has identified two types of genetic resistance to P. infestans.
The first type, called race-specific or vertical resistance, is based on R genes that recognize specific pathogen effectors and trigger a host response to limit initial infection. The second type, called race-unspecific or horizontal resistance, involves a complex genetic architecture and diverse molecular mechanisms that are not fully understood.
Although non-R gene-mediated resistance has been observed in non-host plant species, it is impractical to introduce it into potato germplasm using traditional breeding methods. Therefore, stacking multiple R genes that confer broad-spectrum resistance is the most useful genetic resource for improving potato germplasm; however, these genes may become ineffective as new P. infestans strains emerge.
Traditionally, R genes have been identified through genetic mapping of populations derived from crosses between cultivated potatoes and their wild relatives, but new sequencing technologies have accelerated the process of R gene discovery across a wide phylogenetic range. However, genetic dissection of non-race-specific resistance remains challenging due to its complex genetic architecture and relatively small effect size.
Although studies have identified some genes associated with non-race-specific resistance, most have been identified using reverse genetic approaches, which can have complex pleotropic effects. Therefore, more research is needed to identify practical genetic elements for breeding potato cultivars with this type of resistance.
Emerging technologies in late blight management
Emerging technologies offer new hope for managing potato late blight. While chemical pesticides, forecasting systems, and resistance gene discovery have been used to control the spread of P. infestans, the global prevalence of this disease shows that more needs to be done. In addition to improving existing management practices, new technologies have emerged that show promise for revolutionizing the current PLB management regime.
Scientists have tested several phytochemicals as environmentally friendly biopesticides. Promising candidates include eugenol, matrine, carvacrol, and zeylenone. For example, field tests have shown that applying 0.3% eugenol is comparable to or even superior to chemical pesticides such as mancozeb, resulting in higher potato yields. Eugenol also significantly inhibited the growth of P. infestans on oatmeal agar, and its protective effect can be enhanced by delivery via nanomaterial carriers. Zeylenone, which interferes with the energy metabolism of Phytophthora, could be developed as a botanical fungicide.
Biological control of P. infestans with competitive microbes has also been proposed. For example, Trichoderma strain HNA14 could outcompete the growth of P. infestans in vitro, inhibit its growth by mycoparasitism, and significantly reduce the disease index in the field. Myxococcus fulvus B25-I-3 also showed antagonistic activity against the pathogen, inhibiting its asexual and sexual reproduction. Combinations of different potato-associated Pseudomonas have shown improved bio-control of P. infestans. In addition, some biocontrol agents may facilitate PLB control by promoting host plant defense.
RNA interference (RNAi) is another technology that shows potential for controlling P. infestans. RNAi plays an important role in plant defense against arthropod herbivores and filamentous pathogens. Recent studies have shown that some Phytophthora effectors can inhibit the RNA silencing pathway in plants. Host-induced gene silencing in P. infestans by potato was first demonstrated in 2015, and recent research has suggested that bidirectional small RNA trafficking and functionality may be another battleground between Phytophthora pathogens and their host plants. In the near future, it may be possible to develop non-transgenic small RNA spray-based methods to control P. infestans.
Conclusion and future prospect
The battle against PLB has been long and hard, but significant progress has been made in controlling it through the use of better agricultural techniques, improved detection and forecasting systems, and the widespread use of chemical pesticides. However, with the costs associated with widespread pesticide use and global climate change, these methods are becoming less sustainable. There is a need for collaboration between laboratory scientists, field pathologists, product developers, and growers to develop an efficient, integrated PLB management strategy that is sustainable for potato production worldwide. The focus should be on translating scientific advances and technological developments in PLB research into practical solutions for sustainable potato production.
Source: Suo-meng DONG, Shao-qun ZHOU. Potato late blight caused by Phytophthora infestans: From molecular interactions to integrated management strategies. Journal of Integrative Agriculture, Volume 21, Issue 12, 2022.
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