Plant disease epidemiology

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Plant disease epidemiology is the study of disease in plant populations. Much like diseases of humans and other animals, plant diseases occur due to pathogens such as bacteria, viruses, fungi, oomycetes, nematodes, phytoplasmas, protozoa, and parasitic plants. [1] Plant disease epidemiologists strive for an understanding of the cause and effects of disease and develop strategies to intervene in situations where crop losses may occur. Destructive and non-destructive methods are used to detect diseases in plants. Additionally, understanding the responses of the immune system in plants will further benefit and limit the loss of crops. Typically successful intervention will lead to a low enough level of disease to be acceptable, depending upon the value of the crop.

Contents

Plant disease epidemiology is often looked at from a multi-disciplinary approach, requiring biological, statistical, agronomic and ecological perspectives. Biology is necessary for understanding the pathogen and its life cycle. It is also necessary for understanding the physiology of the crop and how the pathogen is adversely affecting it. Agronomic practices often influence disease incidence for better or for worse. Ecological influences are numerous. Native species of plants may serve as reservoirs for pathogens that cause disease in crops. Statistical models are often applied in order to summarize and describe the complexity of plant disease epidemiology, so that disease processes can be more readily understood. [2] [3] For example, comparisons between patterns of disease progress for different diseases, cultivars, management strategies, or environmental settings can help in determining how plant diseases may best be managed. Policy can be influential in the occurrence of diseases, through actions such as restrictions on imports from sources where a disease occurs.

In 1963 J. E. van der Plank published "Plant Diseases: Epidemics and Control", providing a theoretical framework for the study of the epidemiology of plant diseases. [4] This book provides a theoretical framework based on experiments in many different host pathogen systems and moved the study of plant disease epidemiology forward rapidly, especially for fungal foliar pathogens. Using this framework we can now model and determine thresholds for epidemics that take place in a homogeneous environment such as a mono-cultural crop field. [4]

Elements of an epidemic

Disease epidemics in plants can cause huge losses in yield of crops as well threatening to wipe out an entire species such as was the case with Dutch Elm Disease and could occur with Sudden Oak Death. An epidemic of potato late blight, caused by Phytophthora infestans , led to the Great Irish Famine and the loss of many lives. [5]

Commonly the elements of an epidemic are referred to as the “disease triangle”: a susceptible host, pathogen, and conducive environment. [1] [ page needed ] For a disease to occur all three of these must be present. Below is an illustration of this point. Where all three items meet, there is a disease. The fourth element missing from this illustration for an epidemic to occur is time. As long as all three of these elements are present disease can initiate, an epidemic will only ensue if all three continue to be present. Anyone of the three might be removed from the equation though. The host might out-grow susceptibility as with high-temperature adult-plant resistance, [6] the environment changes and is not conducive for the pathogen to cause disease, or the pathogen is controlled through a fungicide application.

Sometimes a fourth factor of time is added as the time at which a particular infection occurs, and the length of time conditions remain viable for that infection, can also play an important role in epidemics. [1] [ page needed ] The age of the plant species can also play a role, as certain species change in their levels of disease resistance as they mature; in a process known as ontogenic resistance. [1]

If all of the criteria are not met, such as a susceptible host and pathogen are present, but the environment is not conducive to the pathogen infecting and causing disease, a disease cannot occur. For example, corn is planted into a field with corn residue that has the fungus Cercospora zea-maydis, the causal agent of Grey leaf spot of corn, but if the weather is too dry, and there is no leaf wetness the spores of the fungus in the residue cannot germinate and initiate infection.[ citation needed ]

Likewise, if the host is susceptible and the environment favours the development of disease but the pathogen is not present there is no disease. Taking the example above, the corn is planted into a ploughed field where there is no corn residue with the fungus Cercospora zea-maydis, the causal agent of Grey leaf spot of corn, present but the weather means extended periods of leaf wetness, there is no infection initiated.

When a pathogen requires a vector to be spread then for an epidemic to occur the vector must be plentiful and active.

Types of epidemics

Pathogens cause monocyclic epidemics with a low birth rate and death rate, meaning they only have one infection cycle per season. They are typical of soil-borne diseases such as Fusarium wilt of flax. Polycyclic epidemics are caused by pathogens capable of several infection cycles a season. They are most often caused by airborne diseases such as powdery mildew. Bimodal polycyclic epidemics can also occur. For example, in brown rot of stone fruits the blossoms and the fruits are infected at different times.[ citation needed ]

For some diseases the disease occurrence needs to be evaluated over several growing seasons, especially if growing the crops in monoculture year after year or growing perennial plants. Such conditions can mean that the inoculum produced in one season can be carried over to the next leading to a build-up over the years, especially in the tropics where there are no clear-cut breaks between growing seasons.[ citation needed ]

Epidemics under these conditions are called polyetic; they can be caused by both monocyclic and polycyclic pathogens. Apple powdery mildew is an example of a polyetic epidemic caused by a polycyclic pathogen; Dutch Elm disease a polyetic epidemic caused by a monocyclic pathogen.

Detecting diseases

There are many different ways to spot a disease both destructively and non-destructively. In order to understand the cause, affects, and cure for a disease, the non-destructive method is more favorable. They are techniques where sample preparation and/or repetitive processes are not necessary for measuring and observing the conditions of the plants’ health. [7] Non-destructive approaches may include image processing, imaging-based, spectroscopy based, and remote sensing.

Photography, digital imaging, and image analysis technology are useful tools to set up for image processing. Valuable data are extracted from these images and then are analyzed for diseases. But before any analysis happens, image acquisition is the first step. And within this step contains three stages. First, is energy which is the light source of  illuminating from the object of interest. Second, is the optical system such as a camera to focus on the energy. Third, is the energy measured by the sensor. To continue with the image processing, there is a pre-process where one can make certain that there are no factors such as background, size, shape of leave, light, and camera effects the analysis.After the pre-process, image segmentation is used to split the image between regions of disease and non-disease. In these images, there features of color, texture, and shape that can be extracted and used for the analysis. [7]

Imaging-based approaches for the detection has two main methods, fluorescence imaging and hyper-spectral imaging. Fluorescence imaging helps identify the metabolic conditions of the plant. In order to do so, a tool is used to present light onto the chlorophyll complex of the plant. [7] Hyper-spectral imaging is used to obtain reflected images. Such methods consist of the spectral information divergence (SID) where it can assess the spectral reflectance by looking at wavelength bands. [7]

Another non-destructive approach is spectroscopy. This is where the electromagnetic spectrum and matter becomes involved. There are visible and infrared spectroscopy, fluorescence spectroscopy, and electric impedance spectroscopy. Each spectroscopy gives information including the types of radiation energy, the types of material, the nature of interaction, and more. [7]

Finally, the last non-destructive approach is the application of remote sensing in plant diseases. This is where data is obtained without having to be with the plant while observing. There is hyper-spectral and multispectral in remote sensing. Hyper-spectral helps provide high spectral and spatial resolution. Multispectral remote sensing provides the severity of the disease. [7]

As of 2015 there is a need for further development of antibody- and molecular marker-tests for new pathogens and occurrence of known pathogens in new hosts, and also a need for further global integration of quarantine and surveillance. [8]

Immune System

Plants can show many signs or physical evidence of fungal, viral or bacterial infections. This can range from rusts or molds to not showing anything at all when a pathogen invades the plant (occurs in some viral diseases in plants). [9] Symptoms which are visible effects of diseases on the plant consist of changes in color, shape or function. [9] These changes in the plant coordinates with their response to pathogens or foreign organisms that is negatively effecting their system. Even though plants do not have cells that can move and fight foreign organisms and they do not have a somatic adaptive immune system, they do have and depend on innate immunity of each cell and on systemic signals. [10]

In responses to infections, plants have a two-branched innate immune system. The first branch has to recognize and respond to molecules that are similar to classes of microbes, this includes non-pathogens. [11] On the other hand, the second branch responds to pathogen virulence factors, either directly or indirectly to the host. [11]

Pattern recognition receptors (PRRs) are activated by recognition of pathogen or microbial-associated molecular patterns known as PAMPs or MAMPs. These leads to PAMP-Triggered Immunity or Pattern-Triggered Immunity (PTI) where PRRs causes intracellular signaling, transcriptional reprogramming, and biosynthesis of a complex output response that decreases colonization. [11]

In addition, R genes also known as Effector-Triggered Immunity is activated by specific pathogen “effectors” that can trigger a strong antimicrobial response. [11] Both PTI and ETI assist in plant defense through activation of DAMP which is Damage-associated Compounds. [11] Cellular changes or changes in gene expression are activated through ion channel gating, oxidative burst, cellular redox changes, or protein kinase cascades through PTI and ETI receptors. [11]

Impact

Through 2013, invasive tree diseases had killed about 100 million elm trees combined in the United Kingdom and United States and 3.5 billion American chestnut trees. [12]

See also

Related Research Articles

<span class="mw-page-title-main">Epidemic</span> Rapid spread of disease affecting a large number of people in a short time

An epidemic is the rapid spread of disease to a large number of hosts in a given population within a short period of time. For example, in meningococcal infections, an attack rate in excess of 15 cases per 100,000 people for two consecutive weeks is considered an epidemic.

<span class="mw-page-title-main">Plant disease</span> Diseases of plants

Plant diseases are diseases in plants caused by pathogens and environmental conditions. Organisms that cause infectious disease include fungi, oomycetes, bacteria, viruses, viroids, virus-like organisms, phytoplasmas, protozoa, nematodes and parasitic plants. Not included are ectoparasites like insects, mites, vertebrates, or other pests that affect plant health by eating plant tissues and causing injury that may admit plant pathogens. The study of plant disease is called plant pathology.

<i>Magnaporthe grisea</i> Blast, fungal disease of rice & wheat

Magnaporthe grisea, also known as rice blast fungus, rice rotten neck, rice seedling blight, blast of rice, oval leaf spot of graminea, pitting disease, ryegrass blast, Johnson spot, neck blast, wheat blast and Imochi (稲熱), is a plant-pathogenic fungus and model organism that causes a serious disease affecting rice. It is now known that M. grisea consists of a cryptic species complex containing at least two biological species that have clear genetic differences and do not interbreed. Complex members isolated from Digitaria have been more narrowly defined as M. grisea. The remaining members of the complex isolated from rice and a variety of other hosts have been renamed Magnaporthe oryzae, within the same M. grisea complex. Confusion on which of these two names to use for the rice blast pathogen remains, as both are now used by different authors.

<i>Cochliobolus miyabeanus</i> Species of fungus

Cochliobolus miyabeanus is a fungus that causes brown spot disease in rice.

<i>Pyrenophora tritici-repentis</i> Species of fungus

Pyrenophora tritici-repentis (teleomorph) and Drechslera tritici-repentis (anamorph) is a necrotrophic plant pathogen of fungal origin, phylum Ascomycota. The pathogen causes a disease originally named yellow spot but now commonly called tan spot, yellow leaf spot, yellow leaf blotch or helminthosporiosis. At least eight races of the pathogen are known to occur based on their virulence on a wheat differential set.

<i>Gibberella zeae</i> Species of fungus

Gibberella zeae, also known by the name of its anamorph Fusarium graminearum, is a fungal plant pathogen which causes fusarium head blight (FHB), a devastating disease on wheat and barley. The pathogen is responsible for billions of dollars in economic losses worldwide each year. Infection causes shifts in the amino acid composition of wheat, resulting in shriveled kernels and contaminating the remaining grain with mycotoxins, mainly deoxynivalenol (DON), which inhibits protein biosynthesis; and zearalenone, an estrogenic mycotoxin. These toxins cause vomiting, liver damage, and reproductive defects in livestock, and are harmful to humans through contaminated food. Despite great efforts to find resistance genes against F. graminearum, no completely resistant variety is currently available. Research on the biology of F. graminearum is directed towards gaining insight into more details about the infection process and reveal weak spots in the life cycle of this pathogen to develop fungicides that can protect wheat from scab infection.

<i>Macrophomina phaseolina</i> Species of fungus

Macrophomina phaseolina is a Botryosphaeriaceae plant pathogen fungus that causes damping off, seedling blight, collar rot, stem rot, charcoal rot, basal stem rot, and root rot on many plant species.

<i>Sclerotinia sclerotiorum</i> Species of fungus

Sclerotinia sclerotiorum is a plant pathogenic fungus and can cause a disease called white mold if conditions are conducive. S. sclerotiorum can also be known as cottony rot, watery soft rot, stem rot, drop, crown rot and blossom blight. A key characteristic of this pathogen is its ability to produce black resting structures known as sclerotia and white fuzzy growths of mycelium on the plant it infects. These sclerotia give rise to a fruiting body in the spring that produces spores in a sac which is why fungi in this class are called sac fungi (Ascomycota). This pathogen can occur on many continents and has a wide host range of plants. When S. sclerotiorum is onset in the field by favorable environmental conditions, losses can be great and control measures should be considered.

<i>Alternaria solani</i> Species of fungus

Alternaria solani is a fungal pathogen that produces a disease in tomato and potato plants called early blight. The pathogen produces distinctive "bullseye" patterned leaf spots and can also cause stem lesions and fruit rot on tomato and tuber blight on potato. Despite the name "early," foliar symptoms usually occur on older leaves. If uncontrolled, early blight can cause significant yield reductions. Primary methods of controlling this disease include preventing long periods of wetness on leaf surfaces and applying fungicides. Early blight can also be caused by Alternaria tomatophila, which is more virulent on stems and leaves of tomato plants than Alternaria solani.

Peronosclerospora sorghi is a plant pathogen. It is the causal agent of sorghum downy mildew. The pathogen is a fungal-like protist in the oomycota, or water mold, class. Peronosclerospora sorghi infects susceptible plants though sexual oospores, which survive in the soil, and asexual sporangia which are disseminated by wind. Symptoms of sorghum downy mildew include chlorosis, shredding of leaves, and death. Peronosclerospora sorghi infects maize and sorghum around the world, but causes the most severe yield reductions in Africa. The disease is controlled mainly through genetic resistance, chemical control, crop rotation, and strategic timing of planting.

Phialophora gregata is a Deuteromycete fungus that is a plant pathogen which causes the disease commonly known as brown stem rot of soybean. P. gregata does not produce survival structures, but has the ability to overwinter as mycelium in decaying soybean residue.

<span class="mw-page-title-main">Plant disease forecasting</span> Disease management system

Plant disease forecasting is a management system used to predict the occurrence or change in severity of plant diseases. At the field scale, these systems are used by growers to make economic decisions about disease treatments for control. Often the systems ask the grower a series of questions about the susceptibility of the host crop, and incorporate current and forecast weather conditions to make a recommendation. Typically a recommendation is made about whether disease treatment is necessary or not. Usually treatment is a pesticide application.

Pathogenic fungi are fungi that cause disease in humans or other organisms. Although fungi are eukaryotic, many pathogenic fungi are microorganisms. Approximately 300 fungi are known to be pathogenic to humans; their study is called "medical mycology". Fungal infections are estimated to kill more people than either tuberculosis or malaria—about two million people per year.

Biotic stress is stress that occurs as a result of damage done to an organism by other living organisms, such as bacteria, viruses, fungi, parasites, beneficial and harmful insects, weeds, and cultivated or native plants. It is different from abiotic stress, which is the negative impact of non-living factors on the organisms such as temperature, sunlight, wind, salinity, flooding and drought. The types of biotic stresses imposed on an organism depend the climate where it lives as well as the species' ability to resist particular stresses. Biotic stress remains a broadly defined term and those who study it face many challenges, such as the greater difficulty in controlling biotic stresses in an experimental context compared to abiotic stress.

<span class="mw-page-title-main">Plant disease resistance</span> Ability of a plant to stand up to trouble

Plant disease resistance protects plants from pathogens in two ways: by pre-formed structures and chemicals, and by infection-induced responses of the immune system. Relative to a susceptible plant, disease resistance is the reduction of pathogen growth on or in the plant, while the term disease tolerance describes plants that exhibit little disease damage despite substantial pathogen levels. Disease outcome is determined by the three-way interaction of the pathogen, the plant and the environmental conditions.

In biology, a pathogen, in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ.

<span class="mw-page-title-main">Southern corn leaf blight</span> Fungal disease of maize

Southern corn leaf blight (SCLB) is a fungal disease of maize caused by the plant pathogen Bipolaris maydis.

Alternaria black spot of canola or grey leaf spot is an ascomycete fungal disease caused by a group of pathogens including: Alternaria brassicae, A. alternata and A. raphani. This pathogen is characterized by dark, sunken lesions of various size on all parts of the plant, including the leaves, stem, and pods. Its primary economic host is canola. In its early stages it only affects the plants slightly by reducing photosynthesis, however as the plant matures it can cause damage to the seeds and more, reducing oil yield as well.

<i>Stenocarpella maydis</i> Species of fungus

Stenocarpella maydis (Berk.) Sutton is a plant pathogenic fungus and causal organism of diplodia ear and stalk rot. Corn and canes are the only known hosts to date. No teleomorph of the fungus is known.

Phyllachora maydis is a plant pathogen causing ascomycete diseases in maize/corn, and is more commonly referred to as tar spot. Identified by the distinctive development of stroma, this pathogen in itself is of little economic importance in the production of corn. However, the accompanying fungal infection of Monographella maydis, identified by “fish-eye” lesions, was claimed to cause significant foliar damage and subsequently yield reduction. As of 2021 there is insufficient information about this pathogen and its management.

References

  1. 1 2 3 4 Agrios, George (2005). Plant Pathology. Academic Press. ISBN   978-0-12-044565-3.
  2. ↑ Arneson, P.A. (2001). "Plant disease epidemiology: temporal aspects". Plant Health Instructor. American Phytopathological Society. doi:10.1094/PHI-A-2001-0524-01 (inactive 2024-04-08). Archived from the original on 2008-02-23.{{cite journal}}: CS1 maint: DOI inactive as of April 2024 (link)
  3. ↑ Madden, Laurence; Gareth Hughes; Frank Van Den Bosch (2007). Study of Plant Disease Epidemics. American Phytopathological Society. ISBN   978-0-89054-354-2.
  4. 1 2 Drenth, A. (2004). "Fungal epidemics – does spatial structure matter?". New Phytologist. 163 (1). Blackwells: 4–7. doi: 10.1111/j.1469-8137.2004.01116.x . PMID   33873785.
  5. ↑ Ó Gráda, Cormac (2006) Ireland's Great Famine, University College Dublin, ISBN   978-1-9045-5858-3, p. 7
  6. ↑ Schultz, T.R.; Line, R.F. (1992). "High-Temperature, Adult-Plant Resistance to Wheat Stripe Rust and Effects on Yield Components". Agronomy Journal . 84 (2). American Society of Agronomy: 170–175. Bibcode:1992AgrJ...84..170S. doi:10.2134/agronj1992.00021962008400020009x. S2CID   84879649.
  7. 1 2 3 4 5 6 Ali, Maimunah Mohd; Bachik, Nur Azizah; Muhadi, Nur ‘Atirah; Tuan Yusof, Tuan Norizan; Gomes, Chandima (December 2019). "Non-destructive techniques of detecting plant diseases: A review". Physiological and Molecular Plant Pathology. 108: 101426. doi:10.1016/j.pmpp.2019.101426. S2CID   199635227.
  8. ↑ Bebber, Daniel P.; Gurr, Sarah J. (2015). "Crop-destroying fungal and oomycete pathogens challenge food security". Fungal Genetics and Biology . 74. Academic Press: 62–64. doi:10.1016/j.fgb.2014.10.012. ISSN   1087-1845. PMID   25459533.
  9. 1 2 "Signs and symptoms of plant disease: Is it fungal, viral or bacterial?". MSU Extension. 19 December 2012. Retrieved 2020-06-10.
  10. ↑ "Plant Disease: Pathogens and Cycles". CropWatch. 2016-12-19. Retrieved 2020-06-10.
  11. 1 2 3 4 5 6 Jones, Jonathan D. G.; Dangl, Jeffery L. (2006-11-16). "The plant immune system". Nature. 444 (7117): 323–329. Bibcode:2006Natur.444..323J. doi: 10.1038/nature05286 . PMID   17108957.
  12. ↑ Fisher, Matthew C.; Henk, Daniel. A.; Briggs, Cheryl J.; Brownstein, John S.; Madoff, Lawrence C.; McCraw, Sarah L.; Gurr, Sarah J. (2012). "Emerging fungal threats to animal, plant and ecosystem health". Nature . 484 (7393). Nature Portfolio: 186–194. Bibcode:2012Natur.484..186F. doi:10.1038/nature10947. ISSN   0028-0836. PMC   3821985 . PMID   22498624. S2CID   4379694. (MCF ORCID 0000-0002-1862-6402 RID B-9094-2011). (DAH GS AbPV6MYAAAAJ ORCID 0000-0002-1142-3143 Publons 4361029). (CJB RID F-7456-2013). (SJG ORCID 0000-0002-4821-0635). NIHMSID 514851.

Further reading

Crop disease epidemiology

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