Rust (fungus)
Rusts are plant diseases caused by pathogenic fungi of the order Pucciniales. An estimated 168 rust genera and 7,000 species, more than half of which belong to the genus Puccinia, are accepted. Rust fungi are specialized plant pathogens with several unique features. Taken as a group, rust fungi affect many kinds of plants. However, each species has a narrow range of hosts and cannot be transmitted to non-host plants. In addition, most rust fungi cannot be grown in pure culture. A single species of rust fungi may be able to infect two different plant hosts in different stages of its life cycle, may produce up to five morphologically and cytologically distinct spore-producing structures viz. spermogonia, uredinia and basidia in successive stages of reproduction. Each spore type is host specific, can infect only one kind of plant. Rust fungi are obligate plant pathogens. Infections begin when a spore lands on the plant surface and invades its host. Infection is limited to plant parts such as leaves, tender shoots, fruits, etc.
Plants with severe rust infection may appear stunted, chlorotic, or may display signs of infection such as rust fruiting bodies. Rust fungi grow intracellularly, make spore-producing fruiting bodies within or, more on the surfaces of affected plant parts; some rust species form perennial systemic infections that may cause plant deformities such as growth retardation, witch's broom, stem canker, galls, or hypertrophy of affected plant parts. Rusts get their name because they are most observed as deposits of powdery rust-coloured or brown spores on plant surfaces; the Roman agricultural festival Robigalia has ancient origins in combating wheat rust. Rusts are considered among the most harmful pathogens to agriculture and forestry. Rust fungi are major concerns and limiting factors for successful cultivation of agricultural and forestry crops. White pine blister rust, wheat stem rust and coffee rust are examples of notoriously damaging, economically important crops. All rusts are obligate parasites, meaning that they require a living host to complete their life cycle.
They do not kill the host plant but can reduce growth and yield. Cereal crops can be devastated in one season and trees that get infected in the main stem within their first five years by the rust Cronartium quercuum die. Rusts can produce up to five spore types from corresponding fruiting body types during their life cycle, depending on the species. Roman numerals have traditionally been used to refer to these morphological types. 0-Pycniospores from Pycnidia. These serve as haploid gametes in heterothallic rusts. I-Aeciospores from Aecia; these serve as non-repeating, asexual spores, go on to infect the primary host. II-Urediniospores from Uredia; these serve as repeating dikaryotic vegetative spores. These spores are referred to as the repeating stage because they can cause auto-infection on the primary host, re-infecting the same host from which the spores were produced, they are profuse, red/orange, a prominent sign of rust disease. III-Teliospores from Telia; these dikaryotic spores are the survival/overwintering stage of life cycle.
They germinate to produce basidia. IV-Basidiospores from Teliospores; these haploid spores infect the alternate host in Spring. Although these are observed outside of the laboratory. Rust fungi are categorized by their life cycle. Three basic types of life cycles are recognized based on the number of spore states as macrocyclic and microcyclic; the macrocyclic life cycle has all spore states, the demicyclic lacks the uredinial state, the microcyclic cycle lacks the basidial and the aecial states, thus possess only uredinia and telia. Spermagonia may be absent from each type but the microcyclic life cycle. In macrocyclic and demicyclic life cycles, the rust may be either host alternating, i.e. the aecial state is on one kind of plant but the telial state on a different and unrelated plant, or non-host alternating, i.e. the aecial and telial states on the same kind of plant. Heteroecious rust fungi require two unrelated hosts to complete their life cycle, with the primary host being infected by aeciospores and the alternate host being infected with basidiospores.
This can be contrasted with an autoecious fungus which can complete its life cycle on a single host species. Understanding the life cycles of rust fungi allows for proper disease management. There are definite patterns of relationship with host plant groups and the rust fungi that parasitize them; some genera of rust fungi Puccinia and Uromyces, comprise species that are capable of parasitizing plants of many families. Other rust genera appear to be restricted to certain plant groups. Host restriction may, in heteroecious species, apply to both phases of life cycle or to only one phase; the fungi produce asexual spores which disperse by wind, water or by insect vectors spreading the infection. Rust fungi are biotrophs; when airborne spores settle on a plant, weak hydrophobic interactions are formed with the cutin on the plant cell surface, securing it. By a process not understood, the production of mucous like substances called'adhesins' stick the spore to the plant surface. Once attached, the spore germinates by growing a germ tube and locates a stoma by a touch responsive process known as thigmotropism.
This involves growing towards a ridge between the epidermal cells, followed by a perpendi
Amoeba
An amoeba called amoeboid, is a type of cell or unicellular organism which has the ability to alter its shape by extending and retracting pseudopods. Amoebas do not form a single taxonomic group. Amoeboid cells occur not only among the protozoa, but in fungi and animals. Microbiologists use the terms "amoeboid" and "amoeba" interchangeably for any organism that exhibits amoeboid movement. In older classification systems, most amoebas were placed in the class or subphylum Sarcodina, a grouping of single-celled organisms that possess pseudopods or move by protoplasmic flow. However, molecular phylogenetic studies have shown that Sarcodina is not a monophyletic group whose members share common descent. Amoeboid organisms are no longer classified together in one group; the best known amoeboid protists are the "giant amoebae" Chaos carolinense and Amoeba proteus, both of which have been cultivated and studied in classrooms and laboratories. Other well known species include the so-called "brain-eating amoeba" Naegleria fowleri, the intestinal parasite Entamoeba histolytica, which causes amoebic dysentery, the multicellular "social amoeba" or slime mould Dictyostelium discoideum.
Amoebae move and feed by using pseudopods, which are bulges of cytoplasm formed by the coordinated action of actin microfilaments pushing out the plasma membrane that surrounds the cell. The appearance and internal structure of pseudopods are used to distinguish groups of amoebae from one another. Amoebozoan species, such as those in the genus Amoeba have bulbous pseudopods, rounded at the ends and tubular in cross-section. Cercozoan amoeboids, such as Euglypha and Gromia, have thread-like pseudopods. Foraminifera emit fine, branching pseudopods that merge with one another to form net-like structures; some groups, such as the Radiolaria and Heliozoa, have stiff, needle-like, radiating axopodia supported from within by bundles of microtubules. Free-living amoebae may be "testate", or "naked"; the shells of testate amoebae may be composed of various substances, including calcium, chitin, or agglutinations of found materials like small grains of sand and the frustules of diatoms. To regulate osmotic pressure, most freshwater amoebae have a contractile vacuole which expels excess water from the cell.
This organelle is necessary because freshwater has a lower concentration of solutes than the amoeba's own internal fluids. Because the surrounding water is hypotonic with respect to the contents of the cell, water is transferred across the amoeba's cell membrane by osmosis. Without a contractile vacuole, the cell would fill with excess water and burst. Marine amoebae do not possess a contractile vacuole because the concentration of solutes within the cell are in balance with the tonicity of the surrounding water; the food sources of amoebae vary. Some amoebae are live by consuming bacteria and other protists; some eat dead organic material. Amoebae ingest their food by phagocytosis, extending pseudopods to encircle and engulf live prey or particles of scavenged material. Amoeboid cells do not have a mouth or cytostome, there is no fixed place on the cell at which phagocytosis occurs; some amoebae feed by pinocytosis, imbibing dissolved nutrients through vesicles formed within the cell membrane.
The size of amoeboid cells and species is variable. The marine amoeboid Massisteria voersi is just 2.3 to 3 micrometres in diameter, within the size range of many bacteria. At the other extreme, the shells of deep-sea xenophyophores can attain 20 cm in diameter. Most of the free-living freshwater amoebae found in pond water and lakes are microscopic, but some species, such as the so-called "giant amoebae" Pelomyxa palustris and Chaos carolinense, can be large enough to see with the naked eye; some multicellular organisms have amoeboid cells only in certain phases of life, or use amoeboid movements for specialized functions. In the immune system of humans and other animals, amoeboid white blood cells pursue invading organisms, such as bacteria and pathogenic protists, engulf them by phagocytosis. Amoeboid stages occur in the multicellular fungus-like protists, the so-called slime moulds. Both the plasmodial slime moulds classified in the class Myxogastria, the cellular slime moulds of the groups Acrasida and Dictyosteliida, live as amoebae during their feeding stage.
The amoeboid cells of the former combine to form a giant multinucleate organism, while the cells of the latter live separately until food runs out, at which time the amoebae aggregate to form a multicellular migrating "slug" which functions as a single organism. Other organisms may present amoeboid cells during certain life-cycle stages, e.g. the gametes of some green algae and pennate diatoms, the spores of some Mesomycetozoea, the sporoplasm stage of Myxozoa and of Ascetosporea. The earliest record of an amoeboid organism was produced in 1755 by August Johann Rösel von Rosenhof, who named his discovery "Der Kleine Proteus". Rösel's illustrations show an unidentifiable freshwater amoeba, similar in appearance to the common species now known as Amoeba proteus; the term "Proteus animalcule" remained in use throughout the 18th and 19th centuries, as an informal name for any large, free-living amoeboid. In 1822, the genus Amiba was erected by the Frenc
Mycetozoa
Mycetozoa is a grouping of slime molds. It can be divided into dictyostelid and protostelid groups; the mycetozoan groups all fit into the unikont supergroup Amoebozoa, whereas most other slime molds fit into various bikont groups. Dictyostelids are used as examples of cellular communication and differentiation, may provide insights into how multicellular organisms develop. Slime molds like Physarum polycephalum are useful for studying cytoplasmic streaming, they have been used to study the biochemical events that surround mitosis, since all the nuclei in a medium-sized plasmodium divide in synchrony. It has been observed that they can find their way through mazes by spreading out and choosing the shortest path, an interesting example of information processing without a nervous system. Myxomycete plasmodia have been used to study the genetics of asexual cell fusion; the giant size of the plasmodial cells allows for easy evaluation of complete or partial cell fusion. In 2006, researchers at the University of Southampton and the University of Kobe reported that they had built a six-legged robot whose movement was remotely controlled by a Physarum slime mold.
The mold directed the robot into a dark corner most similar to its natural habitat. Slime molds are sometimes studied in advanced mathematics courses. Slime mold aggregation is a natural process that can be approximated with partial differential equations. Members of the Mycetozoa group are able to undergo sexual reproduction either by heterothallic or homothallic mating. An analysis of meiosis-related genes in the Dictyostelium discoideum genome revealed that 36 of the 44 genes tested were present in the genome. One gene, Spo11, was absent in the Mycetozoa, raising questions about the assumed universal role of Spo11 as an initiator of meiosis. Slime Molds Slime Mold Solves Maze Puzzle from abc.net.au Hunting Slime Molds from Smithsonian Magazine "Robot Piloted by a Slime Mold". Slashdot. 2006. Retrieved February 15, 2006. DictyBase is an online informatics resource for Dictyostelium, a cellular slime mould. Nomen.eumycetozoa.com is an online nomenclatural information system of slime moulds of the world.
Photo gallery Introduction to the "Slime Molds" Slime Mold Photos Life cycle of Reticularia lycoperdon at MushooMania.com. Video footage of common slime moulds
Tuber
Tubers are enlarged structures in some plant species used as storage organs for nutrients. They are used for the plant's perennation, to provide energy and nutrients for regrowth during the next growing season, as a means of asexual reproduction. Stem tubers form thickened stolons. Common plant species with stem tubers include yam; some sources treat modified lateral roots under the definition. The term originates from Latin tuber, meaning "lump, swelling"; some sources define the term "tuber" to mean only structures derived from stems. A stem tuber forms from thickened stolons; the top sides of the tuber produce shoots that grow into typical stems and leaves and the under sides produce roots. They tend to form at the sides of the parent plant and are most located near the soil surface; the underground stem tuber is a short-lived storage and regenerative organ developing from a shoot that branches off a mature plant. The offsprings or new tubers are attached to a parent tuber or form at the end of a hypogeogenous rhizome.
In the autumn the plant dies, except for the new offspring stem tubers which have one dominant bud, which in spring regrows a new shoot producing stems and leaves, in summer the tubers decay and new tubers begin to grow. Some plants form smaller tubers and/or tubercules which act like seeds, producing small plants that resemble seedlings; some stem tubers are long-lived, such as those of tuberous begonia, but many plants have tubers that survive only until the plants have leafed out, at which point the tuber is reduced to a shriveled-up husk. Stem tubers start off as enlargements of the hypocotyl section of a seedling but sometimes include the first node or two of the epicotyl and the upper section of the root; the stem tuber has a vertical orientation with one or a few vegetative buds on the top and fibrous roots produced on the bottom from a basal section the stem tuber has an oblong rounded shape. Tuberous begonia and Cyclamen are grown stem tubers. Mignonette vine produces aerial stem tubers on 12-to-25-foot-tall vines, the tubers fall to the ground and grow.
Plectranthus esculentus of the mint family Lamiaceae, produces tuberous under ground organs from the base of the stem, weighing up to 1.8 kg per tuber, forming from axillary buds producing short stolons that grow into tubers. Potatoes are stem tubers. Enlarged stolons thicken to develop into storage organs; the tuber has all the parts including nodes and internodes. The nodes are the eyes and each has a leaf scar; the nodes or eyes are arranged around the tuber in a spiral fashion beginning on the end opposite the attachment point to the stolon. The terminal bud is produced at the farthest point away from the stolon attachment and tubers thus show the same apical dominance as a normal stem. Internally, a tuber is filled with starch stored in enlarged parenchyma like cells; the inside of a tuber has the typical cell structures of any stem, including a pith, vascular zones, a cortex. The tuber is produced in one growing season and used to perennate the plant and as a means of propagation; when fall comes, the above-ground structure of the plant dies, but the tubers survive over winter underground until spring, when they regenerate new shoots that use the stored food in the tuber to grow.
As the main shoot develops from the tuber, the base of the shoot close to the tuber produces adventitious roots and lateral buds on the shoot. The shoot produces stolons that are long etiolated stems; the stolon elongates during long days with the presence of high auxins levels that prevent root growth off of the stolon. Before new tuber formation begins, the stolon must be a certain age; the enzyme lipoxygenase makes a hormone, jasmonic acid, involved in the control of potato tuber development. The stolons are recognized when potato plants are grown from seeds; as the plants grow, stolons are produced around the soil surface from the nodes. The tubers form close to the soil surface and sometimes on top of the ground; when potatoes are cultivated, the tubers are planted much deeper into the soil. Planting the pieces deeper creates more area for the plants to generate the tubers and their size increases; the pieces sprout shoots. These shoots generate short stolons from the nodes while in the ground.
When the shoots reach the soil surface, they produce roots and shoots that grow into the green plant. A tuberous root or storage root, is a modified lateral root, enlarged to function as a storage organ; the enlarged area of the root-tuber, or storage root, can be produced at the end or middle of a root or involve the entire root. It is thus similar in function and appearance to a stem tuber. Examples of plants with notable tuberous roots include the sweet potato and dahlia. Root tubers are perennating organs, thickened roots that store nutrients over periods when the plant cannot grow, thus permitting survival from one year to the next; the massive enlargement of secondary roots represented by sweet potato, have the internal and external cell and tissue structures of a normal root, they produce adventitious roots and stems which again produce adventitious roots. In root-tubers, there are reduced leaves. Root tubers have one end called the proximal end, the end
Botany
Botany called plant science, plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist; the term "botany" comes from the Ancient Greek word βοτάνη meaning "pasture", "grass", or "fodder". Traditionally, botany has included the study of fungi and algae by mycologists and phycologists with the study of these three groups of organisms remaining within the sphere of interest of the International Botanical Congress. Nowadays, botanists study 410,000 species of land plants of which some 391,000 species are vascular plants, 20,000 are bryophytes. Botany originated in prehistory as herbalism with the efforts of early humans to identify – and cultivate – edible and poisonous plants, making it one of the oldest branches of science. Medieval physic gardens attached to monasteries, contained plants of medical importance, they were forerunners of the first botanical gardens attached to universities, founded from the 1540s onwards.
One of the earliest was the Padua botanical garden. These gardens facilitated the academic study of plants. Efforts to catalogue and describe their collections were the beginnings of plant taxonomy, led in 1753 to the binomial system of Carl Linnaeus that remains in use to this day. In the 19th and 20th centuries, new techniques were developed for the study of plants, including methods of optical microscopy and live cell imaging, electron microscopy, analysis of chromosome number, plant chemistry and the structure and function of enzymes and other proteins. In the last two decades of the 20th century, botanists exploited the techniques of molecular genetic analysis, including genomics and proteomics and DNA sequences to classify plants more accurately. Modern botany is a broad, multidisciplinary subject with inputs from most other areas of science and technology. Research topics include the study of plant structure and differentiation, reproduction and primary metabolism, chemical products, diseases, evolutionary relationships and plant taxonomy.
Dominant themes in 21st century plant science are molecular genetics and epigenetics, which are the mechanisms and control of gene expression during differentiation of plant cells and tissues. Botanical research has diverse applications in providing staple foods, materials such as timber, rubber and drugs, in modern horticulture and forestry, plant propagation and genetic modification, in the synthesis of chemicals and raw materials for construction and energy production, in environmental management, the maintenance of biodiversity. Botany originated as the study and use of plants for their medicinal properties. Many records of the Holocene period date early botanical knowledge as far back as 10,000 years ago; this early unrecorded knowledge of plants was discovered in ancient sites of human occupation within Tennessee, which make up much of the Cherokee land today. The early recorded history of botany includes many ancient writings and plant classifications. Examples of early botanical works have been found in ancient texts from India dating back to before 1100 BC, in archaic Avestan writings, in works from China before it was unified in 221 BC.
Modern botany traces its roots back to Ancient Greece to Theophrastus, a student of Aristotle who invented and described many of its principles and is regarded in the scientific community as the "Father of Botany". His major works, Enquiry into Plants and On the Causes of Plants, constitute the most important contributions to botanical science until the Middle Ages seventeen centuries later. Another work from Ancient Greece that made an early impact on botany is De Materia Medica, a five-volume encyclopedia about herbal medicine written in the middle of the first century by Greek physician and pharmacologist Pedanius Dioscorides. De Materia Medica was read for more than 1,500 years. Important contributions from the medieval Muslim world include Ibn Wahshiyya's Nabatean Agriculture, Abū Ḥanīfa Dīnawarī's the Book of Plants, Ibn Bassal's The Classification of Soils. In the early 13th century, Abu al-Abbas al-Nabati, Ibn al-Baitar wrote on botany in a systematic and scientific manner. In the mid-16th century, "botanical gardens" were founded in a number of Italian universities – the Padua botanical garden in 1545 is considered to be the first, still in its original location.
These gardens continued the practical value of earlier "physic gardens" associated with monasteries, in which plants were cultivated for medical use. They supported the growth of botany as an academic subject. Lectures were given about the plants grown in the gardens and their medical uses demonstrated. Botanical gardens came much to northern Europe. Throughout this period, botany remained subordinate to medicine. German physician Leonhart Fuchs was one of "the three German fathers of botany", along with theologian Otto Brunfels and physician Hieronymus Bock. Fuchs and Brunfels broke away from the tradition of copying earlier works to make original observations of their own. Bock created his own system of plant classification. Physician Valerius Cordus authored a botanically and pharmacologically important herbal Historia Plantarum in 1544 and a pharmacopoeia of lasting importance, the Dispensatorium
Oomycete
Oomycota or oomycetes form a distinct phylogenetic lineage of fungus-like eukaryotic microorganisms. They are filamentous, absorptive organisms that reproduce both sexually and asexually. Sexual reproduction of an oospore is the result of contact between hyphae of a male antheridia and female oogonia. Asexual reproduction is the formation of sporangia producing zoospores. Oomycetes occupy both saprophytic and pathogenic lifestyles, include some of the most notorious pathogens of plants, causing devastating diseases such as late blight of potato and sudden oak death. One oomycete, the mycoparasite Pythium oligandrum, is used for biocontrol, attacking plant pathogenic fungi; the oomycetes are often referred to as water molds, although the water-preferring nature which led to that name is not true of most species, which are terrestrial pathogens. The Oomycota have a sparse fossil record. A possible oomycete has been described from Cretaceous amber. "Oomycota" means "egg fungi", referring to the large round oogonia, structures containing the female gametes, that are characteristic of the oomycetes.
The name "water mold" refers to their earlier classification as fungi and their preference for conditions of high humidity and running surface water, characteristic for the basal taxa of the oomycetes. The oomycetes have septa, if they do, they are scarce, appearing at the bases of sporangia, sometimes in older parts of the filaments; some are unicellular, while others are branching. The group was arranged into six orders; the Saprolegniales are the most widespread. Many break down decaying matter; the Leptomitales have wall thickenings that give their continuous cell body the appearance of septation. They bear chitin and reproduce asexually; the Rhipidiales use rhizoids to attach their thallus to the bed of stagnant or polluted water bodies. The Albuginales are considered by some authors to be a family within the Peronosporales, although it has been shown that they are phylogenetically distinct from this order; the Peronosporales too are saprophytic or parasitic on plants, have an aseptate, branching form.
Many of the most damaging agricultural parasites belong to this order. The Lagenidiales are the most primitive; however more this has been expanded considerably. Anisolpidiales Dick 2001 Anisolpidiaceae Karling 1943 Lagenismatales Dick 2001 Lagenismataceae Dick 1995 Salilagenidiales Dick 2001 Salilagenidiaceae Dick 1995 Rozellopsidales Dick 2001 Rozellopsidaceae Dick 1995 Pseudosphaeritaceae Dick 1995 Ectrogellales Ectrogellaceae Haptoglossales Haptoglossaceae Eurychasmales Eurychasmataceae Petersen 1905 Haliphthorales Haliphthoraceae Vishniac 1958 Olpidiopsidales Sirolpidiaceae Cejp 1959 Pontismataceae Petersen 1909 Olpidiopsidaceae Cejp 1959 Atkinsiellales Atkinisellaceae Crypticolaceae Dick 1995 Saprolegniales Achlyaceae Verrucalvaceae Dick 1984 Saprolegniaceae Warm. 1884 Leptomitales Leptomitaceae Kuetz. 1843 Leptolegniellaceae Dick 1971 Rhipidiales Rhipidiaceae Cejp 1959 Albuginales Albuginaceae Schroet. 1893 Peronosporales Salisapiliaceae Pythiaceae Schroet. 1893 Peronosporaceae Warm. 1884 This group was classified among the fungi and treated as protists, based on general morphology and lifestyle.
A cladistic analysis based on modern discoveries about the biology of these organisms supports a close relationship with some photosynthetic organisms, such as brown algae and diatoms. A common taxonomic classification based on these data, places the class Oomycota along with other classes such as Phaeophyceae within the phylum Heterokonta; this relationship is supported by a number of observed differences between the characteristics of oomycetes and fungi. For instance, the cell walls of oomycetes are composed of cellulose rather than chitin and do not have septations. In the vegetative state they have diploid nuclei, whereas fungi have haploid nuclei. Most oomycetes produce self-motile zoospores with two flagella. One flagellum has a "whiplash" morphology, the other a branched "tinsel" morphology; the "tinsel" flagellum is unique to the Kingdom Heterokonta. Spores of the few fungal groups which retain flagella have only one whiplash flagellum. Oomycota and fungi have different metabolic pathways for synthesizing lysine and have a number of enzymes that differ.
The ultrastructure is different, with oomycota having tubular mitochondrial cristae and fungi having flattened cristae. In spite of this evidence to the contrary, many species of oomycetes are still described or listed as types of fungi and may sometimes be referred to as pseudofungi, or lower fungi. Most of the oomycetes produce two distinct types of spores; the main dispersive spores are asexual, self-motile spores called zoospores, which are capable of chemotaxis in surface water. A few oomycetes produce aerial asexual spores, they produce sexual spores, called oospores, that are translucent, double-walled, spherical structures used to survive adverse environmental conditions. Many oomycetes species are economically important, aggressive plant pathogens; some sp
Plant pathology
Plant pathology is the scientific study of diseases in plants caused by pathogens and environmental conditions. Organisms that cause infectious disease include fungi, bacteria, viroids, virus-like organisms, protozoa and parasitic plants. Not included are ectoparasites like insects, vertebrate, or other pests that affect plant health by eating of plant tissues. Plant pathology involves the study of pathogen identification, disease etiology, disease cycles, economic impact, plant disease epidemiology, plant disease resistance, how plant diseases affect humans and animals, pathosystem genetics, management of plant diseases. Control of plant diseases is crucial to the reliable production of food, it provides significant problems in agricultural use of land, water and other inputs. Plants in both natural and cultivated populations carry inherent disease resistance, but there are numerous examples of devastating plant disease impacts such as Irish potato famine and chestnut blight, as well as recurrent severe plant diseases like rice blast, soybean cyst nematode, citrus canker.
However, disease control is reasonably successful for most crops. Disease control is achieved by use of plants that have been bred for good resistance to many diseases, by plant cultivation approaches such as crop rotation, use of pathogen-free seed, appropriate planting date and plant density, control of field moisture, pesticide use. Across large regions and many crop species, it is estimated that diseases reduce plant yields by 10% every year in more developed settings, but yield loss to diseases exceeds 20% in less developed settings. Continuing advances in the science of plant pathology are needed to improve disease control, to keep up with changes in disease pressure caused by the ongoing evolution and movement of plant pathogens and by changes in agricultural practices. Plant diseases cause major economic losses for farmers worldwide; the Food and Agriculture Organization estimates indeed that pests and diseases are responsible for about 25% of crop loss. To solve this issue, new methods are needed to detect diseases and pests early, such as novel sensors that detect plant odours and spectroscopy and biophotonics that are able to diagnose plant health and metabolism.
Most phytopathogenic fungi belong to the Ascomycetes and the Basidiomycetes. The fungi reproduce both sexually and asexually via the production of other structures. Spores may be spread long distances by air or water. Many soil inhabiting fungi are capable of living saprotrophically, carrying out the part of their life cycle in the soil; these are facultative saprotrophs. Fungal diseases may be controlled through the use of other agriculture practices. However, new races of fungi evolve that are resistant to various fungicides. Biotrophic fungal pathogens colonize living plant tissue and obtain nutrients from living host cells. Necrotrophic fungal pathogens infect and kill host tissue and extract nutrients from the dead host cells. Significant fungal plant pathogens include: Fusarium spp. Thielaviopsis spp. Verticillium spp. Magnaporthe grisea Sclerotinia sclerotiorum Ustilago spp. smut of barley Rhizoctonia spp. Phakospora pachyrhizi Puccinia spp. Armillaria spp; the oomycetes are fungus-like organisms.
They include some of the most destructive plant pathogens including the genus Phytophthora, which includes the causal agents of potato late blight and sudden oak death. Particular species of oomycetes are responsible for root rot. Despite not being related to the fungi, the oomycetes have developed similar infection strategies. Oomycetes are capable of using effector proteins to turn off a plant's defenses in its infection process. Plant pathologists group them with fungal pathogens. Significant oomycete plant pathogens include: Pythium spp. Phytophthora spp. including the potato blight of the Great Irish Famine Some slime molds in Phytomyxea cause important diseases, including club root in cabbage and its relatives and powdery scab in potatoes. These are caused by species of Spongospora, respectively. Most bacteria that are associated with plants are saprotrophic and do no harm to the plant itself. However, a small number, around 100 known species, are able to cause disease. Bacterial diseases are much more prevalent in tropical regions of the world.
Most plant pathogenic bacteria are rod-shaped. In order to be able to colonize the plant they have specific pathogenicity factors. Five main types of bacterial pathogenicity factors are known: uses of cell wall–degrading enzymes, effector proteins and exopolysaccharides. Pathogens such as Erwinia species use cell wall–degrading enzymes to cause soft rot. Agrobacterium species change the level of auxins to cause tumours with phytohormones. Exopolysaccharides are produced by bacteria and block xylem vessels leading to the death of the plant. Bacteria control the production of pathogenicity factors via quorum sensing. Significant bacterial plant pathogens: Burkholderia Proteobacteria Xanthomonas spp. Pseudomonas spp. Pseudomonas syringae pv. tomato causes tomato plants to produce less fruit, it "continues to adapt to the tomato by minimizing its recognition by the tomato immune system." Phytoplasma and Spiroplasma are genera of bacteria that lack cell walls and are related to the mycoplasmas, which are human pathogens.
Together they are referred to
