Wheat is a grass cultivated for its seed, a cereal grain, a worldwide staple food. The many species of wheat together make up the genus Triticum; the archaeological record suggests that wheat was first cultivated in the regions of the Fertile Crescent around 9600 BCE. Botanically, the wheat kernel is a type of fruit called a caryopsis. Wheat is grown on more land area than any other food crop. World trade in wheat is greater than for all other crops combined. In 2016, world production of wheat was 749 million tonnes, making it the second most-produced cereal after maize. Since 1960, world production of wheat and other grain crops has tripled and is expected to grow further through the middle of the 21st century. Global demand for wheat is increasing due to the unique viscoelastic and adhesive properties of gluten proteins, which facilitate the production of processed foods, whose consumption is increasing as a result of the worldwide industrialization process and the westernization of the diet.
Wheat is an important source of carbohydrates. Globally, it is the leading source of vegetal protein in human food, having a protein content of about 13%, high compared to other major cereals but low in protein quality for supplying essential amino acids; when eaten as the whole grain, wheat is a source of dietary fiber. In a small part of the general population, gluten – the major part of wheat protein – can trigger coeliac disease, noncoeliac gluten sensitivity, gluten ataxia, dermatitis herpetiformis. Cultivation and repeated harvesting and sowing of the grains of wild grasses led to the creation of domestic strains, as mutant forms of wheat were preferentially chosen by farmers. In domesticated wheat, grains are larger, the seeds remain attached to the ear by a toughened rachis during harvesting. In wild strains, a more fragile rachis allows the ear to shatter and disperse the spikelets. Selection for these traits by farmers might not have been deliberately intended, but have occurred because these traits made gathering the seeds easier.
As the traits that improve wheat as a food source involve the loss of the plant's natural seed dispersal mechanisms domesticated strains of wheat cannot survive in the wild. Cultivation of wheat began to spread beyond the Fertile Crescent after about 8000 BCE. Jared Diamond traces the spread of cultivated emmer wheat starting in the Fertile Crescent sometime before 8800 BCE. Archaeological analysis of wild emmer indicates that it was first cultivated in the southern Levant, with finds dating back as far as 9600 BCE. Genetic analysis of wild einkorn wheat suggests that it was first grown in the Karacadag Mountains in southeastern Turkey. Dated archeological remains of einkorn wheat in settlement sites near this region, including those at Abu Hureyra in Syria, suggest the domestication of einkorn near the Karacadag Mountain Range. With the anomalous exception of two grains from Iraq ed-Dubb, the earliest carbon-14 date for einkorn wheat remains at Abu Hureyra is 7800 to 7500 years BCE. Remains of harvested emmer from several sites near the Karacadag Range have been dated to between 8600 and 8400 BCE, that is, in the Neolithic period.
With the exception of Iraq ed-Dubb, the earliest carbon-14 dated remains of domesticated emmer wheat were found in the earliest levels of Tell Aswad, in the Damascus basin, near Mount Hermon in Syria. These remains were dated by Willem van Zeist and his assistant Johanna Bakker-Heeres to 8800 BCE, they concluded that the settlers of Tell Aswad did not develop this form of emmer themselves, but brought the domesticated grains with them from an as yet unidentified location elsewhere. The cultivation of emmer reached Greece and Indian subcontinent by 6500 BCE, Egypt shortly after 6000 BCE, Germany and Spain by 5000 BCE. "The early Egyptians were developers of bread and the use of the oven and developed baking into one of the first large-scale food production industries." By 3000 BCE, wheat had reached Scandinavia. A millennium it reached China; the oldest evidence for hexaploid wheat has been confirmed through DNA analysis of wheat seeds, dating to around 6400-6200 BCE, recovered from Çatalhöyük.
The first identifiable bread wheat with sufficient gluten for yeasted breads has been identified using DNA analysis in samples from a granary dating to 1350 BCE at Assiros in Macedonia. From Asia, wheat continued to spread across Europe. In the British Isles, wheat straw was used for roofing in the Bronze Age, was in common use until the late 19th century. Technological advances in soil preparation and seed placement at planting time, use of crop rotation and fertilizers to improve plant growth, advances in harvesting methods have all combined to promote wheat as a viable crop; when the use of seed drills replaced broadcasting sowing of seed in the 18th century, another great increase in productivity occurred. Yields of pure wheat per unit area increased as methods of crop rotation were applied to long cultivated land, the use of fertilizers became widespread. Improved agricultural husbandry has more included threshing machines and reaping machines, tractor-drawn cultivators and planters, better varieties.
Great expansion of wheat production occurred as new arable land was farmed in the Americas and Australia in the 19th and 20th centuries. Leaves emerge from the shoot apical meristem in a telescoping fashion until the transition to reprod
Symbiosis is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or parasitic. The organisms, each termed a symbiont, may be of different species. In 1879, Heinrich Anton de Bary defined it as "the living together of unlike organisms"; the term was subject to a century-long debate about whether it should denote mutualism, as in lichens. Symbiosis can be obligatory, which means that one or both of the symbionts depend on each other for survival, or facultative when they can live independently. Symbiosis is classified by physical attachment; when one organism lives on the surface of another, such as head lice on humans, it is called ectosymbiosis. The definition of symbiosis was a matter of debate for 130 years. In 1877, Albert Bernhard Frank used the term symbiosis to describe the mutualistic relationship in lichens. In 1879, the German mycologist Heinrich Anton de Bary defined it as "the living together of unlike organisms".
The definition has varied among scientists, with some advocating that it should only refer to persistent mutualisms, while others thought it should apply to all persistent biological interactions, in other words mutualisms, commensalism, or parasitism, but excluding brief interactions such as predation. Current biology and ecology textbooks use the latter "de Bary" definition, or an broader one where symbiosis means all interspecific interactions. In 1949, Edward Haskell proposed an integrative approach, proposing a classification of "co-actions" adopted by biologists as "interactions". Biological interactions can involve individuals of the same species or individuals of different species; these can be further classified by either the mechanism of the interaction or the strength and direction of their effects. Relationships can be obligate, meaning that one or both of the symbionts depend on each other for survival. For example, in lichens, which consist of fungal and photosynthetic symbionts, the fungal partners cannot live on their own.
The algal or cyanobacterial symbionts in lichens, such as Trentepohlia, can live independently, their symbiosis is, facultative. Endosymbiosis is any symbiotic relationship in which one symbiont lives within the tissues of the other, either within the cells or extracellularly. Examples include diverse microbiomes, nitrogen-fixing bacteria that live in root nodules on legume roots. Ectosymbiosis is any symbiotic relationship in which the symbiont lives on the body surface of the host, including the inner surface of the digestive tract or the ducts of exocrine glands. Examples of this include ectoparasites such as lice. Competition can be defined as an interaction between organisms or species, in which the fitness of one is lowered by the presence of another. Limited supply of at least one resource used by both facilitates this type of interaction, although the competition may exist over other'amenities', such as females for reproduction. Mutualism or interspecies reciprocal altruism is a long-term relationship between individuals of different species where both individuals benefit.
Mutualistic relationships may be either obligate for both species, obligate for one but facultative for the other, or facultative for both. A large percentage of herbivores have mutualistic gut flora to help them digest plant matter, more difficult to digest than animal prey; this gut flora is made up of cellulose-digesting protozoans or bacteria living in the herbivores' intestines. Coral reefs are the result of mutualisms between coral organisms and various types of algae which live inside them. Most land plants and land ecosystems rely on mutualisms between the plants, which fix carbon from the air, mycorrhyzal fungi, which help in extracting water and minerals from the ground. An example of mutualism is the relationship between the ocellaris clownfish that dwell among the tentacles of Ritteri sea anemones; the territorial fish protects the anemone from anemone-eating fish, in turn the stinging tentacles of the anemone protect the clownfish from its predators. A special mucus on the clownfish protects it from the stinging tentacles.
A further example is a fish which sometimes lives together with a shrimp. The shrimp cleans up a burrow in the sand in which both the shrimp and the goby fish live; the shrimp is blind, leaving it vulnerable to predators when outside its burrow. In case of danger, the goby touches the shrimp with its tail to warn it; when that happens both the shrimp and goby retreat into the burrow. Different species of gobies clean up ectoparasites in other fish another kind of mutualism. A non-obligate symbiosis is seen in encru
Sauternes is a French sweet wine from the Sauternais region of the Graves section in Bordeaux. Sauternes is made from Sémillon, Sauvignon blanc, Muscadelle grapes that have been affected by Botrytis cinerea known as noble rot; this causes the grapes to become raisined, resulting in concentrated and distinctively flavored wines. Due to its climate, Sauternes is one of the few wine regions where infection with noble rot is a frequent occurrence. So, production is a hit-or-miss proposition, with varying harvests from vintage to vintage. Wines from Sauternes the Premier Cru Supérieur estate Château d'Yquem, can be expensive, due to the high cost of production. Barsac lies within Sauternes, is entitled to use either name. Somewhat similar but less expensive and less-distinguished wines are produced in the neighboring regions of Monbazillac, Cérons and Cadillac. In the United States, there is a semi-generic label for sweet white dessert wines known as sauterne without the "s" at the end and uncapitalized.
As in most of France, viticulture is believed to have been introduced into Aquitania by the Romans. The earliest evidence of sweet wine production, dates only to the 17th century. While the English had been the region's primary export market since the Middle Ages, their tastes ran to drier wines, starting with clairet in medieval times and shifting to red claret, it was the Dutch traders of the 17th century. For years they were active in the trade of German wines but production in Germany began to wane in the 17th century as the popularity of beer increased; the Dutch saw an opportunity for a new production source in Bordeaux and began investing in the planting of white grape varieties. They introduced to the region German white wine making techniques, such as halting fermentation with the use of sulphur in order to maintain residual sugar levels. One of these techniques involved taking a candle with its wick dipped in the sulphur and burned in the barrel that the wine will be fermenting in; this would leave a presence of sulphur in the barrel that the wine would interact with as it was fermenting.
Being an anti-microbial agent, sulphur stuns the yeast that stimulates fermentation bringing it to a halt with high levels of sugars still in the wine. The Dutch began to identify areas that could produce grapes well suited for white wine production and soon homed in on the area of Sauternes; the wine produced from this area was known as vins liquoreux but it is not clear if the Dutch were using nobly rotted grapes at this point. Wine expert Hugh Johnson has suggested that the unappealing thought of drinking wine made from fungus-infested grapes may have caused Sauternes producers to keep the use of Botrytis a secret. There are records from the 17th century that by October, Sémillon grapes were known to be infected by rot and vineyard workers had to separate rotted and clean berries but they are incomplete in regards to whether the rotted grapes were used in winemaking. By the 18th century, the practice of using nobly rotted grapes in Tokaji and Germany was well known, it seems that at this point the "unspoken secret" was more accepted and the reputation of Sauternes rose to rival those of the German and Hungarian dessert wines.
By the end of the 18th century, the region's reputation for Sauternes was internationally known: Thomas Jefferson was an avid connoisseur. Jefferson recorded that after tasting a sample of Château d'Yquem while President, George Washington placed an order for 30 dozen bottles. Like most of the Bordeaux wine region, the Sauternes region has a maritime climate which brings the viticultural hazards of autumn frost and rains that can ruin an entire vintage; the Sauternes region is located 40 km southeast of the city of Bordeaux along the Garonne river and its tributary, the Ciron. The source of the Ciron is a spring. In the autumn, when the climate is warm and dry, the different temperatures from the two rivers meet to produce mist that descends upon the vineyards from evening to late morning; this condition promotes the development of the Botrytis cinerea fungus. By mid day, the warm sun will help dissipate the mist and dry the grapes to keep them from developing less favorable rot; the Sauternes wine region comprises five communes— Barsac, Bommes and Preignac.
While all five communes are permitted to use the name Sauternes, the Barsac region is permitted to label their wines under the Barsac appellation. The Barsac region is located on the west bank of the Ciron river where the tributary meets the Garonne; the area sits on an alluvial plain with limy soils. In general, Barsac wine is distinguished from other Sauternes in being drier with a lighter body. In years when the noble rot does not develop, Sauternes producers will make dry white wines under the generic Bordeaux AOC. To qualify for the Sauternes label, the wines must have a minimum 13% alcohol level and pass a tasting exam where the wines need to taste noticeably sweet. There is no regulation on the exact amount of residual sugar. Sauternes are characterized by the balance of sweetness with the zest of acidity; some common flavor notes include apricots, peaches but with a nutty note, a typical characteristic of noble semillon itself. The finish can resonate on the palate for several minutes. Sauternes are some of longest-lived wines, with premium examples from exceptional vintages properly kept having the pote
Elms are deciduous and semi-deciduous trees comprising the flowering plant genus Ulmus in the plant family Ulmaceae. The genus first appeared in the Miocene geological period about 20 million years ago, originating in what is now central Asia; these trees flourished and spread over most of the Northern Hemisphere, inhabiting the temperate and tropical-montane regions of North America and Eurasia, presently ranging southward across the Equator into Indonesia. Elms are components of many kinds of natural forests. Moreover, during the 19th and early 20th centuries many species and cultivars were planted as ornamental street and park trees in Europe, North America, parts of the Southern Hemisphere, notably Australasia; some individual elms reached great age. However, in recent decades, most mature elms of European or North American origin have died from Dutch elm disease, caused by a microfungus dispersed by bark beetles. In response, disease-resistant cultivars have been developed, capable of restoring the elm to forestry and landscaping.
There are about 30 to 40 species of Ulmus. Oliver Rackham describes Ulmus as the most critical genus in the entire British flora, adding that'species and varieties are a distinction in the human mind rather than a measured degree of genetic variation'. Eight species are endemic to North America, a smaller number to Europe; the classification adopted in the List of elm species, varieties and hybrids is based on that established by Brummitt. A large number of synonyms have accumulated over the last three centuries. Botanists who study elms and argue over elm identification and classification are called pteleologists, from the Greek πτελέα; as part of the sub-order urticalean rosids they are distant cousins of cannabis and nettles. The name Ulmus is the Latin name for these trees, while the English "elm" and many other European names are either cognate with or derived from it; the genus is hermaphroditic, having apetalous perfect flowers. Elm leaves are alternate, with simple, single- or, most doubly serrate margins asymmetric at the base and acuminate at the apex.
The fruit is a round wind-dispersed samara flushed with chlorophyll, facilitating photosynthesis before the leaves emerge. The samarae are light, those of British elms numbering around 50,000 to the pound. All species are tolerant of a wide range of soils and pH levels but, with few exceptions, demand good drainage; the elm tree can grow to great height with a forked trunk creating a vase profile. Dutch elm disease devastated elms throughout Europe and much of North America in the second half of the 20th century, it derives its name'Dutch' from the first description of the disease and its cause in the 1920s by the Dutch botanists Bea Schwarz and Christina Johanna Buisman. Owing to its geographical isolation and effective quarantine enforcement, Australia has so far remained unaffected by Dutch Elm Disease, as have the provinces of Alberta and British Columbia in western Canada. DED is caused by a micro-fungus transmitted by two species of Scolytus elm-bark beetle which act as vectors; the disease affects all species of elm native to North America and Europe, but many Asiatic species have evolved anti-fungal genes and are resistant.
Fungal spores, introduced into wounds in the tree caused by the beetles, invade the xylem or vascular system. The tree responds by producing tyloses blocking the flow from roots to leaves. Woodland trees in North America are not quite as susceptible to the disease because they lack the root-grafting of the urban elms and are somewhat more isolated from each other. In France, inoculation with the fungus of over three hundred clones of the European species failed to find a single variety possessed of any significant resistance; the first, less aggressive strain of the disease fungus, Ophiostoma ulmi, arrived in Europe from the Far East in 1910, was accidentally introduced to North America in 1928, but was weakened by viruses and had all but disappeared in Europe by the 1940s. The second, far more virulent strain of the disease Ophiostoma novo-ulmi was identified in Europe in the late 1960s, within a decade had killed over 20 million trees in the UK alone. Three times more deadly, the new strain arrived in Europe from the US on a cargo of Rock Elm.
There is no sign of the current pandemic waning, no evidence of a susceptibility of the fungus to a disease of its own caused by d-factors: occurring virus-like agents that debilitated the original O. ulmi and reduced its sporulation. Elm phloem necrosis is a disease of elm trees, spread by leafhoppers or by root grafts; this aggressive disease, with no known cure, occurs in the Eastern United States, southern Ontario in Canada, Europe. It is caused by phytoplasmas. Infection and death of the phloem girdles the tree and stops the flow of water and nutrients; the disease affects cultivated trees. Cutting the infected tree before the disease establishes itself and cleanup and prompt disposal of infected matter has resul
In biology, a spore is a unit of sexual or asexual reproduction that may be adapted for dispersal and for survival for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plants, algae and protozoa. Bacterial spores are not part of a sexual cycle but are resistant structures used for survival under unfavourable conditions. Myxozoan spores release amoebulae into their hosts for parasitic infection, but reproduce within the hosts through the pairing of two nuclei within the plasmodium, which develops from the amoebula. Spores are haploid and unicellular and are produced by meiosis in the sporangium of a diploid sporophyte. Under favourable conditions the spore can develop into a new organism using mitotic division, producing a multicellular gametophyte, which goes on to produce gametes. Two gametes fuse to form a zygote; this cycle is known as alternation of generations. The spores of seed plants, are produced internally and the megaspores, formed within the ovules and the microspores are involved in the formation of more complex structures that form the dispersal units, the seeds and pollen grains.
The term spore derives from the ancient Greek word σπορά spora, meaning "seed, sowing", related to σπόρος sporos, "sowing," and σπείρειν speirein, "to sow." In common parlance, the difference between a "spore" and a "gamete" is that a spore will germinate and develop into a sporeling, while a gamete needs to combine with another gamete to form a zygote before developing further. The main difference between spores and seeds as dispersal units is that spores are unicellular, while seeds contain within them a multicellular gametophyte that produces a developing embryo, the multicellular sporophyte of the next generation. Spores germinate to give rise to haploid gametophytes, while seeds germinate to give rise to diploid sporophytes. Vascular plant spores are always haploid. Vascular plants heterosporous. Plants that are homosporous produce spores of the same type. Heterosporous plants, such as seed plants, spikemosses and ferns of the order Salviniales produce spores of two different sizes: the larger spore in effect functioning as a "female" spore and the smaller functioning as a "male".
Such plants give rise to the two kind of spores from within separate sporangia, either a megasporangium that produces megaspores or a microsporangium that produces microspores. In flowering plants, these sporangia occur within anthers, respectively. Fungi produce spores, as a result of sexual, or asexual, reproduction. Spores are haploid and grow into mature haploid individuals through mitotic division of cells. Dikaryotic cells result from the fusion of two haploid gamete cells. Among sporogenic dikaryotic cells, karyogamy occurs to produce a diploid cell. Diploid cells undergo meiosis to produce haploid spores. Spores can be classified in several ways: In fungi and fungus-like organisms, spores are classified by the structure in which meiosis and spore production occurs. Since fungi are classified according to their spore-producing structures, these spores are characteristic of a particular taxon of the fungi. Sporangiospores: spores produced by a sporangium in many fungi such as zygomycetes.
Zygospores: spores produced by a zygosporangium, characteristic of zygomycetes. Ascospores: spores produced by an ascus, characteristic of ascomycetes. Basidiospores: spores produced by a basidium, characteristic of basidiomycetes. Aeciospores: spores produced by an aecium in some fungi such as rusts or smuts. Urediniospores: spores produced by a uredinium in some fungi such as rusts or smuts. Teliospores: spores produced by a telium in some fungi such as rusts or smuts. Oospores: spores produced by an oogonium, characteristic of oomycetes. Carpospores: spores produced by a carposporophyte, characteristic of red algae. Tetraspores: spores produced by a tetrasporophyte, characteristic of red algae. Chlamydospores: thick-walled resting spores of fungi produced to survive unfavorable conditions. Parasitic fungal spores may be classified into internal spores, which germinate within the host, external spores called environmental spores, released by the host to infest other hosts. Meiospores: spores produced by meiosis.
Examples are the precursor cells of gametophytes of seed plants found in flowers or cones, the zoospores produced from meiosis in the sporophytes of algae such as Ulva. Microspores: meiospores that give rise to a male gametophyte. Megaspores: meiospores that give rise to a female gametophyte. Mitospores: spores produced by mitosis. Fungi in which only mitospores are found are called "mitosporic fungi" or "anamorphic fungi", are classified under the taxon Deuteromycota. Spores can be differentiated by. Zoospores: mobile spores that move by means of one or more flagella, can be found in some algae and fungi. Aplanospores: immobile spores that may potentially grow flagella. Autospores: immobile spores that cannot develop flagella. Ballistospores: spores that are forcibly discharged or ejected from the fungal fruiting body as the result of an internal force, such as buildup of pressure. Most basidiospores are ballistospores, another notable e
Dutch elm disease
Dutch elm disease is caused by a member of the sac fungi affecting elm trees, is spread by elm bark beetles. Although believed to be native to Asia, the disease was accidentally introduced into America and Europe, where it has devastated native populations of elms that did not have resistance to the disease, it has reached New Zealand. The name "Dutch elm disease" refers to its identification in 1921 and in the Netherlands by Dutch phytopathologists Bea Schwarz and Christine Buisman who both worked with Professor Johanna Westerdijk; the disease affects species in the genera Ulmus and Zelkova, therefore it is not specific to the Dutch elm hybrid. The causative agents of DED are ascomycete microfungi. Three species are now recognized: Ophiostoma ulmi, which afflicted Europe from 1910, reaching North America on imported timber in 1928. Ophiostoma himal-ulmi, a species endemic to the western Himalaya. Ophiostoma novo-ulmi, an virulent species from Japan, first described in Europe and North America in the 1940s and has devastated elms in both continents since the late 1960s.
DED is spread in North America by three species of bark beetles: The native elm bark beetle, Hylurgopinus rufipes. The European elm bark beetle, Scolytus multistriatus; the banded elm bark beetle, Scolytus schevyrewi. In Europe, while S. multistriatus still acts as a vector for infection, it is much less effective than the large elm bark beetle, S. scolytus. H. rufipes is inefficient compared to the other vectors. S. schevyrewi was found in 2003 in Utah. Other reported DED vectors include Scolytus sulcifrons, S. pygmaeus, S. laevis, Pteleobius vittatus and Р. kraatzi. Other elm bark beetle species are likely vectors.'Field resistance' is an umbrella term covering the various factors by which some elms avoid infection in the first place, rather than survive it. A clear example would be the European White Elm Ulmus laevis which, while having little or no genetic resistance to DED, synthesizes a triterpene, rendering the bark distasteful to the vector beetles, obliging them to look further afield for more suitable elms.
Another would be the inability of the beetles to see elms which did not break the silhouette.'Weeping' elms are spared infection owing to the beetles' aversion to hanging upside-down while feeding. In an attempt to block the fungus from spreading farther, the tree reacts by plugging its own xylem tissue with gum and tyloses, bladder-like extensions of the xylem cell wall; as the xylem delivers water and nutrients to the rest of the plant, these plugs prevent them from travelling up the trunk of the tree, starving the tree of water and nutrients, therefore killing it. The first sign of infection is an upper branch of the tree with leaves starting to wither and yellow in summer, months before the normal autumnal leaf shedding; this progressively spreads with further dieback of branches. The roots die, starved of nutrients from the leaves. Not all the roots die: the roots of some species, notably the English elm, Ulmus procera, can engage in putting up suckers which flourish for 15 years, after which they too succumb.
Dutch elm disease was first noticed in continental Europe in 1910, spread and extending to all other countries except Greece and Finland. In Britain, the disease was first identified in 1927 by T R Peace on English elm in Hertfordshire; this first strain was a mild one, which killed only a small proportion of elms, more just killing a few branches, had died out by 1940 owing to its susceptibility to viruses. The disease was isolated in The Netherlands in 1921 by Bea Schwarz, a pioneering Dutch phytopathologist, this discovery would lend the disease its name. Circa 1967, a new, far more virulent, strain arrived in Britain via east coast ports on shipments of rock elm U. thomasii logs from Canada destined for the small-boat industry, confirmed in 1973 when another consignment was examined at Southampton Docks. This strain proved both contagious and lethal to European elms; the disease spread northwards, reaching Scotland within 10 years. By 1990 few mature elms were left in Britain or much of continental Europe.
One of the most distinctive English countryside trees, the English elm U. procera Salisb. is susceptible as it is the elm most favoured by the Scolytus beetles. Thirty years after the outbreak of the epidemic, nearly all these trees, which grew to more than 45 m high, are gone; the species still survives in hedgerows, as the roots send up root sprouts. These suckers reach more than 5 m tall before succumbing to a new attack of the fungus. However, established hedges kept low by clipping have remained healthy throughout the nearly 40 years since the onset of the disease in the UK; the largest concentrations of mature elms in Europe are now in The Hague. In 2005, Amsterdam was declared the "Elm City of Europe": the city’s streets and canals are lined with at least 75,000 elms, including several generations of research-elms; some 30,000 of the 100,000 mature trees in The Hague are elms, planted because of their tolerance of salty sea-winds. Since the 1990s, a programme of antifungal injections of the most prominent 10,000 elms, of sanitation felling, has reduc
Phytophthora infestans is an oomycete or water mold, a microorganism that causes the serious potato and tomato disease known as late blight or potato blight. Late blight was a major culprit in the 1840s European, the 1845 Irish, the 1846 Highland potato famines; the organism can infect some other members of the Solanaceae. The pathogen is favored by moist, cool environments: sporulation is optimal at 12–18 °C in water-saturated or nearly saturated environments, zoospore production is favored at temperatures below 15 °C. Lesion growth rates are optimal at a warmer temperature range of 20 to 24 °C; the genus name Phytophthora comes from the Greek φυτό-, meaning: "plant" - plus the Greek φθορά, meaning: "decay, perish". The species name infestans is the present participle of the Latin verb infestare, meaning: "attacking, destroying", from which we get the word "to infest"; the asexual life cycle of Phytophthora infestans is characterized by alternating phases of hyphal growth, sporangia germination, the re-establishment of hyphal growth.
There is a sexual cycle, which occurs when isolates of opposite mating type meet. Hormonal communication triggers the formation of called oospores; the different types of spores play major roles in the survival of P. infestans. Sporangia are spread by wind or water and enable the movement of P. infestans between different host plants. The zoospores released from sporangia are biflagellated and chemotactic, allowing further movement of P. infestans on water films found on leaves or soils. Both sporangia and zoospores are short-lived, in contrast to oospores which can persist in a viable form for many years; the color of potato sign is white. People can observe Phytophthora infestans produce sporangia and sporangiophores on the surface of potato stems and leaves; these sporangia and sporangiophores always appear on the lower surface of the foliage. As for tuber blight, the white mycelium shows on the tubers' surface. Under ideal conditions, the life cycle can be completed on potato or tomato foliage in about five days.
Sporangia develop on the leaves, spreading through the crop when temperatures are above 10 °C and humidity is over 75–80% for 2 days or more. Rain can wash spores into the soil where they infect young tubers, the spores can travel long distances on the wind; the early stages of blight are missed. Symptoms include the appearance of dark blotches on plant stems. White mold will appear under the leaves in humid conditions and the whole plant may collapse. Infected tubers develop grey or dark patches that are reddish brown beneath the skin, decay to a foul-smelling mush caused by the infestation of secondary soft bacterial rots. Healthy tubers may rot when in store. P. infestans survives poorly in nature apart from its plant hosts. Under most conditions, the hyphae and asexual sporangia can survive for only brief periods in plant debris or soil, are killed off during frosts or warm weather; the exceptions involve oospores, hyphae present within tubers. The persistence of viable pathogen within tubers, such as those that are left in the ground after the previous year's harvest or left in cull piles is a major problem in disease management.
In particular, volunteer plants sprouting from infected tubers are thought to be a major source of inoculum at the start of a growing season. This can have devastating effects by destroying entire crops. P. infestans is diploid, with about 11-13 chromosomes, in 2009 scientists completed the sequencing of its genome. The genome was found to be larger than that of most other Phytophthora species whose genomes have been sequenced. About 18,000 genes were detected within the P. infestans genome. It contained a diverse variety of transposons and many gene families encoding for effector proteins that are involved in causing pathogenicity; these proteins are split into two main groups depending on whether they are produced by the water mould in the symplast or in the apoplast. Proteins produced in the symplast included RXLR proteins, which contain an arginine-X-leucine-arginine sequence at the amino terminus of the protein; some RXLR proteins are avirulence proteins, meaning that they can be detected by the plant and lead to a hypersensitive response which restricts the growth of the pathogen.
P. infestans was found to encode around 60% more of these proteins than most other Phytophthora species. Those found in the apoplast include hydrolytic enzymes such as proteases and glycosylases that act to degrade plant tissue, enzyme inhibitors to protect against host defence enzymes and necrotizing toxins. Overall the genome was found to have an high repeat content and to have an unusual gene distribution in that some areas contain many genes whereas others contain few; the highlands of central Mexico are considered by many to be the center of origin of P. infestans, although others have proposed its origin to be in the Andes, the origin of potatoes. A recent study evaluated these two alternate hypotheses and found conclusive support for central Mexico being the center of origin. Support for Mexico comes from multiple observations including the fact that populations are genetically most diverse in Mexico, late blight is observed in native tuber-bearing Solanum species, population