Fusarium wilt of bananas is caused by F. oxysporum f.sp. cubense, a common soil inhabitant. Other formae speciales attack a wide variety of other crops, including cotton, flax, tomatoes, cabbages, peas, sweet potatoes, watermelons and oil palms.;The formae speciales of Fusarium oxysporum each produce three types of asexual spores. The macroconidia (22-36 x 4-5 µm, see Wardlaw, 1961 for measurements) are produced most frequently on branched conidiophores in sporodochia on the surface of infected plant parts or in artificial culture. Macroconidia may also be produced singly in the aerial mycelium, especially in culture. The macroconidia are thin-walled with a definite foot cell and a pointed apical cell. Oval or kidney-shaped microconidia (5-7 x 2.5-3 µm) occur on short microconidiophores in the aerial mycelium and are produced in false heads. Both macroconidia and microconidia may also be formed in the xylem vessel elements of infected host plants, but the microconidia are usually more common. The fungus may be spread by macroconidia, microconidia and mycelium within the plant as well as outside the plant. Illustrations of the conidia have been published (Nelson et al., 1983).;Chlamydospores (9 x 7 µm) are thick-walled asexual spores that are usually produced singly in macroconidia or are intercalary or terminal in the hyphae. The contents are highly refractive. Chlamydospores form in dead host-plant tissue in the final stages of wilt development and also in culture. These spores can survive for an extended time in plant debris in soil.;Mutation in culture is a major problem for those working with vascular wilt isolates of F. oxysporum. The sporodochial type often mutates to a 'mycelial' type or to a 'pionnotal' type. The former has abundant aerial mycelium, but few macroconidia, whereas the latter produces little or no aerial mycelium, but abundant macroconidia. These cultures may lose virulence and the ability to produce toxins. Mutants occur more frequently if the fungus is grown on a medium that is rich in carbohydrates.
Banana;The various symptoms of Fusarium wilt on banana are described and well illustrated by Ploetz and Pegg (1999).;The first external symptoms of Fusarium wilt on bananas is a faint off-green to pale-yellow streak or patch at the base of the petiole of one of the two oldest leaves. The disease can then progress in different ways. The older leaves can yellow, beginning with patches at the leaf margin. Yellowing progresses from the older to the younger leaves until only the recently unfurled or partially unfurled centre leaf remains erect and green. This process may take from 1 to 3 weeks in cultivar 'Gros Michel'. Often the yellow leaves remain erect for 1-2 weeks or some may collapse at the petiole and hang down the pseudostem. In contrast to this 'yellow syndrome', leaves may remain completely green except for a petiole streak or patch but collapse as a result of buckling of the petiole. The leaves fall, the oldest first, until they hang about the plant like a skirt. Eventually, all leaves on infected plants fall down and dry up. The youngest are the last to fall and often stand unusually erect.;Splitting of the base of the pseudostem is another symptom as is necrosis of the emerging heart leaf. Other symptoms include irregular, pale margins on new leaves and the wrinkling and distortion of the lamina. Internodes may also shorten (Stover, 1962, 1972, Jones, 1994, Moore et al., 1995).;The characteristic internal symptom of Fusarium wilt is vascular discoloration. This varies from one or two strands in the oldest and outermost pseudostem leaf sheaths in the early stages of disease to heavy discoloration throughout the pseudostem and fruit stalk in the later disease stages. Discoloration varies from pale yellow in the early stages to dark red or almost black in later stages. The discoloration is most pronounced in the rhizome in the area of dense vascularization where the stele joins the cortex. When symptoms first appear, a small or large portion of the rhizome may be infected. Eventually, almost the entire differentiated vascular system is invaded. The infection may or may not pass into young budding suckers or mature 'daughter' suckers. Where it does, discoloration of vascular strands may be visible in the excised sucker. Usually, suckers less than 1.5 m tall and ca. 4 months old do not show external symptoms. Where wilt is epidemic and spreading rapidly, suckers are usually infected and seldom grow to produce fruit. Above- and below-ground parts of affected plants eventually rot and die.;Fusarium wilt was reported to occur on banana cultivars of the 'Mutika-Lujugira' (AAA genome) subgroup in East Africa above 1400 m. Internal symptoms were much less extensive than those described above and external symptoms more subtle, comprising thin pseudostems and small fingers. Nevertheless, symptomatic plants were recognized by smallholders and were rogued. These mild symptoms were initially believed to be indicative of an attack on a plant whose defences have been weakened as a result of cooler conditions or other predisposing factors at altitude (Ploetz et al., 1994). Given the importance of this banana group, also referred to locally as ÔEast African highland bananasÕ, to local trade and as a staple food, further investigation was merited. This revealed that the disorder also affected non-indigenous banana types, including Cavendish and Bluggoe (which were not affected by Fusarium wilt) and was related to abnormal soil nutrient levels and farm management practice. Discoloration similar to that caused by F. oxysporum f.sp. cubense was observed in vascular tissues of affected plants. Fusarium pallidoroseum (syn. Fusarium semitectum) was consistently isolated from such tissues but found to be non-pathogenic. F. oxysporum was not recovered (Kangire and Rutherford, 2001, Rutherford, 2006).
F. oxysporum f.sp. cubense is one of around 100 formae speciales (special forms) of F. oxysporum which cause vascular wilts of flowering plants (Gerlach and Nirenberg, 1982). Hosts of the various formae speciales are usually restricted to a limited and related set of taxa. As currently defined, F. oxysporum f.sp. cubense affects the following species in the order Zingiberales: in the family Musaceae, Musa acuminata, M. balbisiana, M. schizocarpa and M. textilis, and in the family Heliconeaceae, Heliconia caribaea, H. chartacea, H. crassa, H. collinsiana, H. latispatha, H. mariae, H. rostrata and H. vellerigera (Stover, 1962, Waite, 1963). Additional hosts include hybrids between M. acuminata and M. balbisiana, and M. acuminata and M. schizocarpa.;F. oxysporum f.sp. cubense may survive as a parasite of non-host weed species. Three species of grass (Paspalum fasciculatum, Panicum purpurascens [ Brachiaria mutica ] and Ixophorus unisetus) and Commelina diffusa have been implicated (Waite and Dunlap, 1953).
N. peltata is an aquatic, bottom-rooted perennial plant with round, floating leaves, yellow flowers borne upon peduncles rising above the water's surface, and long branching stolons with adventitious roots beneath the water’s surface. The circular to slightly heart shaped floating leaves are 3-15 cm in diameter on long stalks that attach to underwater rhizomes. The floating leaves have slightly wavy, scalloped margins and are alternately arranged at the stem base but are opposite at the apex (Flora of China, 2002). They are a green to yellow-green colour above, and are often a purple colour on the underside of the leaf. Each peduncle that rises a few inches above the water surface can have two to five flowers, which are bright yellow, have five distinctly fringed petals, and are 3-4 cm in diameter. Both long- and short-styled flower morphs are usually needed to sexually reproduce (Ornduff, 1966). The fruit is a 1.2-2.5 cm beaked capsule that contains many flat, smooth, ovoid seeds with winged margins. The seeds are approximately 0.4 mm thick, 3.8-5.1 mm long, and 2.7–3.0 mm broad (Cook, 1990). The seeds also have winged margins which aid attachment to avian vectors and floatation (Cook, 1990).
N. peltata is an aquatic, bottom-rooted perennial plant with floating leaves, which can grow in dense mats and reproduce prolifically through both vegetative and sexual means. These dense mats have caused many negative environmental and economic impacts, which include displacing native species, reducing biodiversity, decreasing water quality, impeding recreational activities, and diminishing aesthetic value. N. peltata is very difficult to control due to its ability to form a new plant from rhizomes, stolons, separated leaves, or seeds. The dispersal of N. peltata to new locations may be aided by the transport of seeds by avian vectors (Cook, 1990);however, the trade and potential escape of N. peltata through the water garden industry may play a larger role in its spread (Les and Mehrhoff, 1999). N. peltata is declared a noxious weed in New Zealand and parts of North America (NWCB, 2007), and is also declared as invasive in Sweden (Gren et al., 2007). Other species of Nymphoides also have the potential to become invasive, and N. indica and N. cristata have been recorded as problematic in Florida, USA.
Perennial. 50-170 cm tall. Several to many erect stems from a woody, tap-rooted crown are unbranched or sparingly branched distally, villous with septate hairs, thinly arachnoid-tomentose, swollen below the capitula. Leaves are also short-villous and thinly arachnoid, ranging to almost glabrate, dotted with resinous glands. The basal and lower cauline leaves are borne on petioles;blades are oblanceolate to narrowly ovate, 10 to 30 cm, with entire or shallowly dentate margins. Cauline leaves are sessile, shortly decurrent, gradually becoming smaller up the stem, especially the cluster of leaves just below the heads;blades lanceolate to ovate, 5 to 10 cm long, margin entire or slightly undulate, apices acute. Heads subtended by a cluster of reduced leaves. Involucres ovoid to hemispheric, 25-35 mm, surrounded by 3 to 12 rows of layered phyllaries. Phyllary base (body) pale-green to straw-coloured, ovate to broadly lanceolate, glabrous;phyllary appendages erect to spreading, brown to golden, scarious, abruptly expanded forming a cup, 1-2 cm wide, normally concealing the basal parts, lacerate fringed, sometimes tipped by weak spines 1-2 mm, glabrous. Florets many;corollas yellow;corollas of sterile florets slightly expanded, ca. 4 mm;corollas of disc florets ca. 3.5 mm. Cypselae 7-8 mm;pappi of flattened bristles, 5-8 mm long (Roché, 1991;Keil and Ochsmann, 2006).
C. macrocephala is a robust perennial that has been in cultivation for over 200 years. It was reportedly introduced to the UK in 1805 and its presence in the USA dates back to at least 812, when Thomas Jefferson grew it at Monticello from seeds from Philadelphia nurseryman, Bernard McMahon (Anon., 2009). It is still widely used in the horticultural trade. It is listed as a Class A noxious weed in Washington State, USA (USDA-NRCS, 2011) and is a prohibited noxious weed in Alberta, Canada (CIPM, 2011). It threatens natural meadows and spreads in disturbed areas and once established is difficult to control.
C. macrocephala has primarily invaded communities already dominated by perennial plants: meadows;pastures;bluegrass sod;and perennial grass openings in forested areas (Roché, 1991).
Adults are known as tiger mosquitoes due to their conspicuous patterns of very black bodies with white stripes. Also, there is a distinctive single white band (stripe) down the length of the back. The body length is about 3/16-inch long. Like all mosquitoes, Asian tiger mosquitoes are small, fragile insects with slender bodies, one pair of narrow wings, and three pairs of long, slender legs. They have an elongate proboscis with which the female bites and feeds on blood.
The Asian tiger mosquito is spread via the international tyre trade (due to the rainwater retained in the tyres when stored outside). In order to control its spread such trading routes must be highlighted for the introduction of sterilisation or quarantine measures. The tiger mosquito is associated with the transmission of many human diseases, including the viruses: Dengue, West Nile and Japanese Encephalitis.
Culex quinquefasciatus is a medium-sized (approx. 4 mm) mosquito, predominately golden brown in coloration with solid coloured legs and a characteristic white-banded abdomen. The original type specimen collected from the Mississippi River in the southern United States by Thomas Say was lost but type specimens from C.R.W. Wiedemann’s 1828 description of Culex fatigans = quinquefasciatus still exist in the Naturhistorisches Museum of Vienna. A contemporary specimen of Cx. quinquefasciatus from New Orleans has since been designated as a neotype (Belkin, 1977).
Surveillance for Cx. quinquefasciatus generally consists of dip surveys of all suspect larval habitats and selective trapping of adults using mechanized gravid traps baited with infusions of grass, manure or other organic matter (Reiter, 1983). Larval mosquitoes are often identified using morphological keys focused on chaetotaxy - the structure and arrangement of setae - of the siphon and terminal segments of the abdomen (Belkin, 1962;for more current terminology see Harbach and Knight (1980, 1981)). Morphological distinction of Cx. quinquefasciatus from related species is difficult at best but a number of rapid molecular diagnostic assays have been developed (Crabtree et al., 1995;Aspen and Savage, 2003;Smith and Fonseca, 2004).
Culex quinquefasciatus is a peridomestic mosquito seldom found far from human residence or activity, and readily feeds on avian, mammalian or human hosts. The larvae are typically found in the eutrophic water of artificial containers or man-made impoundments including open ponds, ditches and drains containing human or animal sewage. As such, Culex quinquefasciatus was uniquely adapted to the environs of historical sailing ships outfitted for long voyages where polluted water and livestock were common. Since adult mosquitoes can fly short distances to shore (Subra, 1981;LaPointe, 2008) and immature forms could be carried ashore in water casks taken to be refilled (Hardy, 1960), it is likely that this mosquito was spread worldwide by commercial sailing vessels involved in the Atlantic slave trade, Old China trade and American whale oil industry between the 17 and 19 th centuries (Lounibos, 2002). Today, adult Cx. quinquefasciatus are among the most commonly intercepted mosquitoes in passenger airline cabins and their larvae can still be found in exposed cargo (tyres and heavy equipment) and containers on modern ships (Joyce, 1961;Smith and Carter, 1984;Scholte, 2010).
A. calendula is a rosette-forming perennial usually infesting disturbed, urban, and coastal habitats. It prefers a good amount of sun and sandy, well-drained soil. It can grow up to 25 centimeters tall (10 inches) and exhibits purple or yellow daisy-like flowers that can reach 6 centimeters (2.5 inches) in diameter. The plant is pollinated primarily by butterflies. A sterile, vegetatively reproducing yellow-flowered race is not currently regulated in California, but is noted by some to escape from cultivation. This form is now considered a separate species, A. prostrata, sometimes sold in the nursery trade. The invasive A. calendula is regulated in California has purple-tinged disc flowers, is seed-producing, and listed as a category A weed.
H. polysperma is an herbaceous rhizomatous perennial aquatic plant with squarish stems that are ascending or creeping. The stems are mostly submerged, and are usually rooted in the substrate, though can also root freely at floating nodes. The submerged stem is very brittle, and can grow over 6 feet long. The submerged leaves are opposite along the stem, and are sessile with the bases joined at the nodes by ciliated flanges of tissue. The leaves are elliptic to oblong, light green, sparsely hairy, and usually broader towards the tip. Leaves are up to 8 cm long and up to 2 cm wide (UFL-IFAS, 2005), and the leaves on the submersed stem tend to be considerably larger, wider, and lighter in color than those on immersed stem. The small bluish white flower is nearly hidden by leaves in the uppermost leaf axils, and is 2-lipped, with the upper lip being 2-lobed and the lower lip 3-lobed. The fruit is a narrow hairy capsule up to 9mm long, containing 20-30 seeds, each seed being approximately 0.4-0.62 mm long, 0.3-0.5 mm wide, and 0.002-0.06 mm thick. The seeds are compressed, obovate to elliptic to round, with the entire margin narrowly winged. The seed coating is minutely pebbled, glistening, orange-yellow to brown-yellow, and translucent where the seed is particularly thin (FNW Disseminules, 2007).
H. polysperma is an aquatic, mostly submerged, partly immersed plant that can grow to form dense stands and floating mats which cause many negative environmental and economic impacts. Some of these impacts include displacing native plant species, reducing biodiversity, decreasing water quality and flow, clogging irrigation pumps, impeding recreational activities, and diminishing aesthetic value. H. polysperma is extremely difficult and costly to control, and its ability to form new plants vegetatively facilitates its spread to new locations. The trade and potential escape of H. polysperma through the aquarium and water garden industry plays a large role in its spread to new locations, as does the transportation of this plant on recreational equipment or by wildlife moving between water bodies (DCR, 2003). H. polysperma is declared a noxious weed in the United States (USDA-NRCS, 2006), and is currently well established in Florida and parts of Texas. H. polysperma has also been recorded in Virginia, though current status of this population is unknown (Sutton, 1995). H. polysperma has recently been recorded for the first time in Europe (Hussner et al., 2007), and has the potential to spread to new locations throughout the continent.
A cactus-like, usually leafless but evergreen shrub or small tree up to 5 metres high and 15-20 cm in trunk diameter, with fleshy or succulent stems, much branched, hairless throughout, producing abundant milky sap when injured. Stems with whorls of branches nearly to base but on large plants shedding the spiny tissue and developing a rounded brown, fissured trunk. The 3-angled (sometimes 4-angled) branches are mostly joints 10-30 cm long and 2-5 cm across, slightly shiny dark green, with yellowish or whitish streak in the groove of the axis between the angles. The soft cut branches have a light green outer layer less than 0.5 cm thick, which yields latex, and within whitish watery tissue, slightly bitter. Raised leaf bases 0.5 cm high and about 1-2.5 cm apart along the edges of branches correspond to nodes and bear paired spreading gray spines (stipules). Leaves few, scattered, alternate, minute, stalkless, rounded, slightly shiny green, succulent, slightly thick and shedding early, or absent. Flowers small, yellowish green, borne intermittently in small clusters along the stem’s edge (Little et al., 1974).
E. lactea has been widely commercialized as an ornamental plant and due to the presence of spines it is also used as a fence/hedge plant. Many cultivars have been developed for the horticultural trade (USDA-ARS, 2016). It has escaped from cultivation and once naturalized, it often grows forming thickets mostly in disturbed sites, abandoned gardens, deciduous forests, coastal forests, and along roadsides (Little et al., 1974;PIER, 2016;PROTA, 2016). E. lactea spreads by seeds and vegetatively by cuttings and stem fragments (Little et al., 1974). Currently, this species is listed as invasive in Hawaii and Cuba (Oviedo Prieto et al., 2012;PIER, 2016). In Puerto Rico and the Virgin Islands, it is spreading and forming thickets in some places (Little et al., 1974).
Trade involves the transfer of goods or services from one person or entity to another, often in exchange for money. Economists refer to a system or network that allows trade as a market.
An early form of trade, barter, saw the direct exchange of goods and services for other goods and services. Barter involves trading things without the use of money. When either bartering party started to involve precious metals, these gained symbolic as well as practical importance. Modern traders generally negotiate through a medium of exchange, such as money. As a result, buying can be separated from selling, or earning. The invention of money (and later of credit, paper money and non-physical money) greatly simplified and promoted trade. Trade between two traders is called bilateral trade, while trade involving more than two traders is called multilateral trade.
In one modern view, trade exists due to specialization and the division of labor, a predominant form of economic activity in which individuals and groups concentrate on a small aspect of production, but use their output in trades for other products and needs. Trade exists between regions because different regions may have a comparative advantage (perceived or real) in the production of some trade-able commodity—including production of natural resources scarce or limited elsewhere. For example: different regions' sizes may encourage mass production. In such circumstances, trade at market prices between locations can benefit both locations.
Retail trade consists of the sale of goods or merchandise from a very fixed location (such as a department store, boutique or kiosk), online or by mail, in small or individual lots for direct consumption or use by the purchaser. Wholesale trade is defined as traffic in goods that are sold as merchandise to retailers, or to industrial, commercial, institutional, or other professional business users, or to other wholesalers and related subordinated services.
Historically, openness to free trade substantially increased in some areas from 1815 to the outbreak of World War I in 1914. Trade openness increased again during the 1920s, but collapsed (in particular in Europe and North America) during the Great Depression of the 1930s. Trade openness increased substantially again from the 1950s onwards (albeit with a slowdown during the oil crisis of the 1970s). Economists and economic historians contend that current levels of trade openness are the highest they have ever been.