User:Maria Alejandra Caldera/sandbox

Taxonomy
The genus Fusarium was introduced by Link in 1809 (2, 15). Species of this genus can produce disease in humans, animals and plants.The etymology of Fusarium comes from the Latin word for spindle, fusus, based on the canoe-shaped conidia. Fusarium species are poorly defined based on the lack of a large number of morphological characters that can be used for differentiating species. Molecular phylogenetic studies have resulted in the description of sibling species and lineages that are impossible to differentiate morphologically (15).

Many Fusarium species have an anamorph (asexual stage) and teleomorph (sexual stage), such as Fusarium subglutinans (teleomorph:G. subglutinans) and Fusarium verticilliodes (teleomorph: G. moniliformis Sawada). No sexual stage has been reported for Fusarium oxysporum (2).

Fusarium oxysporum Schltdl. was first described by Snyder and Hansen in 1824. F. oxysporum is without a doubt the most economically important species in the genus and is one of the two (F. oxysporum and F. solani) of nine species described by Snyder and Hansen that remains in general use: F. moniliforme, F. roseum, F. lateritium, F. tricinctum, F. nivale, F. rigidiuscula and F. episphaeria (2, 15). It contains a varied range of non-pathogenic, and human and animal pathogenic strains. These can only be differentiated by a pathogenicity test due the lack of morphological variability among strains (2, 6).

Fusarium oxysporum is a species complex, meaning that it is a group of related species for which specific demarcations are largely unclear. This may be due to incomplete reproductive isolation (21). Previous research using rapid rRNA sequencing with two highly variable stretches (138 and 214 nucleotides) of the 5’ end of the 28S-like rRNA molecule has shown that all the F. oxysporum are identical, except for ''F. oxysporum var. redolens''. Further, they concluded that distinguishing between formae speciales and races is not possible with this molecular tool (10). In 2009, O’Donnell et al. constructed a two-locus database for Fusarium oxysporum using partial translation elongation factor (EF-1alpha) gene sequences and almost a full-length sequence of the nuclear ribosomal intergenic spacer region (IGS rDNA). They found some phylogenetic discordance suggesting that the soilborne members of the Fusarium oxysporum complex are genetically diverse and that the use of single-locus data, specifically from nuclear ribosomal DNA, is not recommended for phylogenetic reconstruction and species differentiation (19). On the other hand, they concluded that the IGS rDNA is more promising than a number of genes that have been used in the past for Fusarium oxysporum (e.g polygalacturonase, small subunit ribosomal RNA, β tubulin, MAT1, and MAT2) (19).

Other research has also reported the use of the mating type locus (MAT1) from Fusarium oxysporum and its use in the differentiation between races in Fusarium oxysporum f.sp lycopersici (12). Also, the mating type locus (MAT1 and MAT2) was used for a taxonomic and evolutionary study of Fusarium oxysporum f.sp cubense and suggested that it may have undergone recent sexual reproduction (6).

Phylogenetic studies using DNA sequences of rDNA, translation elongation factor, and mitochondrial small subunit ribosomal RNA gene has suggested the Fusarium oxysporum complex is monophyletic, but that many formae speciales were found to be polyphyletic (6,12). Fusarium is one of the most difficult complexes to classify and additional research is needed to build a consistent phylogeny (6).

Distribution
Fusarium oxysporum species are not just found in tropical areas of the world, they are cosmopolitan in a diversity of environments (2, 10). It has been found from tropical to temperate regions, and in extreme environments such as the arctic and the Sahara desert (10). The first report of Fusarium wilt (Fusarium oxysporum f.sp. cubense) was reported in Australia in 1876 (10), another strain (Fusarium oxysporum f.sp. lycopersici) was described by G.E. Massee in England in 1895 (10). ''Fusarium oxysporum f. sp. cubense'' (Panama disease of banana) has been widely distributed around the world. In some countries is present, but due the lack of research ,further details has not been established, and in others is already localized(8).

Present but not further details
Guatemala, Honduras , Nicaragua , Costa Rica , Belize , Panama , Peru , Ecuador , Colombia, Venezuela ,Mali , Africa , Nigeria , Egypt , Rwanda , Tanzania, Uganda, Congo, Jordan, Mozambique, Madagascar, Canary Islands, Senegal, Ghana, Guinea, Nepal, Bangladesh, Myanmar, China, Asia, Vietnam, Guam , Philippines, and Taiwan.

Localized
Australia, Indonesia, Brazil and some parts of United States

Widespread
Sri Lanka and Mexico

Identification
Fusarium oxysporum is an asexual fungus that produces three types of asexual spores: macroconidia, microconidia and chlamydospores (2, 10, 10, and 15). The macroconidia are produced most often on the surface of infected plants parts or in artificial media. The macroconidia also can be produced singly in the aerial mycelium of culture media. These types of spores are thin walled with a pointed apical cell. Microconidia occur on short microconidiophores and are produced in false heads. Both spores can also be formed in the xylem of infected plants, but microconidia are predominant in infected plant tissue. They can be spread rapidly as large numbers of airborne spores, which act as continual inoculum (16). Chlamydospores are thick walled asexual spores that can persist in the soil and are formed in dead host plant tissue in the final stage of the disease. Chlamydospores remain dormant in the soil until they are stimulated to germinate (10, 15, and 17).

F. oxysporum can be differentiated from F. solani, because it forms microconidia on false heads on long monophialides, and from F. subglutinans, which forms microconidia on polyphialides and does not form chlamydospores(15).

Fusarium oxysporum can be cultured on potato dextrose agar (PDA) revealing a cottonish whitish mycelia that later shows a purple or pinkish pigment (15). Several media have been evaluated from identification such as: carnation leaf agar (CLA), low nutrient agar and Dichloran chloramphenicol peptone agar(3).The most appropriate media was CLA, showing consistent production of macroconidia, microconidia and chlamydospores in the media(3).

Host Range
On plants, Fusarium causes leaf spots, head blights, die-back, and wilt diseases depending on the species and the host. The most important problem is vascular wilt disease caused by Fusarium oxysporum (2, 10). These microorganisms attack a diverse group of plants including: tomato, cabbage, banana, pea, tobacco, sweet potato, cotton, and watermelon. Strains are classified into formae speciales (f.sp.) depending on host specificity. Over 100 formae speciales and races of Fusarium oxysporum have been described (2).

Favorable conditions
Fusarium oxysporum (e. g Fusarium oxysporum f.sp lycopersici) likes warm weather and is more likely to appear on acidic and sandy soils. Favorable conditions for the development of this fungus are: soil and ambient temperature of 28° C, soil moisture for plant growth, low concentration of nitrogen and phosphorus, and a high level of potassium, low light intensity and pH(10, 13, 17 and 20).

Infection
The penetration of the host plant can occur by a wound or directly. Once the pathogen has penetrated, it moves to the vascular tissue and when young roots are the site of infection the pathogen invades the xylem vessel elements, either intercellularly or intracellularly. Fusarium oxysporum is spread throughout the plant by conidia or mycelial growth. As the disease develops the fungus invades tissues near the xylem as cambium, phloem and cortex (13, 17).

Symptoms
''Fusarium oxysporum f.sp. lycopersici is the causal organism of Fusarium'' wilt in tomato. This organism works to block the flow of water by invading the xylem and eventually spreading throughout the plant. In seedling stages, infected plants are stunted and older leaves droop and curve downward turning yellow. The vascular tissue becomes dark brown and infected plants commonly wilt and die. The most common symptom is the yellowing of older leaves and this most of the time develops on one side of the plant. Yellowing affects most of the foliage and also is complemented by wilting during the hottest part of the day. The top of the plant wilts during the day and recover at night, but wilting gets progressively worse until the plant dies (13, 17).

Dissemination
Fusarium oxysporum is a soilborne pathogen and can persist in the soil for several years. It can be disseminated by infected seeds or by plants grown in the infested soil. Chlamydospores can be transported by men, machinery and everything that comes in touch with the infested soil, such as irrigation, animals, and even the wind (13, 17, and 21). Seed transmission of Fusarium oxysporum often occurs when conidia or chlamydospores are carried as surface contaminants or in plant parts that remain with the seed after harvest. In infected tomatoes, vascular invasion by the pathogen often extends into the fruit and onto the surface of the seed and can carry the pathogen for at least 7 months (21).

Management
Fusarium wilt resistance most of the time is hindered by the presence of nematodes in the field. Chesapeake is a highly resistant cultivar, but when Fusarium is mixed with nematodes this resistance can easily break down (13).Examples of Fusarium Oxysporum resistant varieties are:


 * Park's Whopper Improved
 * Better Boy
 * Lemon Boy
 * Enchantment
 * Celebrity
 * Floramerica
 * Sunmaster
 * Mountain Delight
 * Mountain Pride
 * Mountain Spring
 * Sunny
 * Big Beef
 * Beefmaster
 * Sweet Chelsea
 * Small Fry
 * Supersweet 100

The fungicidal control Fusarium wilt is impracticable and uneconomical, but the use of resistant varieties be effective in reducing disease incidence. There are many resistant varieties for race 1 and race 2, but few are resistant to all three races. Although a large number of physiological races of Fusarium oxysporum exist in nature, race 1 and 2 are the most prevalent(13).

Techniques like cleaning farm equipment, avoiding nematode infestation and crop rotation can help to reduce the amount of inoculum (21). This disease does not have a cure but can be prevented using disease-free seed, crop rotation to reduce populations of the fungus in soil, planting in disease-free soil, and destroying infected plant residue. Fusarium wilt affects many plant species belonging to all plant families except the Poaceae.Growing corn, rice or wheat in the field will help to keep away Fusarium wilt for a time. Also, in greenhouses or seed beds, disinfecting or sterilizing the soil with steam helps to reduce the amount of inoculum (13).

Biological control
Difficulty in controlling Fusarium wilt explains why there is a long history of research on biological control of this disease. The existence of soils that naturally limit the incidence of Fusarium has been recognized since the last century. The involvement of saprophytic microflora as a mechanism for the suppression of soil-borne pathogens led to the idea of making soils suppressive by manipulating microbial balance. Enhancing the low level of natural suppressiveness and isolating antagonistic microorganisms that can be mass-produced are two approaches than can be followed to transform a conducive soil into a suppressive soil. The use of organic matter and compost can also help to achieve this goal(1).

Penicillium oxalicum, Penicillium purpurogenum and Aspergillus nidulans have been used to control Fusarium wilt on tomatoes and they have showed damage in the hyphae of the pathogen in vitro and reduction in the numbers of microconidia in the soil(4).

Tomato plants treated with non-pathogenic microorganisms show reduced severity of Fusarium wilt. Susceptible cultivars of tomato plants were treated with non-pathogenic forms of Fusarium oxysporum (F.oxysporum f. sp.dianthi) and inoculated with Fusarium oxysporum f. sp.lycopersici race 1. Results of the experiment showed that plants treated with a mixture of both fungi remained symptomless for over 7 weeks (15). Suppressiveness to Fusarium wilt by nonpathogenic Fusarium strains has been attributed to competition for soil, nutrients, and competition for infection sites (15).

Chemical control
Recent research using neem and willow aqueous extracts has shown a reduction in the incidence of Fusarium wilt in tomato seedlings by increasing the activities of antioxidant defensive enzymes and decreasing the level of lipid peroxidation (5). Soil fumigants such as methyl bromide have a broad spectrum of activity against soilborne pathogens. However, this fumigant is an ozone-depleting substance and both production and usage were phased out in developed nations by the year 2005 and will be phased out in most developing nations in 2015 (17).

Fusarium oxysporum in humans
Fusarium oxysporum is able to produce mycotoxins that can be harmful for humans and animals. One of the diseases produce by Fusarium is called Fusarium keratitis. It produces inflammation of the cornea, ulcers and excessive vascularization (20).Onychomycosis is another disease produced by fungi and some cases Fusarium oxysporum has been reported as the causal organism. This is a fungal infection of toenails or fingernails. Major symptoms are pain, discomfort, and disfigurement(22).