Aspergillus flavus is a saprotrophic and pathogenic fungus with a cosmopolitan distribution. He is best known for the colonization of cereal grains, beans, and tree nuts. Postharvest foams usually develop during harvest, storage, and/or transit. A. flavus infection can occur when the host is still in the field (preharvest), but often shows no symptoms (dormancy) until postharvest storage and/or transportation. In addition to causing pre-harvest and post-harvest infections, many strains produce large numbers of toxic compounds known as mycotoxins, which, when consumed, are toxic to mammals. A. flavus is also an opportunistic human and animal pathogen, causing aspergillosis in individuals with the immune system.
Video Aspergillus flavus
Host
A. flavus is found globally as a saprophyte in the soil and causes disease in many important agricultural crops. Common host pathogens are cereal grains, beans, and tree nuts. Specifically, A. flavus infection causes ear rot to maize and yellow mushroom in beans either before or after harvest. Infection may be present in the field, pre-harvest, postharvest, during storage, and during transit. The common pathogen occurs when the host plant is still in the field; However, symptoms and signs of pathogens are often not seen. A. flavus has the potential to infect seeds with sporulation on the wounded seed. In seeds, pathogens can attack seed embryos and cause infections, which decrease germination and may cause infected seeds to be planted in the field. The pathogen can also blacken the embryo, break the seeds, and kill the seeds, which reduce the value and the price of the grain. The incidence of A. flavus infection increases in the presence of insects and all types of stress on the host in the field as a result of damage. Stress includes rotting stalks, drought, severe leaf damage, and/or less than ideal storage conditions. Generally, excessive moisture conditions and high temperatures of grain and bean storage increase the occurrence of A. flavus aflatoxin production. In mammals, pathogens can cause liver cancer through the consumption of contaminated feed or aspergillosis through invasive growth.
Maps Aspergillus flavus
Symptoms and signs
A. flavus colonies are generally the powder mass of the yellow-green spores on the upper surface and the reddish gold on the lower surface. In grains and nuts, the infection is minimized to small areas, and discoloration and the stupidity of the affected area are often seen. Rapid growth and colonies appear fluffy or textured powders.
The growth of hyphae usually occurs by branching like a thread and producing mycelia. Hyphae is septate and hyaline. Once formed, the mycelium secretes enzymes or proteins that can break down complex nutrients (food). Individual hyphae strands are usually invisible to the naked eye; However, conidia produces thick mycelium mats are often seen. Konidiospores are asexual spores produced by A. flavus during reproduction.
The conidiophores of A. flavus are rough and colorless. Phialides are both uniseriate (arranged in one line) and biseriate.
Recently, Petromyces was identified as the sexual reproductive stage A. flavus , in which ascospor develops in sclerotia. The sexual state of this heterothallic fungus arises when the opposite sexy strain is cultured together. Sexual reproduction occurs between strains that are sexually compatible with different vegetative compatibility groups.
A. flavus is very complex in its morphology and can be classified into two groups based on the size of the resulting sclerotia. Group I consists of strains L with sclerotia greater than 400 m in diameter. Group II consists of S strains with sclerotia less than 400 m in diameter. Both L and S strains can produce the two most common aflatoxins (B1 and B2). Unique to strain S is the production of aflatoxin G1 and G2 which is not normally produced by A. flavus . Strain L is more aggressive than S strains, but produces less aflatoxin. The L strain also has a more acidic homoeostic point and produces less sclerotia than the S strain under more restrictive conditions.
Disease Cycle
A. flavus overwinters on the ground and appears as propagules in decomposing material, either as mycelia or sclerotia. Sclerotia germinates to produce additional hyphae and asexual connective spores. This conidia is said to be the main inoculum for A. flavus . The propulsion on the ground, which is now conidia, is scattered by wind and insects (like the smell of bugs or bug lygus). Konidia can land and infect grains or peas. Spores enter corn through the silk and thus infect the kernel. Conidiofor and conidia are produced in spring from the sclerotial surface. There is a secondary inoculum for A. flavus , the conidia on the leaves and leaves. A. flavus grows on leaves after damage by leaf-eating insects. Insects are said to be sources of inoculum and increase inoculum production.
Environment
A. flavus is unique because it is a thermotolerant disease, so it can survive at temperatures that can not be done by other diseases. A. flavus can contribute to the decomposition of storage, especially when plant material is stored at high moisture levels. A. flavus grows and thrives in hot and humid climates.
Temperature: A. flavus has a minimum growth temperature of 12Ã, Â ° C (54Ã, Â ° F) and a maximum growth temperature of 48Ã, Â ° C (118Ã, Â ° F). Although the maximum growth temperature is about 48Ã, Â ° C (118Ã, Â ° F), the optimum growth temperature is 37Ã, Â ° C (98.6 Ã, Â ° F). A. flavus has rapid growth at 30-55Ã, Â ° C, slow growth at 12-15Ã, Â ° C, and almost stopped growing at 5-8Ã, Â ° C.
Humidity: A. flavus growth occurs at different moisture levels for different plants. For edged cereals, the growth occurred at 13.0-13.2%. For soybeans, growth occurs at 11.5-11.8%. For other plants, growth occurs at 14%. A. flavus growth is prevalent in tropical countries.
Management
To ensure the seeds and legumes remain free of the infected flavus, certain conditions should be included before, during, and after harvest. The humidity level should be kept below 11.5%. The temperature in the storage unit should be kept as low as possible because the pathogen can not grow below 5 ° C. Low temperatures facilitate slower respiration and prevent increased moisture. Fumigants are used to reduce the occurrence and persistence of insects and mites, which help the growth of pathogens rapidly. Sanitation practices include, disposing of old and immature seeds, removing damaged and damaged seeds, and overall cleanliness helps in minimizing colonization and spread of pathogens.
The most common management practice for grains and beans is the use of aeration systems. Air is driven through a storage area with a low flow rate, which removes excess moisture and heat. Airflow regulation allows the water content in the harvested product to remain at a constant level and lower the temperature inside the trash. Temperature levels can be reduced so that insects and mites are inactive, which reduces the rapid growth of pathogens.
Several environmental control practices have been explored to assist with the reduction of A. flavus infections. Resistant plant lines have shown little or no protection against unfavorable environmental conditions. However, good irrigation practices help reduce stress caused by drought, which in turn, reduces the likelihood of pathogen infection. Several studies have been done in identifying specific plant proteins, both pathogenic and drought-resistant proteins, which survive against A. flavus entries.
To protect the bean and corn trees affected by A. flavus , scientists from the Agricultural Research Service found that treating this plant with Pichia anomala yeast reduced the growth of A. flavus . Research shows that treating pistachio trees with P. anomala inhibits the growth of flavus to 97% when compared with untreated trees. [1] Yeast successfully competes with A. flavus for space and nutrition, ultimately limiting its growth.
A. flavus AF36
A. flavus AF36 strains are non-carcinogenic and aflatoxin free and are used as active ingredients in pesticides. AF36 is a fungal antagonist and is applied as a commercial biocontrol for cotton and corn to reduce aflatoxin exposure. AF36 was originally isolated in Arizona and also occurred in Texas. Planted in sterile seeds that function as carriers and sources of nutrients. After application and colonization and in the presence of high humidity, AF36 grows seeds defeating the aflatoxin-producing strains of A. flavus . Nonaflatoxin spores are assisted by wind and insects.
Importance
A. flavus infection will not always reduce yields alone; However, postharvest disease can reduce total crop yield by 10 to 30%, and in developing countries that produce perishable crops, the total loss can be greater than 30%. In seeds and nuts, postharvest disease produces mycotoxin production. The largest economic loss caused by these pathogens is the result of aflatoxin production. In the United States, annual economic loss estimates of peanuts, maize, cottonseed, walnuts, and almonds are less severe compared to Asia and Africa.
After Aspergillus fumigatus , A. flavus is the second leading cause of aspergillosis. Primary infection is caused by spore inhalation; Larger spores have a better chance of staying in the upper respiratory tract. Specific spore size deposition could be a major factor why A. flavus is the common cause of the aetiology of fungal sinusitis and skin infection and noninvasive fungal pneumonia. Countries with dry weather, such as Saudi Arabia and most of Africa, are more susceptible to aspergillosis. Two allergens have been characterized in A. flavus : Asp fl 13 and Asp fl 18. In tropical and warm climates, flavus has been shown to cause keratitis in about 80% of infections. A. flavus infection is usually treated with antifungal drugs such as amphotericin B, itraconazole, voriconazole, posaconazole, and caspofungin; However, some antifungal resistance has been shown in amphotericin B, itraconazole, and voriconazole.
Aflatoxin
In 1960 at a British farm, about 100,000 turkeys died. Further examination of the cause of death shows the main food source, peanut meal, infected with A. flavus . Cultures are isolated, grow in pure culture, and some healthy turkeys are infected. Pure culture isolates cause death in healthy turkeys. The chemical investigation into the cause of death shows the production of four toxic chemicals, called aflatoxins after being found in A. flavus . Turkish autopsy shows aflatoxin targeting the liver and actually killing tissue cells or inducing tumor formation. The discovery of aflatoxin alters the agricultural practice of how grains and legumes are grown, harvested, and stored. New standards for the production of food for human consumption were developed, leading to increased costs in these hosts.
The amount of aflatoxin produced by A. flavus is influenced by environmental factors. If other competitive fungal organisms are present in host plants, aflatoxin production is low. However, if fungal organisms are not competitively present in host plants, aflatoxin production can be very high. Host properties are also an important factor in aflatoxin production. High A. flavus soybean growth produces little aflatoxin. High A. flavus growth is aided by elevated moisture content and warm temperature in peanuts, nutmeg, and chili resulting in high concentrations of aflatoxin. A. flavus growth in spices produces low aflatoxin concentrations as long as the herbs stay dry.
The sensitivity of the species varies greatly with aflatoxin. Rainbow trout is very sensitive at 20 ppb, leading to liver tumor development in half the population. White mice develop liver cancer when exposed to 15 ppb. Piglets, ducklings, and young turkeys afflicted with high doses of aflatoxin become sick and die. Pregnant cows, adult pigs, cows, and sheep aflatoxin exposed to low doses in the long run experience weakening, intestinal bleeding, weakness, diminished growth, nausea, no appetite, and tendencies for other infections.
The four major aflatoxins produced are B1, B2, G1, and G2. The main toxic production is the result of certain strains of A. flavus . Aflatoxin B1 is the most toxic and potent natural hepatocarcinogenic compound. A. flavus also produces other toxic compounds including sterigmatocystin, cyclopiazonic acid, kojic acid ,? -nitropropionic acid, aspertoxin, aflatrem, gliotoxin, and aspergillic acid.
In humans, A. flavus aflatoxin production can cause acute hepatitis, immunosuppression, hepatocellular carcinoma, and neutropenia. The absence of screening arrangements for fungi in countries that also have a high prevalence of viral hepatitis greatly increases the risk of hepatocellular carcinoma.
References
External links
- Aspergillus flavus Genome Order Project
- Aspergillus flavus research
Source of the article : Wikipedia
0 Comments