Aspergillus flavus

Aspergillus flavus is a saprotrophic and pathogenic fungus with a cosmopolitan distribution. It is best known for its colonization of cereal grains, legumes, and tree nuts. Postharvest rot typically develops during harvest, storage, and/or transit. A. flavus infections can occur while hosts are still in the field (preharvest), but often show no symptoms (dormancy) until postharvest storage and/or transport. In addition to causing preharvest and postharvest infections, many strains produce significant quantities 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 immunocompromised individuals. Aspergillus flavus is found globally as a saprophyte in soils and causes disease on many important agriculture crops. Common hosts of the pathogen are cereal grains, legumes, and tree nuts. Specifically, A. flavus infection causes ear rot in corn and yellow mold in peanuts either before or after harvest. Infection can be present in the field, preharvest, postharvest, during storage, and during transit. It is common for the pathogen to originate while host crops are still in the field; however, symptoms and signs of the pathogen are often unseen. A. flavus has the potential to infect seedlings by sporulation on injured seeds. In grains, the pathogen can invade seed embryos and cause infection, which decreases germination and can lead to infected seeds planted in the field. The pathogen can also discolor embryos, damage seedlings, and kill seedlings, which reduces grade and price of the grains. The incidence of A. flavus infection increases in the presence of insects and any type of stress on the host in the field as a result of damage. Stresses include stalk rot, drought, severe leaf damage, and/or less than ideal storage conditions. Generally, excessive moisture conditions and high temperatures of storage grains and legumes increase the occurrence of A. flavus aflatoxin production. In mammals, the pathogen can cause liver cancer through consumption of contaminated feed or aspergillosis through invasive growth. Aspergillus flavus colonies are commonly powdery masses of yellow-green spores on the upper surface and reddish-gold on the lower surface. In both grains and legumes, infection is minimized to small areas, and discoloration and dullness of affected areas is often seen. Growth is rapid and colonies appear downy or powdery in texture. Hyphal growth usually occurs by thread-like branching and produces mycelia. Hyphae are septate and hyaline. Once established, the mycelium secretes degradative enzymes or proteins which can break down complex nutrients (food). Individual hyphae strands are not typically seen by the unaided eye; however, conidia producing thick mycelial mats are often seen. The conidiospores 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 row) and biseriate. Recently, Petromyces was identified as the sexual reproductive stage of A. flavus, where the ascospores develop within sclerotia. The sexual state of this heterothallic fungus arises when strains of opposite mating type are cultured together. Sexual reproduction occurs between sexually compatible strains belonging to different vegetative compatibility groups. Aspergillus flavus is complex in its morphology and can be classified into two groups based on the size of sclerotia produced. Group I consists of L strains with sclerotia greater than 400 μm in diameter. Group II consists of S strains with sclerotia less than 400 μm in diameter. Both L and S strains can produce the two most common aflatoxins (B1 and B2). Unique to the S strains is the production of aflatoxin G1 and G2 which typically are not produced by A. flavus. The L strain is more aggressive than the S strain, but produces more less aflatoxin. The L strain also has a more acidic homoeostatic point and produces less sclerotia than the S strain under more limiting conditions. Aspergillus flavus overwinters in the soil and appears as propagules on decaying matter, either as mycelia or sclerotia. Sclerotia germinate to produce additional hyphae and asexual spores call conidia. These conidia are said to be the primary inoculum for A. flavus. The propagules in the soil, which are now conidia, are dispersed by wind and insects (such as stink bugs or lygus bugs). The conidia can land on and infect either grains or legumes. The spores enter the corn through the silks and thus infect the kernel. Conidiophores and conidia are produced in the spring from sclerotial surfaces. There is a secondary inoculum for A. flavus, which is conidia on leaf parts and leaves. A. flavus grows on leaves after damage by leaf-feeding insects. Insects are said to be a source of inoculum and promote inoculum production.