Symptoms and signs[edit]Aspergillus flavus colonies are commonly powdery masses of yellow-green spores on the upper surface and reddish-gold on the lower surface (underneath). In both grains and legumes, infection is minimized to small areas, and discoloration and dullness of affected area is often seen. Growth is rapid and colonies appear downy or powdery in texture.[5]
Hyphal growth usually occurs by thread-like branching and produces mycelium. 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.[5][6][7]
The conidiophores of A. flavus are rough and colorless. Phialides are both uniseriate (arranged in one row) and biseriate.[5]
Recently, Petromyces was identified as the sexual reproductive stage of A. flavus, where the ascospores develop within sclerotia.[4] The sexual state of this heterothallic fungus arises when strains of opposite mating type are cultured together.[8] Sexual reproduction occurs between sexually compatible strains belonging to different vegetative compatibility groups.
A. 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.[4] 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.[9]