Alternaria brassicicola is a fungal necrotrophic plant pathogen that causes black spot disease on a wide range of hosts, particularly in the genus of Brassica, including a number of economically important crops such as cabbage, Chinese cabbage, cauliflower, oilseeds, broccoli and canola. Although mainly known as a significant plant pathogen, it also contributes to various respiratory allergic conditions such as asthma and rhinoconjunctivitis. Despite the presence of mating genes, no sexual reproductive stage has been reported for this fungus. In terms of geography, it is most likely to be found in tropical and sub-tropical regions, but also in places with high rain and humidity such as Poland. It has also been found in Taiwan and Israel. Its main mode of propagation is vegetative. The resulting conidia reside in the soil, air and water. These spores are extremely resilient and can overwinter on crop debris and overwintering herbaceous plants.
Growth and morphology
The conidia of A. brassicicola are abundant in the outdoor environment from the months of May to late October in the northern hemisphere, peaking in June and again in October. The conidia are dark brown and smooth-walled, up to 60 x 14μm. The conidia are cylindrical to oblong in shape and are muriform and produced in chains of 8-10 spores. They are firmly attached to conidiophores that are olive-brown, septate, and growing to an upper range of 100-200 μm, although this overall length may vary. Conidia are borne in continuous, chain-like structure, but branching at the base has also been observed. Although conidia can be spread by rain, the most common means of spread is through the air. The fungus grows on epidermal leaf wax of plants, particularly those in the Brassicaceae, and prefers an environment with high humidity and temperature range of . Macroscopically, the mycelium exhibits a range of colour: unpigmented when young, to olive-grey, grey-black at maturity. Colonies of A. brassicicola tend to be dark brown or black in colour.
Research history
Historically, much of the early research concerning the fungus was based on plant defense mechanisms. However, once its genome was sequenced, efforts shifted to identifying the genes involved in host-parasite interaction. One of the pioneers for genetic research into Alternaria brassicicola was the Lawrence group at Virginia Bioinformatics Institute and the Genome Center at Washington University. The most common media used for A. brassicicola growth are PDA (potato dextrose agar) and V8 juice-agar. In vitro and under optimal conditions, colonies grow rapidly and appear dark green or white-grey. Spontaneous sporulation occurs at 25°C in darkness on PDA medium.
Growth cycle
Hours after inoculation: 2h: Conidia swells 3h: Germ tube formation observed at the apical or middle cells of conidia 8h: Vesicle of dissolved contents moves from conidial cell to germ tube 20h: Infection of the host cell 48h: Mycelial network develops on the surface 72h: Many chains of conidia can be seen