Alternaria brassicae f. microspora Alternaria brassicae var. minor Alternaria circinans Berk. & M.A. Curtis, (1924) Alternaria oleracea Milbraith, (1922) Helminthosporium brassicae Henn, (1902) Helminthosporium brassicicola Schwein, (1832) Macrosporium cheiranthi var. circinansBerk. & M.A. Curtis, (1875) Macrosporium circinans Macrosporium commune var. circinans Polydesmus exitiosus f. alternarioides J.G. Kühn, (1855) Polydesmus exitiosus f. luxuriosum Sporidesmium exitiosum f. alternarioides Sporidesmium exitiosum f. luxuriosum Sporidesmium septorioides
Alternaria brassicicola is a fungal
conidia reside in the soil, air and water.[3] These spores are extremely resilient and can overwinter on crop debris and overwintering herbaceous plants.[3]
Growth and morphology
Alternaria brassicicola chain-like conidia (left and right)
Legions of A. brassicicola on Chinese Kale
The
epidermal leaf wax of plants, particularly those in the Brassicaceae, and prefers an environment with high humidity and temperature range of 20–30 °C (68–86 °F).[3] Macroscopically, the mycelium exhibits a range of colour: unpigmented when young, to olive-grey, grey-black at maturity.[9][2] Colonies of A. brassicicola tend to be dark brown or black in colour.[2]
Research history
Historically, much of the early research concerning the fungus was based on plant defense mechanisms. However, once its
genes involved in host-parasite interaction.[1] 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.[1] 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.[3]
Growth cycle
Colonies of A. brassicicola on Potato Dextrose Agar after 3 d (l) and 7 d (r).
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
There are three main sources of infection: nearby infected seeds, spores from plant debris in the topsoil and Brassica weeds, and spores moved by wind and air from farther away.
AB toxins, that induce cell death by apoptosis.[3] This results in what look like dents and lesions in the host plant.[3] These are brown, concentric circles with a yellow tinge at the circumference, usually about 0.5-2.5cm in diameter.[11][5][1]Necrosis can generally be observed within 48 hours of infection.[11] The spores can reside on the external seed coat of infected seeds, but the mycelium can also penetrate under the seed coat, where it has the ability to remain viable for several years.[1] Occasionally, it can even penetrate the embryo tissue.[6] The primary mode of transmission is through contaminated seed.[5] Also, the infection is not limited to specific areas of the host plant; it can spread all over and even cause damping off of the seedlings at a relatively early stage.[3] It also affects the host species at various developmental stages.[9] As mentioned above, seedlings exhibit dark stem lesions followed by damping off. Velvety, black spots, resembling soot, can be observed on older plants.[9] Pathogenesis is affected by factors such as: temperature, humidity, pH, reactive oxidation species, host defense molecules.[3]
The most common toxin studied for A. brassicicola is the AB toxin, said to be connected to the
lipases and cutinases are also linked to its pathogenicity, but more evidence of their efficacy is required.[1] One important transcription factor is AbPf2. It regulates 6 of the 139 genes encoding small secretion proteins and may have a role in pathogenesis, specifically cellulose digestion.[1]
Treatments
In order to protect their crops, many individuals pre-treat their seeds with
phenolase activity, high leaf sugar, and thicker wax layers reduce water-borne spore germination. It has been shown that the presence of camalexin in the host plant helps it to disrupt pathogen development. For example, an Arabidopsis mutant in the pad-3 gene that does not produce camalexin is more susceptible to infection. Varying levels show differing levels of resistance.[3] Another suggestion put forth is crop debris management. The aim is to minimize exposure of the crop plants to spores present in the soil by using crop rotation and weed control.[3]
Biological approaches have also been studied. One approach has been to use antagonistic fungi such as Aureobasidium pullulans & Epicoccum nigrum to subdue the effect of A. brassicicola.[3] The plants C. fenestratum and Piper betle also show potent fungicidal activity towards A. brassicicola both in vitro and under greenhouse conditions. These levels are comparable to Iprodione. The active compound, berberine, affects cell wall integrity and ergosterol biosynthesis.[6] Ethanol extracts from the dried roots of Solanum nigrum (black nightshade), traditionally used as herbal remedies in places ranging from the Far East to India and Mexico, show promising anti-fungal activity as well. They seem to suppress conidial germination, possibly by interfering with the AB toxin.[7]
Economic impact
As mentioned previously, Alternaria brassicicola causes severe black spot diseases in a number of ecologically important crops. Often, it occurs in conjunction with Alternaria brassicae. However, it is the more dominant invasive species. These infections lead to a significant loss in viable seeds and produce. The resulting lesions greatly reduce available photosynthetic area, leading to wilt and plant death. Crops like infected cabbages do not last long during storage or transportation.[3] In some cases, yield reductions can be as high as 20-50%.[1] The lack of ability to use fungicides makes it challenging to sustain organic crops in a cost-effective way.[10]
^ abMuto, Machiko (2005). "Control of Black Leaf Spot (Alternaria brassicicola) of Crucifers by Extracts of Black Nightshade (Solanum nigrum)". Plant Pathology Bulletin. 14: 25–34.
^ abSimmons, Emory (2007). An Identification Manual. CBS Fungal Diversity Centre.
^ abcdeMeena, P.D (2010). "Alternaria blight: a chronic disease in rapeseed-mustard". Journal of Oilseed Brassica. 1 (1): 1–11.
