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.[1][2][3] Although mainly known as a significant plant pathogen, it also contributes to various respiratory allergic conditions such as asthma and rhinoconjunctivitis.[4] Despite the presence of mating genes, no sexual reproductive stage has been reported for this fungus.[5][1][3] 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.[3] It has also been found in Taiwan and Israel.[6][7] Its main mode of propagation is vegetative. The resulting 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]
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.[4] The conidia are dark brown[8] and smooth-walled, up to 60 x 14μm.[9][2] The conidia are cylindrical to oblong in shape and are muriform and produced in chains of 8-10 spores.[9] They are firmly attached to conidiophores[4] that are olive-brown, septate, and growing to an upper range of 100-200 μm, although this overall length may vary.[8] Conidia are borne in continuous, chain-like structure, but branching at the base has also been observed.[2] Although conidia can be spread by rain, the most common means of spread is through the air.[4] 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 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]
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.[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]
Hours after inoculation:
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.[3] Infected leaves can spread their spores up to a diameter of 1800m. There are also three major entry points to the host cell: epidermal penetration, stomatal penetration and penetration through an insect.[3] Contact with the host cell triggers the release of various cell wall degrading enzymes which allow the fungus to attach itself to the plant and begin degradation.[10] The suggested mode of attack is through host-specific toxins, primarily 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]
Out of the 10,688 predicted genes from the A. brassicicola genome, 139 encode small secretion proteins that may be involved in pathogenesis, 76 encode lipases and 249 encode glycosyl hydrolases that are important for polysaccharide digestion, potentially damaging host cells. In contrast, mutations in genes such as AbHog1, AbNPS2, and AbSlt2 affect cell wall integrity and make the fungus more susceptible to host defenses. Currently, research is being done to identify the gene(s) responsible for encoding a transcription factor, Bdtf1, important for the detoxification of host metabolites.[1]
The most common toxin studied for A. brassicicola is the AB toxin, said to be connected to the virulence, pathogenicity and host range for the fungus.[3] It is most likely produced during conidial germination and probably linked to the ability of the fungus to infect and colonize Brassica leaves [10] However, recent studies have explored new potential metabolites. For example, this fungus also produces histone deacetylase inhibitors, but these do not have a significant impact on lesion size.[3] Some studies show only a 10% reduction in virulence.[1] Furthermore, alternariol and tenuazonic acid seem to affect mitochondrial-mediated apoptosis pathways and protein synthesis respectively (in the host cell), but again, not to a significant degree. Some cytokines have been linked with the discolouration associated with A. brassicicola infection.[3] Cell wall degrading enzymes like 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]
In order to protect their crops, many individuals pre-treat their seeds with fungicides.[3] The most widespread active ingredients in these fungicides are Iprodione and Strobilurins.[3] In 1995, it was reported that Iprodione most likely acts by mutating two histidine residues in the target site of enzymes.[5] Ultimately, it inhibits germ tube growth.[6] However, the ubiquitous use of fungicides has resulted in the fungus growing increasingly resistant.[6] Thus, different, non-chemical approaches have been explored. People have tried to develop resistant Brassicaceae crops through breeding. However, this has proved challenging due to the difficulty of transferring genes from wild-type to cultivated strains, resulting in genetic bottlenecks. It is further complicated by the probability that resistance seems to be a polygenic trait. There are also some Brassica plants that have developed resistance to the pathogen naturally. High 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]
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]
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.
Alternaria brassicicola es un hongo patógeno de las plantas. Causa manchas en las hojas de la mayoría de brasicáceas.
Alternaria brassicicola[1][2] (spikkelziekte) is een ziekteverwekkende schimmel uit de familie Pleosporaceae van de ascomyceten. De schimmel lijkt veel op Alternaria brassicae, maar veroorzaakt meer schade. Alternaria brassicicola gaat met zaad over. Waardplanten zijn kruisbloemigen, zoals wittekool en bloemkool. Op de bladeren worden lichtbruine en donkerbruine tot bijna zwarte, ronde, met concentrische ringen, 1 - 10 mm grote vlekken gevormd. Rond de vlekken vaak een brede gele zone.
Alternaria brassicicola heeft vertakte, losstaande schimmeldraden. De breedte van de schimmeldraden varieert gemiddeld tussen de 1,5 en de 7,5 micrometer. In het begin zijn deze schimmeldraden doorzichtig maar later krijgen ze een bruine of olijfbruine kleur. De conidioforen komen groepsgewijs of alleen staand voor. Wanneer de conidioforen groepsgewijs voorkomen variëren de aantallen tussen de twee en de twaalf. Deze conidioforen zijn gemiddeld 70 micrometer lang en ze zijn tussen de vijf en de acht micrometer breed.[3] De bleke tot donker olijfbruine, 18 - 130 µm lange en 8 - 30 µm brede conidiën hebben 1 - 6 dwarse en tot 6 in de lengte lopende tussenwanden.[4]
Alternaria brassicicola (spikkelziekte) is een ziekteverwekkende schimmel uit de familie Pleosporaceae van de ascomyceten. De schimmel lijkt veel op Alternaria brassicae, maar veroorzaakt meer schade. Alternaria brassicicola gaat met zaad over. Waardplanten zijn kruisbloemigen, zoals wittekool en bloemkool. Op de bladeren worden lichtbruine en donkerbruine tot bijna zwarte, ronde, met concentrische ringen, 1 - 10 mm grote vlekken gevormd. Rond de vlekken vaak een brede gele zone.
Alternaria brassicicola (Schwein.) Wiltshire – gatunek grzybów z klasy Dothideomycetes[1]. Grzyb mikroskopijny[2].
Pozycja w klasyfikacji według Index Fungorum: Alternaria, Pleosporaceae, Pleosporales, Pleosporomycetidae, Dothideomycetes, Pezizomycotina, Ascomycota, Fungi[1].
Po raz pierwszy takson ten zdiagnozował w 1832 roku L. Schweinitz nadając mu nazwę Helminthosporium brassicicola. Obecną, uznaną przez Index Fungorum nazwę nadał mu w 1947 roku S.P. Wiltshire[1].
Znana jest tylko anamorfa tego gatunku. Na podstawie budowy molekularnej i ultrastrukturalnej ustalono, że teleomorfą jest któryś z gatunków grzybów zaliczany do rodzaju Lewia[3].
Grzybnię łatwo można hodować na sztucznych pożywkach. Tworzy na nich rozproszoną, ciemnooliwkową kolonię złożoną z septowanych, początkowo hialinowych, potem brunatnobrązowych lub oliwkowych gładkich strzępek o szerokości 1,5-7,5 μm. Konidiofory powstają pojedynczo, lub w grupach po 2-12, a czasami nawet więcej. Wznoszą się powyżej grzybni, są zazwyczaj proste, czasami zagięte, cylindryczne, ale często lekko nabrzmiałe przy podstawie, septowane, barwy od blado do średnio oliwkowej. Mają długość do 70 μm, szerokość 5-8 μm. Konidia powstają zwykle w łańcuszkach po 20 lub więcej sztuk. Łańcuszki te czasami rozgałęziają się. Konidia są proste, cylindryczne lub owalne, zazwyczaj nieco zwężające się w kierunku wierzchołka. Maja zaokrągloną podstawę i posiadają krótki dziobek. Posiadają 1-6, wyjątkowo do 11 przegród poprzecznych, na których są przewężone. Barwa od blado do ciemnooliwkowej i brunatnej, powierzchnia gładka, długość 18-130 μm, szerokość 8-30 μm. Długość dzioba nie przekracza 1/6 długości całego konidium[2].
Konidiogeneza zachodzi w temperaturze 8 do 30° C. Dojrzałe zarodniki powstają po 14 do 43 godzinach. W optymalnej temperaturze 16-24° C czas ten wynosi 13 godzin. Warunkiem jest stałe zwilżenie wodą przez okres co najmniej 9-18 godzin, lub względna wilgotność powietrza wynosząca co najmniej 91,5%. Konidiogeneza zachodzi również na martwych szczątkach roślin. W jednym z badań, zakażone liście rzepaku i kapusty umieszczone na zewnątrz na glebie, wytwarzały żywe zarodniki tak długo, dopóki tkanki liści nie uległy zniszczeniu. W przypadku rzepaku było to do 8 tygodni, a dla kapusty do 12 tygodni[5]
Zarodniki A. brassicicola wnikają do tkanek żywiciela przez rany spowodowane pracami polowymi czy żerowaniem szkodników, lub przez aparaty szparkowe, ale potrafią spenetrować także zdrową epidermę. Odróżniają się tym od drugiego gatunku wywołującego czerń krzyżowych – Alternaria brassicae)[6]. Kiełkują w temperaturze 7° C do 30° C. W temperaturze 15° C po 10 godzinach inkubacji wykiełkowało 98% zarodników. Przy temperaturze 35° C i wyższej zarodniki nie kiełkują[5]. Do kiełkowania poza tym niezbędna jest wilgoć. Czas zwilżenia wynosi co najmniej 5 godzin. Do infekcji dochodzi także przy wilgotności względnej powietrza wynoszącej 95-100% i utrzymującej się stale przez 12-20 godzin[6]. Optymalna temperatura dla rozwoju choroby wynosi 30 °C[5].
Grzybnia i zarodniki A. brassicola mogą na powierzchni porażonych nasion przetrwać do 2 lat, wewnątrz nasion nawet 12 lat, gdy nasiona są przechowywane w temperaturze 10° C przy wilgotności względnej 50%. Na porażonych martwych częściach roślin mogą przetrwać do 12 tygodni[5]. W takich warunkach wytwarzają przetrwalnikowe chlamydospory, które są odporne na przemarzanie i odwodnienie. Najdłużej zachowują żywotność w niskich temperaturach (ok. 3° C)[5].
Gatunek kosmopolityczny, szeroko rozprzestrzeniony na całym świecie. Saprotrof i pasożyt roślin z rodziny kapustowatych (krzyżowych)[2]. Jest jednym z patogenów wywołującym u nich chorobę o nazwie czerń krzyżowych. Występuje częściej i jest bardziej patologiczny niż drugi patogen tej choroby – Alternaria brassicae[3].
Alternaria brassicicola (Schwein.) Wiltshire – gatunek grzybów z klasy Dothideomycetes. Grzyb mikroskopijny.
Alternaria brassicicola je grzib[7], co go nojprzōd ôpisoł Ludwig David von Schweinitz, a terŏźnõ nazwã doł mu Wiltshire 1947. Alternaria brassicicola nŏleży do zorty Alternaria i familije Pleosporaceae.[8][9] Żŏdne podgatōnki niy sōm wymianowane we Catalogue of Life.[8]
Alternaria brassicicola je grzib, co go nojprzōd ôpisoł Ludwig David von Schweinitz, a terŏźnõ nazwã doł mu Wiltshire 1947. Alternaria brassicicola nŏleży do zorty Alternaria i familije Pleosporaceae. Żŏdne podgatōnki niy sōm wymianowane we Catalogue of Life.
