Alternaria black spot of cruciferous vegetables, encouraged by different species of Alternaria, remains an increasing threat to Brassicaceae crops throughout the world, including Poland. Brassica plants are attacked by conidia of A. brassicae (Berk.) Sacc., A. brassicicola (Schw.) Wiltsh. The pathogens have a wide spectrum of hosts, such as head cabbage, Chinese cabbage, cauliflower, broccoli, and other crucifers including cultivated and wild grown plants. Alternaria pathogens usually cause damping-off of seedlings, spotting of leaves of cabbages, blackleg of heads of cabbages, and spotting of cauliflower curds and broccoli florets. In oilseed rape, A. brassicae is the dominant invasive species, while in the cruciferous vegetables, both species, A. brassicae, and A. brassicicola are encountered. Infected seeds with spores on the seed coat or mycelium under the seed coat are the main means of distribution for these pathogens. The fungus can overwinter on susceptible weeds or crop debris and on seed plants, as well as on stecklings.
Methods for disease prevention and control are based on combining agricultural management practices with chemical control. Using disease-free seeds or seeds treated with fungicides can greatly reduce disease incidence. After appearance of the first symptoms of disease, stringent fungicide spray program is an effective way to reduce losses. Many authors seem to agree, that the most economically feasible method of disease control is the development of resistant Brassicaceae crops varieties, as transgenic approach proved unsuccessful. Due to our increasing understanding of pathogen-host plant interactions, identification of resistance sources, and assessment of the resistance trait inheritance mode, breeding programs of Brassica crops for Alternaria resistance can be enhanced. This is of particular importance since recent years experience dynamic development of ecological and integrated plant production with an emphasis on plant biotic stress resistance. Highly resistant genetic resources have not been reported in Brassica cultivated species, although some varieties differ in their resistance/susceptibility level.
In the world the second largest oil seed crop is brassica after soybean (FAO, 2010; Raymer, 2002). Out of 37 species of brassica genus only 4 are highly cultivated species for oil and vegetables are for Brassica rapa L., B. juncea (L.) Czern. & Cosson, B. napus L., and B. carinata A. Braun. (Raymer, 2002; Rakow, 2004; Sovero, 1993). Two species derived from Oleiferous brassica that is B. napus L. and B. campestris L. (syn. B. rapa L.) B. campestris is also mentioned to by such local names in specific countries as toria, sarson, summer turnip rape, and Polish rape. All rapeseed-contributing cultivated Brassica spp. are highly polymorphic including oilseed crops, root crops, and vegetables such as Chinese cabbage, broccoli, and Brussels sprouts. B. compestris are also grown under European and Canadian conditions, both in summer and winter.
Production technology:
Rapeseed is cultivated over an area of 28.23 million hectares with production of about 58.21 Mt, by this production rapeseed ranks on third number in oil seed production after palm oil and soybean (FAO, 2010). Canada is the largest producer of and contributes about 22% of the total world production of oil (FAO, 2010). In Pakistan after cotton, rapeseed-mustard is the second most important source of oil in Pakistan. It is cultivated over an area of 307,000 hectares with annual production of 233,000 tones and contribute about 17% to the domestic production of edible oil (PASC). As we know that everything has some aspects in this case the aspect is the attack of pathogen on brassica plant.
Major diseases of brassica:
Fungus and fungus-like diseases
• Blackleg (Leptosphaeria maculans)
• Clubroot (Plasmodiophora brassicae)
• Damping off (Fusarium or Pythium species)
• Downy mildew (Hyaloperonospora parasitica)
• Leaf spot/target spot (Alternaria species)
• Powdery mildew (Erysiphe cruciferarum)
• Ring spot (Mycosphaerella brassicicola)
• White blister (Albugo candida)
• Alternaria brassicae (Berk.) Sacc.,
• Alternaria brassicicola (Schw.) Wiltsh.,
• Alternaria raphani Groves & Skolko.
Pathogen biology:
The primary source of infection of dark leaf spot in UK is A. brassicae and A. brassicicola. The four species or alternaria can cause major negative influence on the Cruciferous plants (Brassicaceae) these species are A. brassicae (Berk.) Sacc., A. brassicicola (Schw.) Wiltsh., A. raphani Groves and Skolko, and A. alternata (Fr.) Kreissler. The most common alternaria species are A. brassicicola and A. brassicae that can cause disease on oleiferous and vegetable (oleraceous) Brassicas, the A. brassicicola is the dominant invasive species of the vegetable Brassicas, while the oleiferous crucifers are primary hosts for the A. brassicae fungus (Maude & Humpherson-Jones 1980, Humpherson-Jones 1989). These both species cause infection on leaves. The most common disease in the crucifer’s plant is dark spot that is located in tropical and sub-tropical regions.
• Kingdom Fungi
• Phylum Ascomycota
• Subdivision Pezizomycotina;
• Class Dothideomycetes;
• Order Pleosporales;
• Family Pleosporaceae;
• Subfamily mi-tosporic Pleosporaceae;
• Genus Al-ternaria
Both species are cosmopolitan in nature but occur sporadically. A. brassicicola is the predominant species on cabbage grown in New York (King, S. R. 1994). A. raphani is wide spread in Northern hemisphere.
Crops Affected:
Cruciferous vegetables, including cabbage, broccoli, cauliflower, Brussels sprouts, Chinese cabbage, kale, kohlrabi, rutabaga, turnip and cruciferous weeds.
Severe economic losses cause by A. brassicicola and A. brassicae (HumphersonJones ; Maude 1982, HumphersonJones 1989). In Europe, only the seed losses due to both pathogens were expected to be 86% in B. oleracea in several years (Maude ; Hampherson-Jones 1980, Humpherson-Jones 1989).
Epidemiology factors:
Infected seeds are the primary source of infection or non-decomposed plant debris in the upper soil layers with over-wintering hyphae or spores. Second important source of the pathogen are the weeds of brassica, which increase infestations as pathogen host plants (Humpherson-Jones 1989). During the vegetation period, the rain and wind-transported fungal conidial spores are also an important source of infection. The conidial majority are released during harvest and in the cleaning of infected leaves; such released spores are then spread approximately within 1,800 m (Humpherson-Jones ; Maude 1982). Under Sophisticated climate, highest Alternaria spore presence are detected in the air mainly in June and July (Nowakowska et al. 2011).
Infection mechanism:
Three ways of Alternaria infection have been reported: Through epidermis penetration, through stomata, and through insects- or agro-technique-derived host plants wounding. Regardless of their means of entry, A. brassicicola and A. brassicae exhibit distinct differences in the host plant tissue penetration. A. brassicae invades host plants solely through their stomata, while for A. brassicicola, direct plant tissue penetration prevails over stomatal infections. Hyphae of both pathogens develop well on the epidermis, directly beneath the leaf waxes, and exhibit low cell penetration ratio.
During plant infection, both Alternaria fungi produce and exude phytotoxins that belong to HST clade (host-specific toxins or host-selective toxins; Parada et al. 2008, Wight et al. 2009). These compounds play a major role in the pathogenesis by determining the host plant spectrum, as well as the isolates’ virulence and pathogenicity levels (Nishimura & Kohmoto 1983). To date, two HSTs have been In addition to the above, A. brassicae produces a number of phytotoxins (destruxin B and derivatives, such as homodestruxin B, desmethyldestruxin B, and destruxin B2) responsible for typical black spot symptoms, such as necrotic and chlocharacterized in detail: AB toxin of A. brassicicola (Otani et al. 1998) and ABR of A. brassicae (Parada et al. 2008). Both phytotoxins are proteins with a suggested role in evoking disease symptoms on the infected cruciferous plants.
In addition to the above, A. brassicae produces a number of phytotoxins (destruxin B and derivatives, such as homodestruxin B, desmethyldestruxin B, and destruxin B2) responsible for typical black spot symptoms, such as necrotic and chlorotic lesions. Destruxin B was shown to be a major one responsible for inducing necrotic lesions on plant leaves and eliciting the phytoalexins brassilexin and sinalbin A (Sodelade et al. 2012). Majority of researchers classify destruxin B as an HST (Sodelade et al. 2012), while others have demonstrated its unspecific character and questioned its role in the initial host plant colonization (Buchwaldt & Green 1992, Parada et al. 2008). While some isolates of A. brassicae produce destruxin B as their sole toxin, others are capable of production of their derivatives (homodestruxin B, desmethyldestruxin B, and destruxin B2; Parada et al. 2008). Depudecin, an eleven-carbon linear polyketide and histone deacetylase (HDAC) inhibitor made by A. brassicicola, proved a minor virulence factor.
• Life cycle:Since both pathogens survive on crop debris, seeds, and in association with weed hosts (Humpherson-Jones & Maude 1982, Humpherson-Jones 1989), crop debris management Alternaria fungi proliferation is vegetative in character, and takes place by means of conidial spores, airborne and found in the soil and water, as well as indoors and on objects. Sexual recombination (teleomorphy) occurs very rarely.
• Spore size: dark-brown spots of different sizes (0.5 cm to several cm in diameter) appear on the leaves.
• Factors on which growth depends: Development of the pathogen’s infection structures and of the disease symptoms on the oleraceous Brassicas depends primarily on the incubation temperature (Bassey ; Gabrielson 1983), and relative air humidity. pores of both Alternaria species germinate in a rather broad temperature range, germination effectiveness is correlated with the temperature (Degenhardt et al. 1982).
Optimal hyphae growth temperature for A. brassicae is 18-24?, and for A. brassicicola 20- 30?. According to the in vitro studies, sporulation of A. brassicae is temperature-dependent: At 8-24ºC, fullydeveloped spores are detectable at 24 or 14 h, respectively. Temperature spectrum of A. brassicicola sporulation is broader (8-30ºC), with fully developed spores detectable after 43 or 14 h, respectively. High air humidity (95-100% RH) lasting at least 9-18 h is a crucial requirement displayed by both pathogen species during plant infection (Humperson-Jones ; Phelps 1989). Reports of massive infestations under air temperatures of 20-27ºC and constant plant moisture of at least 5 h, or RH exceeding 95% lasting at least 12-20 h are commonplace (reviewed in Bart ; Thomma 2003). Sporulation can occur over a wide range of temperatures and is optimal at 20 to 30°C. Spore dispersal occurs during the warmest and driest part of the day, and night dispersal is rare. In England, the highest spore catches of A. brassicicola occurred after a period of rain or prolonged leaf wetness (lasting more than 3 h) with a mean temperature above 13°C.
Alternaria black spot symptoms appear on all host plant parts and at every developmental stage. A. brassicicola and A. brassicae cause the damping off of the crucifer’s seedlings. Elongated brownings develop on the sub-cotyledonous part of the stem and on the cotyledons, often leading to narrowing and breaking of the stems, and thus, to seedlings’ decease. Most often infected are the lower, older leaves of head cabbage. Chinese cabbage, cauliflower, broccoli, and of other crucifers. Infected cauliflower curds or broccoli florets develop slight dents with brownish spots covered with black bloom of spores. In these plants, infection usually remains on the surface and does not reach deep in the curd or the floret; however, symptomatic cauliflower or broccoli florets lose their commercial value. In case of radish, turnip, or rutabaga, the disease affects the root thickenings as well; the disease symptoms manifest themselves as brown rots only during their storage. Under favorable conditions, lesions become covered with brown-black downy-like bloom of sporulating hyphae. A. brasicicola-derived spots are darker and less regular in shape compared with those of A. brassicae origin.
Both most common Alternaria pathogens usually cause black spot disease, manifested by damping-off of seedlings, spotting of leaves of cabbages, blackleg of heads of cabbages (head cabbage and Chinese), and spotting/browning of cauliflower curds and broccoli florets.
Control and management:
Alternaria prevention and control methods include combining the proper agro-technique with chemical protection. An essential disease prevention method is production of healthy seeds, obtained from plantations with heavy fungicide protection. In the 2-year lasting crucifers seed production periods, good effects of protection against Alternaria infections during the 1st year of growth were expedited by fungicides containing iprodione as an active ingredient (Maude et al. 1984, Survilien? et al. 2010). In Poland, the only product containing it is Seed Protector T 75DS WS (Zaprawa Nasienna). Since both pathogens survive on crop debris, seeds, and in association with weed hosts (Humpherson-Jones ; Maude 1982, Humpherson-Jones 1989), crop debris management (for example through crop rotation and deep tillage) and use of clean seed and proper weed control should alleviate the disease. After appearance of disease symptoms, one may achieve limitation of the infection by repeated spray with fungicides containing strobilurines as active ingredients (Amistar 250 SC, Signum 33 WG, Zato 50 WG) and fungicides based on iprodione (Rovral FLO 255 SC) (Maude et al. 1984, Survilien? et al. 2010). This method, however, carries an economic disadvantage and may prove ineffective under pathogen infection-conducive weather conditions, particularly among the seed crops. An alternative protection method to be employed is use of antagonistic fungi; deployment of Aureobasidium pullulans and Epicoccum nigrum on the crucifers leaves reduced the infection level under controlled conditions (Pace ; Campbell 1974). Field studies concerning the biological control efficacy are yet to be carried out.