Sclerotinia Stem Rot Disease and its Management in Brassica Sp.
Authors: N. C. Gupta, Amos Samkumar, M. Rao
Sclerotinia sclerotiorum (Lib.) de Bary is a ubiquitous phytopathogenic fungi which has the broadest host range of any known plant pathogen. It nearly affects more than 400 species of 275 genera which includes cruciferous vegetables like cabbage and cauliflower, tomato, potato, sunflower, soybean, lettuce and also predominantly affects major oil seed crops such as canola and Indian mustard (Brassica rapa and Brassica juncea). India is one of the leading producers of oil seed Brassicas accounting for 11.12 % of the world's rapeseed-mustard production, and ranks third in the world next to China and Canada and this pathogen is a menace during late flowering and maturity of plants resulting up to 80% loss of yield causing a major drop in oil seed economy
Mode of infection
The disease infestation of this devastating necrotrophic pathogen can be called as watery soft rot, white mold or Sclerotinia Stem rot depending on the host it infects. In winters it affects within the plants directly from mycelia and it forms hyphal aggregates called sclerotia in stem and soil (long term survival structures) during spring, once the conditions are favourable the sclerotia germinate to form Apothecia to produce Ascus and releases Ascospores in the end of spring or early summer. The mode of infection of this fungus invades through the stomata and sub-stomatal chamber of plants by invasion of either Ascospores or mycelium which progresses rapidly through the leaf tissues. Pathogen infestation can be also found from the early falling of petals which measures the infection pressure significant with the weather conditions.
It is already well known that Oxalic acid is known to be directly involved during infection and host colonization and acts as a major virulence factor secreted from the pathogen. Several associated candidate activities and their genes have also been characterized from the Sclerotinia sclerotiorum genome. (1980 isolate, Broad institute) These include NADPH oxidase (nox1, nox2), superoxide dismutase (sod1), catalase (cat1), glutathione metabolism genes (gama glutamyl transpeptidase (ggt1, and others), pH signal regulators (pac1), and ROS resistance regulators (yap1)
The invasion and pathogenesis is mediated by a couple of cellulolytic and pectinolytic enzymes which has a role in penetration, maceration , nutrient acquisition , plant defense induction and symptom expressions, all enzymes are optimally active under acidic conditions provided by oxalic acid. Some of the major enzymes involved are pectin methyl esterase, acid proteases and aspartyl proteinase all plays a role in degradation of cell wall proteins and inactivation or inhibition of plant defence response proteins.
Management practices and Crop improvement in Brassica sp:
Some traditional management practices include ploughing the crop debris and top soil deeply to remove the sclerotia in soil followed by crop rotation with non-host plant species every season but severe infestations can be controlled by applying the fungicide Carbendazim 250g/l and foliar spray application in field. There are no complete resistant Brasica spp lines developed through either traditional breeding or molecular breeding till date but some cultivars at different geographical location have shown considerable amount of tolerance against the pathogen in field conditions. The tolerant cultivars was analysed by measuring the lesion size developed after artificial infection correlating with the stem diameter. Some hybrids were also developed by crossing susceptible and tolerant cultivars but none of the lines developed against Sclerotinia isolates showed complete resistance against the pathogen.
Genetically modified approaches employs Agrobacterium tumefacience-mediated transformation, protoplast culture, somatic hybridization and microplast techniques should be exploited for developing transgenic plants of crops with superior resistance to Sclerotinia. Several strategies including detoxification defence, activation and general inhibition have potential to engineer Sclerotinia resistance. It is highly necessary to understand disease epidemic in variable environmental conditions. In recent years scientists have developed transgenic Brassica sp harbouring various genes like chitinases, glucanases, polygalacturonases inhibiting protein (PGIP) to produce resistance trait against the pathogen and other genetically modified approaches like Overexpression of WRKY transcription regulation factors, genes from MAPK Signalling cascade and genes involved in oxalic acid production like oxalate oxidase under a strong constitutive promoter were developed to confer resistance against Sclerotinia sclerotiorum. More emphasis will be given in identifying new novel resistant genes and elucidating molecular mechanisms to develop complete resistance against this devastating pathogen of major oilseed crops in near future.
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