Brassicaceae cover crops reduce Aphanomyces pea ro

Authors: Shakhawat Hossain Göran Bergkvist Robert Glinwood Kerstin Berglund Anna Mårtensson Sara Hallin Paula Persson

Publish Date: 2015/04/02

Volume: 392, Issue: 1-2, Pages: 227-238

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Abstract

Brassicaceae cover crops can be used to suppress soilborne pathogens The aim was to investigate the effect of different brassicas with different glucosinolate profiles on the development of Aphanomyces pea root rot in subsequent pea plants and the genetic potential of freeliving N2fixing bacteria and ammonia oxidising bacteria AOB and archaea AOA performing key soil ecosystem servicesThe Brassicaceae species Brassica juncea and Sinapis alba and nonBrassicaceae species Secale cereale were grown for 11weeks in Aphanomyces euteiches infested soil at low and high nitrogen N fertiliser doses After removing both shoots and roots of the cover crops peas were grown as a bioassay to evaluate Aphanomyces pea root rot development Soil was sampled before harvesting the cover crops and at the end of the bioassay Volatile compounds were collected in the rootsoil environment before harvesting the Brassicaceae cover crops to determine the concentration of isothiocyanates The abundance of genes involved in N2fixing bacteria and ammonia oxidation in AOA and AOB were assessedPea root rot disease severity was reduced in Brassicaceae grown soil at the high N fertiliser dose This was associated with increased growth of the cover crops The growth of Brassicaceae did not suppress the abundance of Ncycling microbial communities but rather increased the AOB at the end of the bioassay most likely due to increased N availability The disease suppressive effect was higher with S alba than with B juncea and this coincided with a more diverse composition and higher concentration of aliphatic ITCs released from S alba roots Fewer nodules were formed after the Brassicaceae crops especially Sinapis albaBrassicaceae cover crops particularly S alba can be used to control soilborne pathogens without major side effects on the genetic potential of beneficial soil microorganisms involved in N cycling However less nodule formation after brassicas indicates an effect on rhizobium activityThe persistent and globally distributed soilborne pathogen Aphanomyces euteiches is an oomycete causing root rot in legumes which is a severe problem in commercial pea Pisum sativum L production Papavizas and Ayres 1974 Gaulin et al 2007 Persson 2008 Pea root rot is difficult to control without long intervals 6–8 years between pea crops in the crop rotation Biofumigation using plants from the Brassicaceae family is however of interest since many Brassicaceae species produce sulphurcontaining secondary metabolites known as glucosinolates GSLs Sang et al 1984 Fahey et al 2001 Hossain et al 2012 When the GSLs are hydrolysed by the endogenous enzyme myrosinase volatile isothiocyanates ITCs thiocyanates and watersoluble nitriles and epithionitriles are formed Brown and Morra 1997 Kiddle et al 2001 These products are toxic to soilborne pathogens Kirkegaard et al 2000 Potter et al 2000 van Dam et al 2009 but the suppressive effect depends on their chemical composition Smolinska et al 2003 Matthiessen and Shackleton 2005 and concentration Angus et al 1994 Sarwar et al 1998 Hossain et al 2014 For the pea root rot pathogen A euteiches invitro experiments have demonstrated reduced hyphal growth caused by volatiles from Brassica napus rapeseed meal cv Dwarf Essex Dandurand et al 2000 and B juncea cv Corron shoot tissue Hossain et al 2014 Several field studies have shown that incorporation of B napus or Sinapis alba white mustard plant tissue can reduce the incidence or development of root rot in subsequent pea crops Chan and Close 1987 Muehlchen et al 1990Most examples of biofumigation using brassicas are based on macerating the plants and incorporating them into the soil to achieve a sudden boost of toxic volatiles Angus et al 1994 Kirkegaard et al 2000 Less is known about the effects of living and growing roots of brassicas on A euteiches A few studies have shown that intact Brassicaceae roots have an impact on fungal and bacterial communities Rumberger and Marshner 2004 Bressan et al 2009 and a negative effect on fungal spore germination Schreiner and Koide 1993 Moreover the amount of GSLs is usually higher in root than shoot tissue Rosa 1997 van Dam et al 2009 Borek et al 1996 found a higher concentration of the myrosinase enzyme in rhizospheric soil of brassicas than in nonrhizospheric soil and demonstrated that this enzyme actively hydrolysed GSLsSince the GSL hydrolysis products can suppress plant pathogens there is concern over their toxic effect on beneficial organisms including the rhizobium bacteria that are essential for pea root nodule formation for the fixation of atmospheric dinitrogen gas N2 Kirkegaard et al 1999 Peas had fewer root nodules when Brassicaceae shoot tissues had been incorporated before sowing Muehlchen et al 1990 suggesting that GSL hydrolysis products inhibited bacteria essential for nodule formation and the rhizobial community associated with living Brassicaceae roots was significantly influenced by changes in GSL root profiles Bressan et al 2009 Other soil microorganisms related to the cycling of N and their functions may be affected by GSL hydrolysis products Bending and Lincoln 2000 found that synthetic ITCs inhibited nitrification activity and decreased growth of nitrifying bacteria especially the ammonia oxidisers that perform the first step in the nitrification process These organisms are known to be sensitive to a range of environmental disturbances and have therefore been used as indicator organisms for different types of soil perturbation Wessén and Hallin 2011

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