Abstract
Aims Brassicaceae cover crops can be used to suppress soil-borne 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 free-living N2-fixing bacteria and ammonia oxidising bacteria (AOB) and archaea (AOA) performing key soil ecosystem services. Methods The Brassicaceae species Brassica juncea and Sinapis alba and non-Brassicaceae species Secale cereale were grown for 11-weeks 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 root-soil environment before harvesting the Brassicaceae cover crops to determine the concentration of isothiocyanates. The abundance of genes involved in N2-fixing bacteria and ammonia oxidation in AOA and AOB were assessed. Results Pea 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 N-cycling 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 alba. Conclusions Brassicaceae cover crops, particularly S. alba, can be used to control soil-borne 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 activity.
Highlights
The persistent and globally distributed soil-borne 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)
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
To evaluate potential negative side-effects on the soil microbial community, we investigated effects on the genetic potential of freeliving N2-fixing bacteria and ammonia oxidising bacteria (AOB) and archaea (AOA) by quantifying genes of key enzymes involved in N2 fixation and ammonia oxidation
Summary
The persistent and globally distributed soil-borne 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). When the GSLs are hydrolysed by the endogenous enzyme myrosinase, volatile isothiocyanates (ITCs), thiocyanates and water-soluble 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). 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 1990)
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