When the same crop is grown time and time again in a rotation, yield often suffers. Understanding such ‘yield declines’ has been a source of much interest across the globe, because halting them would boost output from some of the world’s most important crops. 

Physical and chemical factors certainly play a role in the decline but biological factors are believed to be particularly significant, especially soil-borne pathogens. Technological advances mean researchers can now screen thousands of root samples relatively easily in the quest to hunt down potential pathogens. 

Oilseed rape (OSR) is a crop that suffers from yield decline in close rotations, yet the underlying causes are poorly understood. Now results from an AHDB-funded PhD studentship, conducted at Harper Adams University (HAU), have shed light on some interesting soil-borne pathogens associated with this important break crop. 

Alex McCormack (pictured) was awarded the studentship in 2013 and he spent the following three years getting up close to the life beneath OSR fields. HAU had a large set of suitable OSR root samples already, so he did not need to start from scratch in his search for candidate pathogens. Alex used Next Generation Sequencing (NGS), a novel molecular technique, to examine the fungal species present in the samples and found that fungal communities differed surprisingly little, with generalist fungal species, that live on dead or decaying organic matter, dominating. Of the pathogenic species present, Rhizoctonia solani was the most abundant, a finding consistent with studies on OSR and vegetable brassica crops across the world. 

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R. solani, which is primarily a damping-off disease, can result in the death of OSR seedlings before or soon after they emerge. Furthermore, as a complex of different ‘strains’, more commonly known as anastomosis groups (AGs), it was important to understand which were causing the underlying problem. 

Alex then used real-time PCR, another molecular approach, to examine the root samples for the presence and quantity of several of the most commonly cited AGs. He found AG2-1 was the most common (present at 60 per cent of sites tested). Controlled environment experiments also showed AG 2-1 isolates caused visible disease symptoms on young OSR seedlings, while other AGs tested caused little to no appreciable symptoms. Glasshouse experiments also showed that very little AG2-1 inoculum was needed to result in appreciable disease and seedling death. 

As with many pathogens, R. solani is highly adaptive and has long-lived survival structures, which makes it fairly robust. Indeed, results from Alex’s molecular tests found that cultural methods – such as rotation length and cultivation approach – had limited impact on the presence of this pathogen.

Although Alex’s work did not crack the yield-decline conundrum, it did highlight the need to pay more attention to Rhizoctonia and the need to identify novel cultural and chemical control options. 

Partly in response to his findings, AHDB, in conjunction with the UK government’s Agri-Tech Catalyst fund, invested in a three-year programme of work. This latest phase of research will look to identify genetic traits associated with resistance to R. solani in OSR, increase knowledge of disease epidemiology and yield loss, and investigate the potential of low-dose seed treatments targeted at soil-borne disease.

Information about the work can be accessed via cereals.ahdb.org.uk/rhizoctonia