Soil borne disease research projects

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By Heather Chalmers 

The continued presence of soil and seed-borne diseases in potato crops can be partly mitigated by sowing crops in paddocks with good soil structure, Plant and Food Research work shows.   

Previous potato yield gap work from 2012-15 identified that soil- borne disease and soil physical quality were two major yield limiting factors.  

In 2015, a Sustainable Farming Fund project in Canterbury was set up to help quantify these results. According to Plant and Food research associate at Lincoln, Steve Dellow, year two of the project focused on cropping history and what impact that had on potato yields. For this research, 15 commercial fields were selected that were to be planted in two cultivars, Russet Burbank or Innovator.  

Fields were chosen based on four categories. A diseased field was defined as one which had grown potatoes once within the last 10 years. Clean paddocks were those that had not grown potatoes for more than 10 years.  

These were further broken down into paddocks with good or poor soil structure. Paddocks with a good soil structure had a cropping history that was predominantly restorative, notably long-term pasture. Paddocks with poor soil structure had been continually cropped.  

In terms of soil-borne disease, the trial showed that cropping history influenced the prevalence of Rhizoctonia stem canker. 

“If you had grown potatoes in the last 10 years you had an increased chance of having Rhizoctonia stem canker. Coming out of long-term pasture also led to an increased incidence of stem canker.” 

“For Spongospora (the pathogen causing powdery scab and root galls) the trend was the same.”  

Despite increased disease incidence, no yield reduction was noted, said Steve Dellow.  

“Marketable yield was greater, at an average 10 tonne higher, from paddocks that had come out of long-term pasture compared with land that had been annually cropped.”   

The second part of the research project looked at soil physical qualities to explain this yield difference.   

Each paddock was given a 10-year crop history score, based on how restorative it was and how much organic matter was returned to improve the soil structure. An in-field visual assessment was also made of soil structural quality and how favourable this was for root penetration.    

“With those two methods we got a relatively good correlation with yield. 

“As you move from a non-restorative, annually cropped paddock history to a more permanent pasture and soil structure improved, you got an increase in yield.  

“This was more so for Russet Burbank, which was more sensitive to poorly structured soils. 

“Based on the analysis, we can say that about half of the yield variability can be accounted for by soil structure.  

“Even though we had better soil structure from having more permanent pasture in the crop rotation we still have more disease. However, as the plants have more access to water, air and nutrients they can defend themselves more against the presence of that disease.  

“These soil quality measures could help farmers to make more informed decisions about which paddocks could potentially yield better for potato production,” Steve Dellow said.  

Potato seed line health

In related research from 2016-18, Alex Michel and his team monitored the health of seed potato lines.  

In year one, six commercial Agria and five Innovator lines were monitored and compared for in-season health, compared with following daughter main crops.  

The seed was also grown on as whole seed in disease-free potting mix in a controlled, disease-inducing environment to estimate the potential presence of seed-borne pathogens.   

Incidence and severity of Rhizoctonia stem canker and Spongospora root galling was low in the seed crops and low to moderate in the glasshouse plants. This trend was noted in subsequent replicated trials and in several commercial crops nationwide, using seed collected from the monitored lines. However, one daughter commercial crop became highly diseased, as it was stressed by drought and flood conditions (Manawatu).   

These results show that if seed-borne inoculum can be minimised through the supply chain, the impact on well-managed crops can be minimal, Alex Michel said.   

The continued presence of soil and seed-borne diseases in potato crops can be partly mitigated by sowing crops in paddocks with good soil structure, Plant and Food Research work shows.   

Previous potato yield gap work from 2012-15 identified that soil- borne disease and soil physical quality were two major yield limiting factors.  

In 2015, a Sustainable Farming Fund project in Canterbury was set up to help quantify these results. According to Plant and Food research associate at Lincoln, Steve Dellow, year two of the project focused on cropping history and what impact that had on potato yields. For this research, 15 commercial fields were selected that were to be planted in two cultivars, Russet Burbank or Innovator.  

Fields were chosen based on four categories. A diseased field was defined as one which had grown potatoes once within the last 10 years. Clean paddocks were those that had not grown potatoes for more than 10 years.  

These were further broken down into paddocks with good or poor soil structure. Paddocks with a good soil structure had a cropping history that was predominantly restorative, notably long-term pasture. Paddocks with poor soil structure had been continually cropped.  

In terms of soil-borne disease, the trial showed that cropping history influenced the prevalence of Rhizoctonia stem canker. 

“If you had grown potatoes in the last 10 years you had an increased chance of having Rhizoctonia stem canker. Coming out of long-term pasture also led to an increased incidence of stem canker.” 

“For Spongospora (the pathogen causing powdery scab and root galls) the trend was the same.”  

Despite increased disease incidence, no yield reduction was noted, said Steve Dellow.  

“Marketable yield was greater, at an average 10 tonne higher, from paddocks that had come out of long-term pasture compared with land that had been annually cropped.”   

The second part of the research project looked at soil physical qualities to explain this yield difference.   

Each paddock was given a 10-year crop history score, based on how restorative it was and how much organic matter was returned to improve the soil structure. An in-field visual assessment was also made of soil structural quality and how favourable this was for root penetration.    

“With those two methods we got a relatively good correlation with yield. 

“As you move from a non-restorative, annually cropped paddock history to a more permanent pasture and soil structure improved, you got an increase in yield.  

“This was more so for Russet Burbank, which was more sensitive to poorly structured soils. 

“Based on the analysis, we can say that about half of the yield variability can be accounted for by soil structure.  

“Even though we had better soil structure from having more permanent pasture in the crop rotation we still have more disease. However, as the plants have more access to water, air and nutrients they can defend themselves more against the presence of that disease.  

“These soil quality measures could help farmers to make more informed decisions about which paddocks could potentially yield better for potato production,” Steve Dellow said.  

Potato seed line health

In related research from 2016-18, Alex Michel and his team monitored the health of seed potato lines.  

In year one, six commercial Agria and five Innovator lines were monitored and compared for in-season health, compared with following daughter main crops.  

The seed was also grown on as whole seed in disease-free potting mix in a controlled, disease-inducing environment to estimate the potential presence of seed-borne pathogens.   

Incidence and severity of Rhizoctonia stem canker and Spongospora root galling was low in the seed crops and low to moderate in the glasshouse plants. This trend was noted in subsequent replicated trials and in several commercial crops nationwide, using seed collected from the monitored lines. However, one daughter commercial crop became highly diseased, as it was stressed by drought and flood conditions (Manawatu).   

These results show that if seed-borne inoculum can be minimised through the supply chain, the impact on well-managed crops can be minimal, Alex Michel said.