Extending the scope of modern potato diagnostics and their interpretation

This project investigated aspects of potato disease diagnostics that remained unclear from two previous diagnostic projects. 

Soil sampling for Rhizoctonia solani

Whilst a successful soil sampling protocol had been established for detection and quantification of black dot and powdery scab, it was clear from testing commercial fields that it was inadequate for Rhizoctonia solani. 15 fields were sampled using the standard protocol for black dot and powdery scab and tested for the presence of R. solani AG3 (the main pathogen causing stem canker and black scurf).  From the results two fields were chosen for intensive sampling to ascertain the pattern of distribution of the pathogen.  From an analysis of the distribution, and using data from elsewhere, four new sampling protocols were tested along with the existing protocol in three fields.  At the same time optimum sampling depth was examined.

It was concluded that low levels of inoculum may escape detection but be sufficient to cause disease, if environmental conditions favour development of the pathogen.  It was suggested that sampling should contiune to be on a 4ha basis  but an increased number of sampling points (200) should be adopted and only the top 5cm of soil should be sampled.

Proficiency testing of labs offering diagnostic services

An inter-laboratory comparison was carried out on four pathogens, Colletotrichum coccodes, Spongospora subterranea, Rhizoctonia solani AG3 and Pectobacterium atrosepticum.  Standard spiked samples of soil or potato peel were constructed and dispersed to the three laboratories of participating organisations for testing blind.  Some limited variation in testing can be expected and on the whole there was consistency in testing between laboratories. 

Relationship between a soil test result for S. subterranea and powdery scab disease risk 

Two varieties were used: one very susceptible (Agria or Estima) and one moderately susceptible (Nicola).  Root infection was evident in both varieties from soon after emergence and was present at high levels throughout the period of monitoring. Root galling was observed around 4 weeks after tuber initiation and at least 5 weeks after root infection.  Agria showed significantly more gall symptoms than Nicola, and Estima the least. This is in agreement with previous EU trials where root galling was monitored across several sites.

Powdery scab symptoms were first observed in week 8 of sampling, around 6 weeks after tuber initiation. Overall, Estima was the most susceptible cultivar followed by Agria and then Nicola.  DNA of S. subterranea was detectable in symptomless tubers from the time of tuber formation at both sites, showing that infection takes place at a very early stage of development. 

To improve the interpretation of diagnostic test results for Rhizoctonia solani and Spongospora subterranea