The importance of potato mop top virus (PMTV) in Scottish seed potatoes

Potato mop-top virus (PMTV) causes spraing, necrotic arcs or flecks in the flesh of tubers.  Infection is invariably linked with transmission by Spongospora subterranea, the cause of powdery scab.  

In order to assess the effectiveness of potential control measures and to aid growers with decisions on affected crops, the objectives of this project were to determine the extent of PMTV infection in Scottish seed potatoes, the effect on yield and quality, the transmission rate from seed to daughter tubers and the relative importance of seed and soil inoculum on disease development.

 Results

The transmission of PMTV from seed to daughter tubers in cvs Cara, Maris Piper, Nicola, Rooster, Slaney and Winston was assessed over two years at SASA’s Gogarbank Farm. Transmission from seed to daughter tubers was found to be less efficient than for aphid-borne viruses, with less than half the daughter tubers derived from PMTV-infected seed being infected by PMTV. Infection in daughter tubers from PMTV-free seed was generally very low or absent. In 2005, yield of daughter tubers from PMTV-infected seed was reduced significantly compared with that from PMTV-free seed but no difference was found in 2004. The development of symptoms on the growing plant varied with cultivar and year, probably because of differences in the summer temperatures in the two years.

 The importance of soil and seed inoculum on PMTV infection over two successive generations at over 25 sites was examined using cv. Cara as a model in which the initial seed came from a common source. The amount of PMTV infection in initial seed was very low, <1.0%, in both years. Powdery scab was present in the majority of daughter crops. A high incidence of powdery scab was not generally correlated with a high incidence of PMTV. However, when PMTV infection was found, it was rare for powdery scab to be absent. No correlation was observed between the amount of infection by PMTV or powdery scab on seed tubers and that on the daughter tubers. Soil would, therefore, appear to be the main source of infection of potato crops by PMTV and high amounts of tuber infection occurred after one growing season.  

A random selection of crops from 4 widely grown cultivars (cvs Hermes, Maris Piper, Saturna and Nicola) known to be susceptible to PMTV were selected from four major regions of Scotland covering 16 counties. The majority of crops were free of PMTV infection and spraing.  

In a glasshouse experiment with five cultivars, the development of powdery scab was much greater at 12°C than at 19°C or 27°C, where it was rare/absent. The incidence of PMTV infection in tubers was, however, similar at 12°C and 19°C but spraing was virtually absent at 19°C, even although the same post harvest treatment had been applied to all tubers to enhance its development. 

In summary, PMTV was not particularly prevalent, even on known susceptible cultivars. Although its occurrence is linked to the powdery scab organism, there was no correlation between the amount of tubers affected by powdery scab and that infected by PMTV. Soil is the principal source of infection and can infect a considerable amount of tubers potentially resulting in a crop being unmarketable because of spraing. Planting infected seed tubers in clean land also brings a risk of introducing PMTV into the soil.

To improve the understanding of PMTV in terms of transmission rate, disease development, yield and quality and the role of the powdery scab fungus (Spongospora subterranea) in infection.