The importance of potato mop-top virus (PMTV) in Scottish seed potatoes
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The key aim of this research was to determine the extent of PMTV infection in Scottish seed potatoes and the critical factors which influence infection. The research incorporated a survey of PMTV infection in susceptible cultivars in Scotland, a glasshouse trial to determine the role of temperature in the transmission of PMTV from the soil to the host plant, and field trials which studied the transmission rate from seed to daughter tubers and the relative contribution of seed and soil inoculum to disease development. The survey of Scottish seed crops showed that PMTV occurs in all regions of Scotland but is not particularly prevalent even on known susceptible cultivars. The incidence of crops infected by PMTV differed greatly amongst the regions, with more crops grown in Central Scotland being infected than elsewhere. Although the occurrence of PMTV is linked to the powdery scab organism, there is no correlation between the occurrence of powdery scab and PMTV infection. Temperature was found to be an important factor in the occurrence of symptoms of PMTV infection. The incidence of PMTV infection in tubers was similar at 12°C and 19°C but spraing was absent at 19°C. Transmission from seed to daughter tubers was found to be inefficient, with less than half the daughter tubers derived from PMTV-infected seed being infected by PMTV. However, high incidences of tuber infection were often present in crops after one growing season indicating that soil inoculum is the main source of PMTV infection. Seed-borne inoculum is also of great importance as planting infected seed tubers in clean land brings a risk of introducing PMTV into the soil.