Aim: To determine if field trials, using PMTV-free tubers planted into naturally infested field soils, can be used to provide reliable information on variety susceptibility to PMTV.
At present, only limited and anecdotal evidence exists on the relative susceptibility of potato cultivars to potato mop top virus (PMTV) and to symptom expression on plants and tubers. Therefore work is needed to establish the relative susceptibility of the most popular cultivars, particularly the extent to which infection may occur without symptom development (symptomless infection).
However, the complex epidemiology of PMTV makes it more difficult to assess cultivar susceptibility employing the type of pot-tests generally used for disease testing.
Ten GB grown cultivars were planted at two sites, in each of two years and a programme of testing samples of soil and tubers pre-planting and post-harvest was carried out. The recommendations from the work include:
1. Testing potato varieties for susceptibility to PMTV should concentrate on assessing susceptibility to transmission and spraing over one season by exposing test material to infection from inoculum in the growing medium. The development of foliar symptoms is significantly influenced by environmental conditions and is a poor indicator of PMTV infection and, thus, not a suitable tool for assessing varietal resistance.
2. If a field test is used for varietal testing, the plot area should be tested for PMTV before planting to ensure that inoculum is uniformly distributed across the intended area. However, the current test using tomato as bait plant followed by detection of PMTV in roots by real time PCR cannot give an accurate assessment of the amount of infestation in the soil because the bait plant acts as an enrichment step distorting the amount of inoculum originally in the sample. A pot test, using amended compost, could be considered as an alternative to conducting field tests but further work is required to establish the reproducibility of the test.
3. Tests should be conducted for more than one year because differences amongst varieties in susceptibility to PMTV infection, as with the ten tested here, may be relatively small and differences in respect to spraing appeared to be variable amongst years and trials. The reasons for this variability in spraing reaction are unclear and factors affecting spraing development may merit further investigation, both for testing and grower management purposes.
4. Planting PMTV-infected seed in clean land gave variable results in terms of infesting the soil and the risks involved for growers merit further study.
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