Abstract
Clubroot (Plasmodiophora brassicae) is a serious soil-borne disease in brassica crops world-wide. We report on a time series of soil samples from Swedish long-term fertility trials started in 1957, 1963 and 1966, which were analyzed for the amount of P. brassicae DNA. The crop rotations included a brassica crop every 4 or 6 years. All experimental sites with a 4-year rotation of oilseed rape, except one with calcium carbonate in the soil profile, showed high (>1000 fg DNA g−1 soil) levels of P. brassicae DNA after 9, 11 and 12 rotations. In contrast, detectable levels (>5 fg DNA g−1 soil) of P. brassicae were found only at one of five sites with a 6-year rotation of spring oilseed rape. In years with high levels of P. brassicae DNA, low yield was reported and a subsequent decline in P. brassicae DNA in soil was observed. Different NPK (nitrogen/phosphorus/potassium) fertiliser regimes resulted in similar P. brassicae DNA levels. The robustness and reliability of the method applied was verified by analyses of soil from individual plots compared with a mixture of plots and by repeated analyses of selected samples, which showed that P. brassicae DNA remained stable during dry storage.
Highlights
Clubroot in brassica crops, caused by the obligate endoparasite Plasmodiophora brassicae (Woronin), is recognised as a serious soil-borne disease associated with appreciable yield losses (Wallenhammar 1998; Strelkov and Hwang 2014)
The quantitative PCR (qPCR) method provides new possibilities to analyse the presence of pathogen DNA in archived samples and to re-evaluate observations in long-term field experiments
We examined clubroot disease development over time using a qPCR assay on archived soil samples from two series of crop rotation experiments with brassica crops running for 42 years
Summary
Clubroot in brassica crops, caused by the obligate endoparasite Plasmodiophora brassicae (Woronin), is recognised as a serious soil-borne disease associated with appreciable yield losses (Wallenhammar 1998; Strelkov and Hwang 2014) It is one of the economically most important diseases of cruciferous crops found throughout oilseed rape and vegetable brassica growing areas world-wide (Dixon 2009a, 2014). Field experiments were established in the 1950s-1960s in different parts of the country and comprised two crop rotations; one including a brassica crop and one including livestock manure, along with 16 combinations of manure and inorganic NPK (nitrogen/phosphorus/potassium) fertiliser. These fertilisers are compared in a 4-year rotation in southern Sweden and a 6-year rotation in central Sweden. Soil samples have been taken every 4th and 6th year in all crop rotations since the start of the experiments and stored dry
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