Abstract
Sclerotinia stem rot (SSR) is the most important disease of oilseed Brassica crops in Norway. Fungicide applications should be aligned with the actual need for control, but the SSR prediction models used lack accuracy. We have studied the importance of precipitation, and the role of petal and leaf infection for SSR incidence by using data from Norwegian field and trap plant trials over several years. In the trials, SSR incidence ranged from 0 to 65%. Given an infection threshold of 25% SSR, regression and Receiver Operating Characteristics (ROC) analysis were used to evaluate different precipitation thresholds. The sum of precipitation two weeks before and during flowering appeared to be a poor predictor for SSR infection in our field and trap plant trials (P = 0.24, P = 0.11, respectively). Leaves from three levels (leaf one, three, five), and petals were collected at three to four different times during flowering from nine field sites over two years and tested for SSR infection with real-time PCR. Percentage total leaf and petal infection explained 57 and 45% of variation in SSR incidence, respectively. Examining the different leaves and petals separately, infection of leaf three sampled at full flowering showed the highest explanation of variation in later SSR incidence (R2 = 65%, P < 0.001). ROC analysis showed that given an infection threshold of 45%, both petal and leaf infection recommended spraying when spraying was actually needed. Combining information on petal and leaf infection during flowering with relevant microclimate factors in the canopy, instead of the sum of precipitation might improve prediction accuracy for SSR.
Highlights
Sclerotinia stem rot (SSR), caused by the ascomycete fungus Sclerotinia sclerotiorum (Lib.) de Bary and in some instances S. subarctica nom. prov. (Brodal et al 2017; Taylor et al 2018) is one of the major diseases in Brassica oilseed crops (Buchwaldt 2007)
Sclerotinia sclerotiorum has a wide host range of over 400 plant species (Boland and Hall 1994) and yield losses of up to 50–60% have been recorded in oilseed rape (Pope et al 1989; Nordin et al 1992)
>25 a OSR, Oilseed rape; TR, Turnip rape b GS, Growth stage c SSR, Sclerotinia stem rot d Average of the two weather stations e The predicted SSR of above 25% is based on ‘sum of precipitation’ above 25 mm, while SSR is predicted to be below 25%, when the ‘sum of precipitation’ is below 25 mm development of different S. sclerotiorum and S. subarctica isolates on the stem by our research team
Summary
Sclerotinia stem rot (SSR), caused by the ascomycete fungus Sclerotinia sclerotiorum (Lib.) de Bary and in some instances S. subarctica nom. prov. (Brodal et al 2017; Taylor et al 2018) is one of the major diseases in Brassica oilseed crops (Buchwaldt 2007). Sclerotinia stem rot (SSR), caused by the ascomycete fungus Sclerotinia sclerotiorum (Lib.) de Bary and in some instances S. subarctica nom. (Brodal et al 2017; Taylor et al 2018) is one of the major diseases in Brassica oilseed crops (Buchwaldt 2007). Sclerotinia sclerotiorum has a wide host range of over 400 plant species (Boland and Hall 1994) and yield losses of up to 50–60% have been recorded in oilseed rape (Pope et al 1989; Nordin et al 1992). Due to increased cropping and little rotation with non-SSR hosts in major oilseed Brassica production areas such as Germany and Canada, annual yield losses due to SSR are likely to increase and will require improved control methods (Derbyshire and Denton-Giles 2016)
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