Reducing the impact of sclerotinia disease on arable rotations, vegetable crops and land use
Sclerotinia disease, caused by the fungus Sclerotinia sclerotiorum, has two key phases susceptible to control: long-lived resting bodies (sclerotia) in soil, and airborne spores, produced when sclerotia germinate to produce mushroom-like apothecia, starting in spring. This project investigated control during these key phases, with two main objectives:  To improve timing of fungicide applications, based on weather data and/or airborne inoculum detection, and  To quantify the effect of soil management and crop rotation on sclerotinia disease.
Two forecasting models were tested for predicting infection by sclerotinia. First, a sclerotial germination model correctly predicted onset of germination by region (SW earliest, NW latest) and by year, but with an error of + two weeks for individual sites. The model provides a useful forecast guide to the main onset of spore inoculum release by region. Second, a weather-based infection model was tested with sprays applied according to forecast alert dates. The model gave 76–96% control of sclerotinia disease in oilseed rape, equivalent to the best control determined in retrospect with a standard fungicide timing. During experiments, sclerotinia inoculum was measured by petal testing, and high infection (90–100%) in oilseed rape was associated with 15–30% stem rot. Zero or very low petal infection often resulted in low stem rot. Air samples from Burkard spore traps were tested for the presence of sclerotinia DNA using quantitative PCR tests. Peaks of inoculum detected by rooftop spore traps, in the same region as the experimental field sites, generally coincided with the timing of inoculum peaks from air samples from spore traps within-field, suggesting that that regional spore traps can indicate infection risk. The key factors for assessing sclerotinia risk in-field were: infection model alerts, petal infection and forecast temperature and rain.
Optimum crop rotations for sclerotinia control were modelled using dynamic programming, aimed at maximising profits while reducing sclerotinia. The model showed that only one non-susceptible crop in a rotation is needed to prevent long term build-up of sclerotia while also providing the greatest financial benefit. The model confirmed that rotation gives the greatest financial benefits for high sclerotinia pressure, but is also the best financial strategy for low sclerotinia.
In experiments where sclerotial production on different crops was investigated, carrots produced several thousand sclerotia/m2 (high plant density, small sclerotia), whereas oilseed rape produced a few hundred (lower plant density and large sclerotia). Numbers in lettuce, peas, beans and potatoes were intermediate. It appears that an infected carrot crop may pose the highest risk to following crops in the rotation, but there may be a modifying effect of sclerotial size, i.e. small sclerotia produce fewer apothecia. The amount of sclerotinia spore inoculum produced in spring was similar in oilseed rape crops following ploughing or minimum-tillage, as determined from testing petals for sclerotinia presence.
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