Epidemiology of light leaf spot (Pyrenopeziza brassicae) on winter oilseed rape (Brassica napus)

Forecasts of the severity of light leaf spot of winter oilseed rape are needed to help growers with their decisions on fungicide applications at times when sprays are needed to control light leaf spot, but the disease is difficult to diagnose. A thorough understanding of stages in development of Pyrenopeziza brassicae contributing to light leaf spot epidemics on winter oilseed rape and how these stages in development are affected by weather factors can be used to develop models, which can predict epidemic progress under different weather conditions. A risk forecast system is currently in use, which on a regional level predicts leaf disease incidence in March based on a regression with pod disease incidence in July, summer temperatures and winter rain. This regional forecast system can be made more specific for individual winter oilseed rape crops by incorporating models which can predict the effects of weather conditions on stages in development of P. brassicae . The main aims of this research project were firstly to investigate the light leaf spot epidemic cycle and secondly to identify stages in the development of P. brassicae , which are critical for epidemic progress and investigate how these stages in development are affected by weather factors.Air-borne ascospores of P. brassicae released from stem and pod debris from previous winter oilseed rape crops are the main primary inoculum initiating light leaf spot epidemics on winter oilseed rape in the UK. Secondary spread of light leaf spot occurs by conidia, which are dispersed over short distances by splash during rain showers. Several cycles of conidial dispersal, infection and sporulation can occur during late autumn, winter and spring. In spring and summer, both ascospores and conidia are present when stem, flower and pod infections occur. Ascospores are produced on infected leaf debris underneath crops and ascospores, which are dispersed upwards in crops by wind, could cause infections of stems, flowers and pods. Conidia can be dispersed upwards in crops by rain splash and thereby cause infections at a higher canopy levels in crops. Also, it has been suggested that conidia can cause latent infections of primordia during late winter, which become apparent after stem extension in spring. Thus, light leaf spot epidemics are polycyclic and both wind-dispersed ascospores and splash-dispersed conidia contribute to disease progress.The sexual stage of P. brassicae develops only after senescence of oilseed rape tissues. After a short phase of saprophytic growth on the senesced tissues, globular structures of immature apothecia develop. The apex of a globular structure depresses before the disk expands and asci are able to release ascospores. Apothecia develop to maturity at temperatures from 6 to 18 °C when debris is wet, but do not develop at or above 22 °C. As temperature decreases from 18 to 6 °C the rate of apothecial development decreases, but the time to apothecial decay increases and thereby increases the duration of time over which mature apothecia are present on debris. An interruption in wetness delays apothecial development and decreases the number of mature apothecia, but does not inhibit apothecial development. The effects of temperature on the time to apothecial development and the time to apothecial decay have been described in models. The model describing the effects of temperature on apothecial development successfully predicted the time to the first observation of mature apothecia on pod debris, which was incubated under natural conditions in a field plot. The model describing the effects of temperature on apothecial decay over-predicted the time when apothecia were present on the pod debris outdoors. It is suggested that rapid wetting and drying of debris causes apothecia to release their ascospores and when the ascospores have been released apothecia decay.Infection by conidia of P. brassicae is affected by temperature and leaf wetness duration. On oilseed rape (cv. Bristol), infections can occur at temperatures ranging from 4 to 20 °C, but not at or above 24 °C. Infections are successful only if leaf wetness duration is longer than a minimum length of time. The minimum leaf wetness duration, which is required for infection, is temperature-dependent. At temperatures ranging from 12 to 20 °C, the minimum leaf wetness duration required for infection is 6 h, and increases to 10, 10-16 or 16-24 h as temperature decreases to 8, 6 or 4 °C, respectively. Measurements of leaf wetness duration in previous field experiments suggest that leaf wetness duration in the UK is frequently longer than the minimum leaf wetness duration required for infection. Thus, leaf wetness duration is often not a factor limiting to infection. The latent period of P. brassicae is shortest at c. 16 °C and increases as temperature increases from 16 to 20 °C or as temperature decreases from 16 to 4 °C. Leaf wetness duration also affected latent period at 6 and 8 °C; latent period of P. brassicae decreased when leaf wetness duration increased.The effects of temperature and leaf wetness duration on light leaf spot development following single conidial infections in controlled environments were described in a model with the parameters maximum disease severity, maximum rate of increase in disease severity and latent period (time from inoculation to 37% of the maximum disease severity; this corresponds with the steepest slope of the Gompertz curve). This model predicted well the latent period of an independent data set for disease severity with time after conidial infections in controlled environments at 12 or 18 °C after 16 or 48 h leaf wetness duration. However, levels of disease severity were not predicted well by the model. The part of the model describing the effect of temperature on the latent period of P. brassicae was tested with data for outdoor pot plants showing increases in disease severity in relation to rainfall events. The model predictions for a latent period following 'heavy' rainfall events (≥2 mm of rain h ^-1 ) for > 0.5 h corresponded well with observed maxima in the increase in disease severity, but less 'heavy' rain events did not correspond well with the observed maxima in the increase in disease severity.The models predicting the effects of weather on the time to development of mature apothecia of P. brassicae and the time to the greatest increase in disease severity following conidial infection events can now be incorporated in systems forecasting light leaf spot severity on winter oilseed rape. By incorporating these models based on relationships with weather in the existing regional risk forecasting system, the effects of local variation in weather on epidemic progress can be predicted, and forecasts can become more specific to individual winter oilseed rape crops. For the model predicting the time to development of mature apothecia measurements of temperature and debris wetness are input; for the model predicting the time to the greatest increase in disease severity following conidial infection events measurements of temperature and rain intensity are input. Although temperature is measured by many growers, rain intensity and debris wetness are usually not measured by growers. Rain intensity and debris wetness can probably be measured by farm advisors for several farmers within a region. Therefore, although the newly developed models have the potential to predict the development of P. brassicae at an individual crop level, this potential is often not obtainable because of the lack of equipment to measure specific weather parameters