| Abstract: | ABSTRACT Severe Cercospora leaf spots epidemics in sugar beet during the late 1980s and early 1990s in southern Germany prompted us to initiate investigations on the epidemiology of the causal agent, Cercospora beticola. The data set involved 69 field trials (1993 to 2003) focusing on factors affecting the epidemic onset of this disease. Observations were made at weekly intervals, recording the calendar week when canopy closure occurred (growth stage according to BBCH scale = 39) and symptom development by assessing the percentage of infected leaf area on a single-leaf basis (n = 40 plants). These monitoring trials revealed that epidemic onset varied between early July and mid-September. Hence, the target was to identify the reasons for this variation in order to deduct the most suitable approach for predicting epidemic onset. Differences in cultivar resistance explained part of epidemic onset variability, as did different timings of canopy closure, presumably due to associated microclimate changes. Moreover, meteorological variables were considered as potential reasons for variation in epidemic onset. The weather-dependent infection probability was assessed by daily infection values (DIV) in the range from 0 to 1 using hourly weather data. For calculating DIVs, the temperature effect was quantified by the proportions of the latent period (LP) relative to the optimum at 20 to 25 degrees C, established by artificial inoculation of sugar beet plants in growth cabinets. Artificial infection experiments further established that air relative humidity (RH) >95% or leaf wetness was required for infection and subsequent lesion development. Under field conditions, the probability of leaf wetness was 75% at RH >90%. Therefore, DIVs were set to 0 for RH
|
