Epidemiology of chocolate spot of faba bean
Assess chocolate spot and other diseases of faba bean incidence and severity in Saskatchewan and Alberta. Determine conditions for chocolate spot disease development.
Surveys for foliar diseases of commercial faba bean fields in Saskatchewan and Alberta were completed in 2017, 2018 and 2019. All faba bean crops surveyed had foliar lesions in all years, but disease severity was low across all fields. Botrytis species that cause chocolate spot were commonly present but inflict higher disease severity in wet years. Furthermore, Stemphylium blight was common, sometimes more so than chocolate spot, and it’s etiology and impact on yield need further study. Information on chocolate spot risk periods and weather factors leading to inoculum release under prairie conditions is lacking. Therefore, spore trapping in field trials was conducted in 2017, 2018 and 2019 at three locations in Alberta to determine the duration of inoculum discharge and infectious periods under field conditions by using 3-week-old faba bean plants as trap plants. Results indicated that infectious periods varied greatly by location and year, and that multiple infectious periods can occur within a growing season. Data analysis showed that temperature and dew point temperature significantly affected disease development and progression. High temperatures were consistently negatively correlated with DS while all other factors were positively correlated to DS, indicating that cooler temperatures and wetter environments contribute to infectious periods. However, threshold levels or specific ranges of weather variables could not be directly calculated to predict infectious periods based on correlations to disease severity, as there was too much variability in the patterns from year to year. Experiments were also performed to determine the optimum inoculum concentration, temperature and leaf wetness durations necessary for the disease development. The results from the experiments confirmed finding from the field trapping experiments, as disease was highest at cooler night time temperatures of 10 – 15oC, and exposure to dew point temperature of 5oC for 3 hours. Minimum inoculum concentration required to cause disease was 103 conidia/ml but maximum disease occurred at inoculum concentrations greater than 105 conidia/ml. In conclusion, because inoculum was present throughout the growing season, it did not appear to be closely correlated to any specific weather patterns. This means that disease development, or progression of an infection to disease symptoms, is more heavily dependent on environmental conditions, and particularly cool night-time temperatures and dew periods. These environmental conditions should be considered as risk factors for chocolate spot development and fungicide timing, since inoculum is always present.