Assessing the intensity of Fusarium-damage in wheat: A comparison of selected disease parameters during disease development and the role of fungicides

Disease severity (% of Fusarium-damaged spikelets), disease incidence (% of Fusarium-damaged wheat heads), Fusarium graminearurn DNA (pg per ng total DNA), deoxynivalenol (DON) content (mg DON per kg grain dry mass) and yield (t per ha) were recorded in a field experiment carried out in Northern Germany. Plants (susceptible cultivar Ritmo) were inoculated at anthesis by overhead application of a E graminearum mycelial suspension during a 6.7 mm precipitation event. Fungicides were applied around the time of wheat anthesis to generate different levels of Fusarium-damage. Infected heads were sampled in two-week-intervals post inoculation. Disease incidence increased almost linearly with time, reaching levels of about 80% at four weeks post inoculation. Disease severity increased in a slightly exponential manner with time, reaching levels of about 60% after four weeks. Six weeks after inoculation, Fusarium head blight symptoms could not be distinguished from blighting of wheat heads due to plant senescence anymore. The ratio of F. graminearum DNA and total DNA increased exponentially with time up to four weeks post inoculation. Relationships between fungal DNA and yield or DON were closer when DNA was extracted two weeks post anthesis in comparison with four weeks post anthesis. Hence, quantification of fungal DNA seems to be advantageous for assessing FHB in wheat at early stages of disease development. In the middle of disease development, disease severity allowed the most precise estimation of yield and DON contents. Towards the end of disease development, the DON content was the most useful variable to describe the intensity of Fusarium-damage, because it could be evaluated in senescent plant material. Yield of infected plants increased from about 6.5 to 10 t ha(-1) upon fungicide (Caramba, Folicur, or Input) application at two days before or two days after anthesis. Effects were smaller when fungicides were applied earlier or later. At equal application time, equal disease severity level, and equal fungal DNA concentration, effects of fungicides on yield were not significantly different (P = 0.734) whereas effects on the DON content were (P < 0.001).Folicur applied around the time of wheat anthesis decreased the DON content of the grain from about 14 mg kg(-1) to 9 mg kg(-1) whereas Caramba and Input decreased the DON content to about 5 mg kg(-1).