Quantifying the effect of reduced doses of propiconazole (Tilt) and initial disease incidence on leek rust development

The effect of reduced doses of propiconazole (Tilt) and different initial inoculation levels on leek rust caused by Puccinia allii was quantified by three parameters: (i) rate of plant-to-plant spread (b), (ii) the time taken for 50% plant infection (m) and, (iii) rate of appearance of new pustules. All propiconazole doses tested reduced values for b compared with an unsprayed treatment but lower doses had little effect on new pustule appearance. Although b values were different for the same propiconazole dose at two field sites with different epidemics, values compared with the unsprayed treatments were similar. Fungicide efficacy was therefore measured independently of disease level in the field. The parameter b for rust spread was also directly related to marketable field at harvest. Initial rust incidence levels of <0.1% infected plants reduced b compared with higher levels (0.5-5%). This suggests that rust spread occurs at a maximum rate unless incidence is extremely law. Reduced doses of propiconazole also suppressed spore germination of P. allii and the appearance of rust pustules in inoculated plants in the glasshouse. The value of the parameters identified is discussed in relation to leek rust control, epidemiology and the effect of weather.