Bioassays using detached tobacco leaves to determine the sensitivity of Peronospora tabacina to fungicides

Two bioassay methods are described which use detached tobacco leaves to measure the sensitivity of Peronospora tabacina to systemic fungicides. Tobacco leaves (13–15 cm²), treated with fungicides before or after detachment from the plant, were inoculated with sporangia in water drops and, after incubation in beakers and Petri plates, the disease severity and/or production of sporangia was determined 4–7 days after treatment with the fungicides. Of 15 systemic fungicides applied to detached leaves, eight N‐phenylamides at 0.066−1.0 μg ml⁻¹ controlled blue mould; metalaxyl was the most effective fungicide. Isolates of P. tabacina, collected in the field from tobacco plants grown in soil treated with metalaxyl, were not resistant to the fungicide applied to detached leaves prior to inoculation. The fungicide, applied to leaves before detachment, was used to measure the efficacy of five systemic N‐phenylamide fungicides sprayed on the basal and unsprayed distal portions of the leaves. Blue mould was controlled on the basal portion of the leaf by all the fungicides at 0.66−1.0 μg ml⁻¹, but it required the application of 3–30 times more chemical on the basal portion to achieve comparable blue mould control on the distal part of the leaf.