Quantitative relationships between inoculum of Melampsora larici-epitea and corresponding disease on Salix

Six Salix clones were inoculated with urediniospores of four isolates of Melampsora larici-epitea at five inoculum levels using a leaf-disc method. Disease reactions were recorded using a digital camera; the number and size of uredinia were examined using image analysis software; and spore yield per leaf disc was measured. In three Salix / Melampsora combinations, S.  ×  mollissima 'Q83'/Q1 (LET4); S. viminalis '78183'/V1 (LET1); and S.  ×  stipularis /V1, pustule numbers increased as inoculum density became higher. In the remainder, S. viminalis 'Mullatin'/V1; S.  ×  calodendron /DB (LET3); and S. burjatica 'Korso'/K (LR1), pustule numbers initially increased, then decreased as inoculum densities exceeded 140–360 spores per disc. Calculated infection efficiency ranged from 0·11 to 0·20 on the three willows inoculated with V1: 0·16–0·68 for S.  ×  calodendron /DB; 0·20–0·55 for 'Q83'/Q1; and 0·07–0·48 for Korso/K. In single-spore inoculations, up to 10% of spores produced single uredinia. Infection efficiency increased sharply between inoculum densities of 1–40 spores per leaf disc. Spore yield was more closely correlated to pustule area (accounting for 61·2% variance for the combined data) than to the number of pustules (42·7% variance). For spore yields in relation to pustule numbers, clone-specific individual lines having different intercepts and slopes fitted significantly better than either a single line for all the tested willows, or parallel lines fitted to each clone ( P  < 0·001). For spore yields in relation to pustule area, clone-specific individual parallel lines were significantly better than a single line ( P  < 0·001).