INFLUENCE OF CLONAL SUSCEPTIBILITY, LEAF AGE AND INOCULUM DENSITY ON INFECTIONS BY MELAMPSORA-LARICI-POPULINA RACES E1 AND E3

A study was conducted on the influence of clonal susceptibility, leaf age and inoculum density on the incubation and presporulation periods, and on the intensity of attack of Melampsora larici-populina races El and E3, under controlled conditions, on artificially inoculated popla r leaf discs. Both races were more virulent on susceptible clones and on 5- to 15-day-old leaves. On susceptible clones the intensity of att ack increased progressively with the increase of inoculum density. Whe re there was complete coverage of the leaf surface, uredia were smalle r in size as a result of the increase in density. The incubation perio d and, above all, the average presporulation period of susceptible clo nes proved to be longer with 1- to 3-day-old leaves and, generally, wi th the oldest. The epidemiological consequences of these results are d iscussed.