COMPARATIVE EFFECTS OF TEMPERATURE AND INTERRUPTED WET PERIODS ON GERMINATION, PENETRATION, AND INFECTION OF PUCCINIA-RECONDITA F SP TRITICI AND P-STRIIFORMIS ON WHEAT SEEDLINGS

Under optimal temperature and nonlimiting wetness duration, the infection efficiency (defined as the proportion of inoculated urediniospores causing lesions on wheat seedling leaves) was 12 times greater for Puccinia recondita f. sp. tritici than for P. striiformis. Penetration of both species, however, was similarly affected by a 1-h dry period interrupting a 24-h wet period 1, 2, 4, 6, 8, or 16 h after inoculation at several temperatures between 5 and 30 C. Appressoria from germinated P. r. tritici urediniospores prior to penetration were unable to survive the I-h dry period. An interruption of the wet period by a dry period did not affect ungerminated urediniospores, which were able to infect leaves during a subsequent dew period. The minimal continuous dew period necessary for infection increased from 4 to 6 h at optimal temperatures (8 C for P. striiformis, 15 C for P. r. tritici) to at least 16 h at suboptimal temperatures. Infection by P. r. tritici occurred over a wide range of temperatures (5-25 C), whereas infection by P. striiformis was restricted to a narrower range (5-12 C). Percentage of infection as a function of the duration of the continuous dew period was described by a Richards's function with temperature-dependent parameters. For a dry period interrupting the 24-h dew period before the minimal continuous dew period necessary for infection, percentage of infection at specific temperatures was fitted by a negative exponential function of time of interruption. If the dry period occurred after the minimal dew duration for infection, percentage of infection was the same as with a continuous wet period.