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 infect ion efficiency (defined as the proportion of inoculated urediniospores causing lesions on wheat seedling leaves) was 12 times greater for Pu ccinia recondita f. sp. tritici than for P. striiformis. Penetration o f both species, however, was similarly affected by a 1-h dry period in terrupting 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 germinate d P. r. tritici urediniospores prior to penetration were unable to sur vive the I-h dry period. An interruption of the wet period by a dry pe riod did not affect ungerminated urediniospores, which were able to in fect leaves during a subsequent dew period. The minimal continuous dew period necessary for infection increased from 4 to 6 h at optimal tem peratures (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. st riiformis was restricted to a narrower range (5-12 C). Percentage of i nfection as a function of the duration of the continuous dew period wa s described by a Richards's function with temperature-dependent parame ters. For a dry period interrupting the 24-h dew period before the min imal continuous dew period necessary for infection, percentage of infe ction at specific temperatures was fitted by a negative exponential fu nction of time of interruption. If the dry period occurred after the m inimal dew duration for infection, percentage of infection was the sam e as with a continuous wet period.