EFFECTS OF TEMPERATURE AND LEAF WETNESS ON THE LATENT PERIOD OF RHYNCHOSPORIUM-SECALIS (LEAF BLOTCH) ON LEAVES OF WINTER BARLEY

Effects of temperature and leaf wetness on the latent period of Rhynch osporium secalis (leaf blotch) on winter barley were examined in contr olled environment experiments. At 100% relative humidity (continuous l eaf wetness) the mean length of the latent period was c. 24 days at 5 degrees C, c. 19 days at 10 degrees C, c. 16 days at 15 degrees C and c. 13 days at 20 degrees C. The mean number of days between the appear ance of the first and the last lesions was c. 13 days at 5 degrees C, c. 6 days at 10 degrees C, c. 5 days at 15 degrees C and c. 3 days at 20 degrees C. A negative curvilinear regression of latent period on te mperature accounted for 99% of the variance. The mean area of lesions per leaf was 38 mm(2) at 5 degrees C, 46 mm(2) at 10 degrees C, 24 mm( 2) at 15 degrees C and 24 mm(2) at 20 degrees C. At 10 degrees C, afte r a 48 h wet infection period, the interruption of leaf wetness for 5 or more days at any time during the next 15 days of the latent period did not decrease subsequent lesion area. However, absence of leaf wetn ess after these 15 days, at the onset of sporulation, did decrease the area of lesions which developed.