Effects of temperature and wetness duration on infection of oilseed rape leaves by ascospores of Leptosphaeria maculans (stem canker)

In controlled environment experiments, ascospores of Leptosphaeria maculans (stem canker) infected oilseed rape (cv. Nickel) leaves and caused phoma l eaf spots at temperatures from 8 degrees C to 24 degrees C and leaf wetness durations from 8 h to 72 h. The conditions that produced the greatest numb ers of leaf spot lesions were a leaf wetness duration of 48 h at 20 degrees C; numbers of lesions decreased with decreasing leaf wetness duration and increasing or decreasing temperature. At 20 degrees C with 48 h of leaf wet ness, it was estimated that one out of four spores infected leaves to cause a lesion whereas with 8 h of leaf wetness only one out of 300 spores cause d a lesion. As temperature increased from 8 degrees C to 20 degrees C, the time from inoculation to the appearance of the first lesions (a measure of the incubation period) decreased from 15 to 5 days but leaf wetness duratio n affected the length of the incubation period only at sub-optimal temperat ures. Analyses suggested that, within the optimal ranges, there was little effect of temperature or wetness duration on incubation period expressed as degree-days; the time until appearance of 50% of the lesions was ca. 145 d egree-days. A linear regression of % leaves with lesions (P-l) (square-root transformed) on % plants with lesions (P-p) accounted for 93% of the varia nce: root P-l=1.31+0.061P(p). This relationship was also investigated in wi nter oilseed rape field experiments in unsprayed plots from October to Apri l in 1995/96 (cv. Envol), 1996/97 (cv. Envol), 1997/98 (cvs Bristol and Cap itol) and 1998/99 (cvs Apex, Bristol and Capitol) seasons. The linear regre ssion of % leaves with lesions (square-root transformed) on % plants with l esions accounted for 90% of the variance and had a similar slope to the con trolled environment relationship: root P-l=0.81+0.051P(p). These results we re used to examine relationships between the development of phoma leaf spot on plants in winter oilseed rape crops, the incubation period of L. macula ns and the occurrence of infection criteria (temperature, rainfall) in the autumns of 1996, 1997 and 1998.