Effect of temperature on latent period of septoria leaf blotch on winter wheat under outdoor conditions

Batches of two winter wheat cultivars (Riband and Apollo) were inoculated with conidia of Mycosphaerella graminicola at weekly intervals over a 2 year period. Following 72 h incubation, plants were placed in ambient temperatures ranging between -7 and 32degreesC with mean batch temperatures of 2.9-20.2degreesC. Latent period until the first visible symptoms ranged between 11 and 42 days. The relationship between development of lesions and accumulated thermal time was described using a shifted cumulative gamma distribution model. The model provided good estimates of lesion development with r(2)>0.92 for both cultivars. Base temperatures, below which the pathogen did not develop, were estimated from the model as approximately -2.4degreesC for the two cultivars. Latent period was estimated as being 250 and 301 degree-days above the estimated base temperature, when defined as time from inoculation to first lesion and time to 50% of maximal lesions, respectively, for cv. Riband. The values for cv. Apollo were similar, but with estimates of thermal time periods c. 5% higher. The relationship between mean temperature and inverse latent period, expressed as days either to first lesion or to 50% of maximal lesions, was best described by a linear regression with r(2)>0.96 for both cultivars. The opportunity for plants to outgrow disease was reduced when prolonged periods of cold temperature occurred, because the base temperature for growth of the pathogen was less than that for the crop.