Temperature alteration of phenotypic expression of spring wheat resistance to Stagonospora nodorum spot

Abstract

The spring wheat Triticum aestivum L. cultivars Fortuna, Newana, and Manitou, previously known as susceptible, moderately resistant, and resistant, respectively, to Stagonospora nodorum spot, caused by Phaeosphaeria nodorum, were maintained at postinoculation temperatures of 12, 18, 20, 24 or 28°C. At 20°C, reactions were similar to those previously reported for all cultivars with Fortuna being the most severely attacked by the fungus. However, resistance to Stagonospora nodorum spot was markedly influenced by temperatures of 24 °C; the Manitou reaction changed from resistant to susceptible. The moderately resistant reaction of Newana was unchanged with temperatures of 18 to 28°C, Minimal disease severity occurred at 12°C in all germplasms evaluated. There were no significant differences in percentage of leaf area showing symptoms among cultivars at 28 °C. Maximum disease development and the highest number all lesions per cm² were observed in plants of Fortuna and Newana kept at 18-24 °C and in plants of Manitou kept at 24 °C. Incubation period was reduced as the temperature increased from 12 to 24 °C in all germplasms tested. These data suggest that temperature might be one factor responsible for contrasting results in resistance tests and for the absence of durable resistance to Stagonospora nodorum spot in Brazil.